What Is Shale Gas?

Shale gas refers to organic-rich, mature dark-colored shale or high-carbon mud shale. Due to the adsorption of organic matter or the existence of cracks and matrix pores in the rock, it has stored and preserved certain biological genesis of commercial value. , Natural gas due to pyrolysis and a mixture of the two. ^[1]

Shale gas is found in unconventional natural gas in the reservoir rock series mainly composed of organic-rich shale. It is a continuously generated biochemical gas, thermogenic gas, or a mixture of the two. It can exist in natural cracks and pores in a free state, exist on the surface of kerogen and clay particles in an adsorbed state, and store a small amount in cheese in a dissolved state In roots and asphaltenes, the proportion of free gas is generally 20% to 85%. ^[2]

Chinese name: shale gas
Foreign name: Shale Gas
Category: Energy and Chemical

China reserves: 36.1 trillion cubic meters
Features: Self-storage
Cause: Thermal cracking and biogenic

Introduction to shale gas

Shale gas is a natural gas resource that can be exploited in shale layers. China's shale gas reserves are relatively large. The formation and enrichment of shale gas has its own unique characteristics, which are often distributed in shale source rock strata with large thickness and wide distribution in the basin. Compared with conventional natural gas, shale gas development has the advantages of long production life and long production cycle. Most of the shale gas produced has a wide distribution range, a large thickness, and generally contains gas. This allows shale gas wells to be stable for a long period of time. Rate of gas production. ^[3]

Shale gas is found in organic-rich mud shale and its interlayers. It is an unconventional natural gas with adsorption and free state as its main existence mode. It is mainly composed of methane. It is a clean and efficient energy resource and chemical raw material. It is widely used in residential gas, urban heating, power generation, automobile fuel and chemical production. In the shale gas production process, drainage is generally not required, and the production cycle is long, generally 30 to 50 years. The exploration and development success rate is high, and it has high industrial economic value. ^[4] According to forecasts, the resources of China's major basins and regions are about 36 trillion cubic meters, with great economic value and broad prospects for resources. ^[5]

Characteristics of shale gas

Shale gas, like deep basin gas and coal bed gas, belongs to "continuous" accumulation of unconventional natural gas. ^[1]

Shale gas generation model The formation, adsorption, and dissolution of natural gas in shale have the same mechanism and process as coalbed methane. As shown in the figure, the natural gas produced by biological or thermal maturation first meets the need for adsorption of organic matter and the surface of rock particles. At this time, the shale gas formed is mainly stored in the shale in an adsorbed state. When the amount of adsorbed gas and dissolved escape gas reach saturation, rich shale gas desorbs into the matrix pores. As a large amount of natural gas is generated, the internal pressure of shale rises, and gap formation and discharge occur. Free natural gas enters shale fractures and accumulates. ^[1]

Shale lithology is mostly asphaltene or organic-rich dark, black mud shale and high-carbon mud shale. The rock composition generally includes 30% to 50% clay minerals and 15% to 25% silty sand ( Quartz particles) and 4% to 30% organic matter. Because of the characteristics of shale, natural gas in shale exists in many ways, mainly including two forms, namely free state (largely existing in shale pores and fractures) and adsorption state (largely existing in clay Minerals, organic matter, kerogen particles and pore surfaces), among which the natural gas existing in the adsorbed state accounts for more than 20% (BarnettShale) to 85% (LewisShale) of the total natural gas occurrence. ^[1]

Shale gas shale gas formation

Shale gas formation causes

Previous studies on the genesis of shale gas in the five major shale gas basins in the United States have shown that shale gas can evolve from the following two ways. ^[6]

1. Thermal cracking genesis (naturally generated)

Formation pathway There are three ways for the formation of pyrogenic gas in shale (pictured): kerogen is decomposed into gas and asphalt; the asphalt is decomposed into oil and gas (step 1 and step 2 are the primary cracking); oil is decomposed into gas, High carbon content coke or asphalt residue (secondary cracking). The last step depends mainly on the amount of oil remaining in the system and the adsorption of the reservoir. Barnett shale gas in the Fort Worth Basin in Texas is derived from kerogen thermal degradation and secondary cracking of residual oil, mainly due to the secondary cracking of residual oil. Because of this, Barnett shale gas Has greater resource potential. ^[1]

Shale gas is natural gas extracted from shale layers. The main body is located in dark mud shale or high-carbon mud shale. Shale gas is mainly present in mudstones, high-carbon mudstones, shales, and powders in an adsorbed or free state. Natural gas in sandy interlayers, which can be generated on natural gas bodies at various stages of organic origin, exists in fractures, pores, and other storage spaces in a free phase state (about 50%), and in an adsorbed state (about 50%) It exists on the surface of kerogen, clay particles and pores, and is stored in a small amount in kerogen, asphaltenes and petroleum in a dissolved state. Natural gas also exists in interbedded siltstones, silt mudstones, muddy siltstones, and even sandstone formations. After natural gas generation, it accumulates nearby in the source rock formation, which is typical

Shale gas extraction The in situ accumulation pattern is quite different from oil shale, oil sands, and asphalt. Unlike conventional reservoir gas reservoirs, shale is both a source rock for natural gas generation, and a reservoir and caprock that accumulates and preserves natural gas. Therefore, black shales and high-carbon mudstones with high organic matter content are often the best conditions for shale gas development. ^[2]

2.Biogenic gas

Generally refers to the gas that is directly degraded by bacteria during the biochemical stage of shale formation. There are also biogases that are later transformed by gas reservoirs. For example, the Antrim shale gas in the Michigan Basin in the United States is the thermally degraded gas generated during kerogen maturation and the biogenic gas generated in the methanogen's metabolic activities. The latter is mainly the latter. The reason may be that well-developed fracture systems not only allow natural gas and primitive formation water carrying a large number of bacteria to enter the Antrim Shale, but also atmospheric intrusion from the aquifer in the Upper Pleistocene glacier drift, which is beneficial to the formation of bacterial methane. . ^[1]

Shale gas formation conditions

1.Sedimentary environment

Faster sedimentary conditions and a more closed reducing environment are important conditions for the formation of black shale. The faster sedimentation rate can enable a large amount of organic-rich shale to be deposited before being destroyed by oxidation, and the lack of oxygen in the water body can inhibit the activity of microorganisms and reduce its damage to organic matter. For example, the organic-rich black shale of the Barnett Formation in the Fort Worth Basin is deposited in a deep-water (120-215 m) foreland basin. It has hypoxic-anaerobic characteristics below the storm wave base and hypoxic zone (OMZ), and communicates with the open sea. limited. ^[1]

2.Effective thickness

Widely distributed shale is an important condition for shale gas formation. At the same time, the effective thickness of the deposit is a prerequisite to ensure sufficient organic matter and sufficient storage space. The thicker the shale, the stronger the shale's capping ability, which is conducive to the preservation of gas and thus to the formation of shale gas . The net shale thickness of the five major shale gas exploration and production areas in the United States is 9.14 to 91.44 meters, and the average thickness of the Barnett shale and Lewis shale with higher gas production is more than 30.48 meters. ^[1]

3. Total organic carbon content (TOC)

The total organic carbon content is an important index for evaluating the abundance of source rocks, and is also an important parameter for measuring the intensity and amount of hydrocarbon generation. Organic carbon content changes with lithology. For clay-rich mud shales, the organic carbon content is the highest due to the large amount of adsorption. Therefore, the lower limit of organic content of shale as a potential source rock is When the type of organic matter in the source rock is better and the degree of thermal evolution is higher, the corresponding lower limit of the organic carbon content is lower. For the lower limit of organic carbon content in argillaceous source rocks, domestic and foreign opinions are basically the same, ranging from 0.4% to 0.6%, while the lower limit of organic carbon content in argillaceous source rocks is different. A large number of research results show that gaseous hydrocarbon molecules are small, have strong solubility in water, and are easy to migrate. The lower limit of organic carbon content of gas source rocks is much lower than that of oil source rocks. The five major shale gas systems in the United States have high total organic carbon content and a large distribution range (0.5% to 25%), which can be divided into two types. The TOC content of Antrim shale and New Albany shale is high, and is generally distributed in The TOC content of Ohio shale, Barnett shale, and Lewis shale is between 0.45% and 4.7%. ^[1]

4, kerogen type and maturity

In different sedimentary environments, kerogen formed from different sources of organic matter has obvious differences in composition, and its properties and potential for oil and gas generation are also very different. Therefore, studying the type (property) of kerogen is an important part of oil and gas geochemistry, and it is also the basis for evaluating the potential of kerogen oil and gas. The kerogen type is a parameter to measure the hydrocarbon generation ability of organic matter. Different types of kerogen also determine whether the product is mainly oil or gas. Generally, type and type 2 kerogen are mainly oil, and type kerogen are mainly gas. Looking at the types of shale kerogen in the US shale gas basin, mainly type kerogen and type kerogen, there are also some type kerogen, and different types of kerogen type shale have generated a considerable amount of gas. There are reasons to believe that kerogen type is not a key factor in determining gas production. The abundance of dispersed organic matter and the type of hydrocarbon-forming parent matter in sedimentary rocks are the material basis for oil and gas generation, and the maturity of organic matter is the key to oil and gas generation. Only when kerogen has reached a certain degree of maturity can a large amount of hydrocarbon generation and expulsion begin. Different types of kerogen have different amounts of hydrocarbon generation at different stages of thermal evolution. In the low maturity stage (0.4% to 0.6%), organic matter can be transformed into hydrocarbons. The thermal maturity distribution of shales in the five major shale basins in the United States ranges from 0.4% to 2.0%. It can be seen that shale gas is generated throughout the organic hydrocarbon generation process. With the increase of maturity, the crude oil produced in the early stage begins to crack into gas. The reason why the Barnett shale in the United States has a large gas content is mainly due to the hydrocarbon generation volume (results of organic matter abundance, hydrocarbon generation potential, and shale thickness), maturity, and continued cracking of some liquid hydrocarbons. The lower the maturity of the Barnett shale area, the lower its gas production may be because less gas is generated and the flow of residual hydrocarbons blocks the pores. The cracking of kerogen and oil in many highly mature Barnett shale areas has resulted in a significant increase in gas production, leading to large gas flow in shale gas wells. Therefore, maturity is a key geochemical parameter for evaluating the similarity of high-flow shale gas. ^[1]

Factors affecting shale gas accumulation

1.Porosity

In conventional reservoirs, porosity is an important aspect to describe reservoir characteristics. The same is true for shale reservoirs. As a reservoir, shale mostly shows low porosity (<10%), of course, it can also have a large porosity, and a lot of free gas is stored in these pores. Even in older rock formations, free gas Can also fill 50% of pores. Free gas content is closely related to the size of pore volume. In general, the larger the pore volume, the greater the amount of free gas contained. ^[1]

2. Crack development

The mineral composition of shale is relatively complicated, and the quartz content is high, and most of them are clay-grained, often appearing in the form of striae. The shale with high organic matter and quartz content is relatively brittle, and it is easy to form natural fractures and Inducing fractures is conducive to natural gas seepage, indicating that lithology and rock mineral composition are the main intrinsic factors controlling the degree of fracture development. ^[1]

Because shale has the characteristics of low porosity and low permeability, gas production is not high, and those open rectangular natural fractures make up for this deficiency and greatly increase shale gas production. Fractures improve the seepage capacity of mud shale. Fractures are both a storage space and a seepage channel, and are a necessary way for shale gas to flow from the matrix pores to the bottom of the well. Not all high-quality source rocks can form fractured oil and gas reservoirs with economic exploitation value. Only those brittle shales with low Poisson's ratio, high elastic modulus, and rich in organic matter are the primary exploration targets for shale gas resources. ^[1]

3. Organic carbon content

In the fractured shale gas system, there is a linear relationship between the gas adsorption capacity of the shale and the total organic carbon content of the shale. ^[1]

At the same pressure, shale with higher total organic carbon content has significantly higher methane adsorption capacity than shale with lower content. In addition to being adsorbed on the surface of organic matter, shale gas can also be adsorbed on the surface of clay (dry). When the organic carbon content is close to the same pressure, the amount of gas adsorbed by shales with high clay content is higher than that with shales with low clay content. And as pressure increases, so does the gap. ^[1]

4. Formation pressure

Formation pressure is also one of the factors affecting shale gas production. Studies have shown that there is a positive correlation between formation pressure and adsorbed gas. The higher the formation pressure, the greater the shale adsorption capacity and the higher the adsorbed gas content. The free gas content also increases with increasing pressure, and the two are basically linear. It is worth noting that before the pressure of 6.89 MPa, the increase of the adsorbed gas content with the pressure was obvious, but after that, the increase was not so obvious, similar to the conventional tight gas reservoir. Of course, pressure gradients will also vary in different regions due to differences in organic matter content and storage capacity of surrounding rock. ^[1]

In addition to the above-mentioned influencing factors, organic matter type, maturity, etc. can also affect shale gas content. ^[1]

Shale gas accumulation

Shale gas has undergone complex and variable accumulation processes, and is an important component and representative of natural gas accumulation mechanism sequences. According to different accumulation conditions, shale gas accumulation can be represented by typical adsorption mechanism, piston migration and accumulation mechanism, or displacement migration and accumulation mechanism. According to the different accumulation mechanism, the natural gas accumulation process can be divided into three main stages, and the first two stages are the shale gas accumulation process. ^[7]

Accumulation process The first stage is the generation and adsorption of natural gas. This stage occurs in the early stage of reservoir formation, which is the same as the mechanism of coalbed methane accumulation. Due to the adsorption capacity of organic carbon and other substances in the shale, during the shale gas generation process, a small amount of natural gas initially generated was adsorbed by organic carbon and other substances, so only the adsorbed natural gas remained in the shale layer (Figure A). ^[7]

The second stage is the formation and discharge of natural gas. This stage is at the peak of gas generation, which is similar to the mechanism of root-edge gas formation. With the large-scale generation of natural gas, organic carbon in shale cannot completely adsorb it, so unadsorbed natural gas accumulates in the shale layer in a free state. With the continuous generation of shale gas, the accumulated large amount of free gas will form a high pressure due to expansion, until the rock formation ruptures and micro-fractures occur. Because the cracks or pores generated at this time are extremely small, shale gas cannot flow freely inside the shale formation. Under the action of subsequent strong hydrocarbon generation, ie, gas expansion, shale gas will be driven and pushed from the high-pressure area at the bottom to the relatively low-pressure area at the upper part along the upslope of the structure, so that the formation is in a large area containing gas. . The natural gas generated at this stage is not affected by buoyancy and appears as a piston-like migration and accumulation feature (Figure B). ^[7]

The third stage is the replacement and transportation of natural gas. If the amount of natural gas generation continues to increase and there are suitable reservoirs outside the shale formation, then under the buoyancy effect, natural gas will migrate from the shale formation to the reservoir along the fracture in a displacement manner, thereby forming a conventional natural gas reservoir (Figure C ). ^[7]

During the formation of shale gas reservoirs, the adsorption mechanism and the piston migration and accumulation mechanism work together to control the change in the proportion of the space occupied by the adsorbed and free natural gas in the shale gas reservoir. Therefore, the mechanism of shale gas accumulation is essentially a dynamic balance between the occurrence states of natural gas in shale pores. The existence of adsorbed natural gas in shale is determined by its own rock characteristics and has no direct relationship with preservation conditions, so there are no special requirements for preservation conditions after shale gas accumulation. The gas anomalies monitored in the marine strata of the Sichuan Basin have also confirmed that even multi-stage tectonic movements will not have much impact on shale gas reservoirs. ^[7]

Shale Gas Research in China

Chinese scholars' research on shale gas started late, and the earliest domestic reports on shale gas appeared in the fourth issue of "Foreign Logging Technology" in 1996. However, until 2002, after Li Darong and other classic articles translated from JBCurtis, "Fractured shale-gas systems" were published, many domestic scholars began to pay attention to shale gas. ^[7]

Since 2004, outstanding petroleum geological science and technology work represented by Professor Zhang Jinchuan of China University of Geosciences, Professor Li Xinjing and Wang She of China Petroleum Exploration and Development Research Institute, Professor Wang Hongyan and Professor Li Jingming of Langfang Branch of China Petroleum Exploration and Development Research Institute, and Professor Pan Renfang of Yangtze University The author started related research on the basic theory of shale gas exploration and development, and achieved promising results in shale gas reservoir formation mechanism, reserve evaluation, resource classification, and shale gas percolation mechanism, laying a foundation for China's shale gas exploration and development. In 2009, China launched the project "Shale Gas Resource Potential and Optimal Selection of Favorable Areas in China's Key Areas"; in 2010, China carried out research work in three steps, the first to launch the "Sichuan, Chongqing, Guizhou and Hubei Shale Gas"

Professor Zhang Jinchuan instructs Pengshui to drill the first shale gas well The Investigation Pilot Pilot Area carried out key investigations, the second was to conduct shale gas resource surveys in the Lower Yangtze, Jiangsu, Anhui, and Zhejiang regions, and the third was to conduct preliminary surveys and research on shale gas resources in northern regions (North China, Northeast, and Northwest); in 2011, combined As a result of previous surveys and research, the Ministry of Land and Resources conducted pilot experiments in five projects in Sichuan, Chongqing, Guizhou and Hubei, five in the Upper Yangtze and Yunnan, Guizhou, Guiyang and Southeast, Northwest, Qinghai-Tibet, and East-Northeast The region continued to carry out resource potential surveys, and launched five key projects related to shale gas exploration and development. ^[7]

In April 2012, the Ning 201-H1 well in the Ning 201-H1 area of Changning area tested 15 × 10 ⁴ cubic meters of shale gas per day and achieved a breakthrough in China s shale gas exploration and commercial development. In November of the same year, Sinopec in the Jiaoshiba area of Fuling in southeast Sichuan. Well Jiao 1HF was tested in the Wufeng Formation-Longmaxi Formation to obtain 20.3 × 10 ⁴ cubic meters of shale gas per day. ^[8]

In 2014, China's first proven shale gas geological reserve was 1067.5 × 10 ⁸ cubic meters. By the end of 2017, the Fuling shale gas field had accumulated proven geological reserves of more than 6000 × 10 ⁸ cubic meters. The cumulative production capacity of shale gas is 100 × 10 ⁸ cubic meters, and the annual output of shale gas reaches 60.4 × 10 ⁸ cubic meters. ^[8]

Through the practice of exploration and development, Chinese scholars have proposed the understanding of binary enrichment of marine shale gas in complex structural areas, structural sweets and continuous sweet area shale gas enrichment models. Exploration and evaluation technology systems such as gas selection area selection and target optimization are becoming more mature, and related development technologies such as shale gas reservoir description, productivity evaluation, and optimization of development parameters have been initially formed; excellent horizontal well drilling, pumping bridge plugs, and perforating points Stage fracturing, synchronous fracturing, zipper fracturing and other technologies are becoming more mature, possessing technical capabilities for the development of shallow marine facies shale gas at a scale of 3500 meters; forming a mountain well factory operation mode, greatly improving construction efficiency, and a single platform Compared with single-hole drilling, the drilling and construction periods have been shortened by more than 30% compared with the same period of the previous year. The supporting production has formed a clean production technology system for waste slag, waste liquid and waste gas recycling and harmless treatment. In the development of key fracturing equipment, a 3000-type fracturing vehicle with independent intellectual property rights has been formed, and the research and development, testing, manufacturing system and application specifications of domestic high-power fracturing units have been established; the self-developed naked eye packer, Downhole fracturing tools such as bridge plugs have achieved industrial mass production. Taking the exploration and development of Fuling shale gas field as an example, nearly one hundred technical standards and specifications have been formed, and 39 national patents have been authorized, including 12 invention patents. China s scientific and technological progress in the field of shale gas has received high attention and recognition from the global industry. In 2014, the Fifth World Shale Oil and Gas Summit awarded Chinese companies the Shale Oil and Gas International Pioneer Award. The "Efficient Exploration and Development of Rock Gas Field" project won the first prize of National Science and Technology Progress Award. ^[8]

Distribution of shale gas resources

World reserves and distribution of shale gas

Global shale gas resources are very rich and widely distributed. According to the National Petroleum Commission (NPC), by the end of 2009, the global shale gas resources were approximately 456.2 trillion cubic meters, accounting for nearly 50% of the global unconventional gas resources. Compared with conventional natural gas, the resource potential of shale gas may even be significantly greater than conventional natural gas. Mainly distributed in North America, Central Asia and China, the Middle East and North Africa, Pacific countries, Latin America, and other regions (see the table below). ^[3]

Estimated shale gas resources in major regions of the world
area	Resources / trillion cubic meters	% Of global share
North America	108.79	23.8
Central Asia and China	99.9	21.9
Middle East and North Africa	72.15	15.8
Pacific countries	65.5	14.4
Latin America	59.95	13.1
other areas	49.94	11

North America has the most, but its abundance is low, and the ratio of technical recoverable volume to total resources is low. At the same time, shale gas reservoirs have the characteristics of low porosity and low permeability, which is difficult to mine and requires high-level drilling. And completion technology. When the shale gas production began in the 21st century, horizontal drilling technology and water-based hydraulic fracturing technology were mostly used to improve the recovery factor. ^[7]

Organic shale is widely distributed in 48 states in the United States, and has abundant shale gas resources. According to the estimates of the National Oil and Gas Resources Commission, the total amount of shale gas resources in the United States is approximately 14.2 trillion to 19.8 trillion cubic meters, and the recoverable resources are approximately 3.62 trillion cubic meters. As of the end of 2008, the United States had proven shale gas reserves. 928.9 billion cubic meters. ^[3]

European shale gas reserves can reach 15 trillion cubic meters. Swedish alum schist, northern Germany, the Netherlands, and southern British Posidonia schist are similar to the US shale. The shale in Europe is mainly black or dark brown, which means that during the formation of the rock layer, it is rich in organic substances possessed by the organisms growing in the soil, and most of these organic substances have become the current shale gas. If shale reserves can be confirmed, this will change the fate of European countries' long-term dependence on Russian gas. Currently, one quarter of the natural gas supply in EU member states comes from Russia. Latin American shale gas resources are concentrated in countries such as Argentina, Mexico and Brazil. ^[3]

Shale gas reserves and distribution in China

China's shale gas resources are abundant, but development is still in its infancy. The state is actively promoting the development and utilization of shale gas. ^[9]

In 2018, the Ministry of Natural Resources' Mineral Resources Protection and Supervision Working Group revealed that from September 2014 to April 2018, in less than 4 years, China's cumulative additional proven geological reserves of shale gas exceeded 1 trillion cubic meters, with a production capacity of 135 Billion cubic meters, with a cumulative gas production of 22,580 million cubic meters. From September 2014 to April 2018, in less than 4 years, four complete shale gas fields in Fuling, Weiyuan, Changning, and Weirong have been proven in the Sichuan Basin, and the cumulative new proven geological reserves of shale gas have broken through. It has a trillion cubic meters, a production capacity of 13.5 billion cubic meters, and a cumulative gas production of 22,580 billion cubic meters. ^[10]

Although China's shale gas exploration is still in its infancy, preliminary comparisons of reservoir-forming conditions show that many basins in China and shale gas reservoirs in the eastern United States

Shale gas distribution The geological conditions are similar, so the exploration potential is huge. China's shale gas development area can be divided into four regions roughly corresponding to the plate, namely the southern region, the central and eastern regions, the northwestern region, and the Qinghai-Tibet region, all of which have good prospects for shale gas exploration (see figure). ^[7]

A total of 8 sets of source rock formations dominated by black shales in the Yangtze platform area of southern China: Upper Sinian Doushantuo Formation, Lower Cambrian Qiongzhusi Formation, Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation, Middle Devonian Luofu Formation, Lower Carboniferous Hezhou Formation, Lower Permian Qixia Formation, Upper Permian Longtan Formation and Dalong Formation, Lower Triassic Qinglong Formation. The above horizons are widely distributed, shallowly buried, large in thickness, rich in organic matter, high in maturity, and have excellent shale gas accumulation conditions. Therefore, they are favorable areas for shale gas development and have great exploration potential. Among them, the Sichuan Basin has undergone complex tectonic changes during the evolution of the Craton and Foreland Basin, forming similar tectonic evolution characteristics and geological conditions similar to typical shale gas basins in the United States. The Lower Paleozoic source rocks have widely distributed, It is characterized by large thickness, high maturity, micro-fracture development, and strong organic hydrocarbon generation, and there is a lot of evidence indicating that the basin is rich in shale gas. For example, the black brushstone shale of the Lower Silurian Longmaxi Formation in this basin is the most developed, and the analysis and test data of newly drilled shallow shale gas wells in the southern Changning structure also confirms that the Longmaxi Formation has the advantage of forming a shale gas reservoir Conditions; the volume method is used to preliminarily estimate the shale gas resources of the Longmaxi Formation in the Sichuan Basin and adjacent areas to be 4.0 × 10 ⁸ to 12.4 × 10 ⁸ cubic meters, showing the huge exploration potential of shale gas in this area; in 2009, in The first shale gas strategic investigation well drilled in Chongqing Pengshui, well Yuyu 1 saw a good shale gas display, and the later experimental analysis results of Yu page 1 well also show that the geological conditions in the southeast area of Chongqing are complicated and high. The steep tectonic zone has good shale gas accumulation geological conditions, and it also indicates that the Upper Yangtze and even the entire Yangtze region may be a favorable potential area for shale gas development. Therefore, in general, the Lower Cambrian and Lower Silurian shales in eastern and southern Sichuan (including southwestern Sichuan) have the most potential, while the western Hubei-East Chongqing region and even the entire middle-lower Yangtze region are distributed by shale gas. Important area. ^[7]

The shale gas in the central and eastern regions may be distributed at the bottom of the main oil and gas layers, from the Mesozoic to the Paleozoic in the region. The Cretaceous in the Songliao Basin, the Triassic in the Ordos Basin, and the shallower Paleogene in the Bohai Bay Basin all contribute to the development of shale gas. Among them, the Mesozoic shale in the Ordos Basin has a buried depth of less than 3 kilometers, a thickness of> 5 meters, and an organic carbon mass fraction of> 2%, which is very favorable for the development of shale gas. In the northwestern region, subject to the characteristics of today's basins, the Jurassic, Triassic, and shallow Paleozoic mud shales distributed at the edge of the basin have large thickness and organic carbon content, and they have shale gas. Exploration potential. The Meso-Paleozoic shale formations in the Qinghai-Tibet region have large thickness, high organic matter content, and moderate thermal evolution of organic matter, which also has the potential for shale gas exploration.

On August 24, 2018, the natural gas resources in the earth's shale formation were comparable to the recoverable reserves of conventional natural gas. "Although China s shale gas development started late, it is the third country to form a scale and industry after the United States and Canada, and its output can reach 10 billion cubic meters of energy in the near future. Zhang Dawei, director of the Mineral Resources Reserve Evaluation Center, said so. ^[11]

Shale gas evaluation criteria

There are five conditions in the core area for economic development of shale gas, which usually refer to effective shales with TOC values greater than 2%, within the gas window, and brittle mineral content greater than 40%. The effective shale thickness is about 30 ~ 50 meters. When the effective shale is continuously developed more than 30 meters, when the intermittent development or TOC value is less than 2%, the cumulative thickness of more than 50 meters is sufficient to meet the requirements of commercial development. ^[12]

Shale gas development and utilization

Shale gas extraction technology

Shale gas development

Compared with conventional natural gas, shale gas reservoirs generally have low porosity and low permeability. Generally, the permeability is less than 1 × 10 ³ m, and the porosity is only 4% ~ 5%. Natural gas is large and requires hydraulic fracture of the reservoir to be exploited. The use of hydraulic fracturing and horizontal well technology can greatly increase the production of shale gas reservoirs in shale reservoirs. ^[13]

1. Vertical well technology

Vertical wells are the main means of early exploitation of shale gas. When drilling shallow wells less than 1000 meters, the under-balanced spin method with a faster drilling speed is generally used to effectively reduce the damage to the formation. When drilling deep wells of 1000 to 2500 meters, light drilling fluid is used Conventional rotary drilling while drilling. ^[14]

2. Horizontal well technology

With the great success of 7 horizontal test wells in the Barnett Shale Formation in the Fort Worth Basin, Texas, USA, in 2002, horizontal wells were vigorously promoted in the industry and quickly became the main drilling method for shale gas production. ^[14]

Horizontal wells increase the possibility of contact with fractures in shale formations and increase the contact area with gas in reservoirs. At the same time, horizontal wells reduce the surface facilities, extend the range of exploitation, and avoid the interference of adverse ground conditions. The output is 3 ~ 5 times of straight well. ^[13]

At present, domestic horizontal well drilling technologies mainly include control pressure drilling, low pressure underbalanced air drilling and rotary steerable drilling. Controlled pressure drilling technology can well overcome the wall collapse problem; low pressure and under-balanced air drilling technology has mature applications, rotary steering drilling technology, good wellbore purification effect, high precision of well path control, and strong displacement extension ability Important direction of technological development. Domestic shale gas horizontal well completion technology has developed to a certain extent. The main completion technologies include: hydraulic jet perforation completion, combined bridge plug completion, and mechanical combined completion technology. Combined bridge plug completion is the most commonly used and time-consuming completion method for shale gas completions. In cased wells, a combined bridge plug is used to separate sections and perform perforation and fracturing; for example, hydrojet perforation Completion is the use of high-speed ejected fluid to penetrate casing and rocks, without the need for lower packers and bridge plugs, which can shorten the completion time; mechanical combined completion uses a special sliding sleeve mechanism and expansion packer, which is suitable for With horizontal open-hole fracturing in restricted flow fracturing, cementing and staged fracturing can be completed in one trip to the pipe string. ^[9]

Compared with vertical wells, horizontal wells have higher construction costs, but because they can significantly increase the contact area with fractures in shale formations, effectively improve the flow state of shale gas in the reservoir and increase gas production rates, so horizontal wells It is 2 to 3 times higher than the vertical well in terms of initial production rate, control reserves and final evaluation of recoverable reserves. ^[14]

3. Fracturing stimulation technology

Due to the ultra-low permeability and low porosity of shale gas reservoirs, horizontal wells need to undergo multi-stage large-scale hydraulic fracturing treatment to ensure economic production of shale gas reservoirs. Fracturing stimulation technology is one of the core technologies for the successful development of shale gas. ^[13]

According to the fracturing medium, it can be divided into foam fracturing and hydraulic fracturing.

1. Foam fracturing. Fracturing methods using liquid nitrogen or carbon dioxide foam as fracturing agents are generally used in shale reservoirs with shallow burial depths and low formation pressures. They have high fracture-making efficiency, low filtration loss, sand carrying and return ability It has the advantages of strong friction coefficient, low damage to the reservoir, but high requirements for injection pressure, simple form of fracture generation, difficulty in providing more channels for gas migration, and high cost. ^[14]

2. Hydraulic fracturing. Water is used as a fracturing agent. It is often used in reservoirs with large burial depth and high formation pressure. It requires less proppant, does not require surfactants and stabilizers, and rarely requires a pump to pressurize, so the cost is relatively high. low. Hydraulic fracturing is a clean fracturing technology that can clean fractures and increase the permeability of the reservoir to a certain extent. ^[14]

According to the fracturing method, it can be divided into the following types.

1. Horizontal well fracturing. That is, multi-stage fracturing operations are performed on relatively long horizontal well sections, forming multiple hydraulic fractures, effectively increasing the fracture network to increase gas production. Segmented fracturing can effectively reduce costs, especially when combined with pre-fluid drilling, which can minimize the time and cost of mobilization, dissolution, material handling, shortening preparation time and downtime between each pumping operation. ^[14]

2. Repeat fracturing. With the passage of time and the release of pressure, the fractures maintained by the proppant formed by the initial fracturing will gradually close, resulting in a severe reduction in shale gas production. Repeated fracturing of shale reservoirs to restore productivity is repeated fracturing. ^[14]

3. Simultaneous fracturing. Simultaneous fracturing is generally carried out simultaneously or interactively by two or more wells. The principle of the shortest migration distance between fracturing fluid and proppant under high pressure is used to increase the density and area of fracturing fractures to increase production. the goal of. ^[14]

China's decades of horizontal well development experience accumulated in the conventional oil and gas field has also accelerated the current process of domestic shale gas technology development. Horizontal well hydraulic fracturing technology is widely used in domestic conventional oil and gas development, especially multi-stage fracturing technology, repetitive fracturing technology, and fresh water fracturing technology. There are many examples of successful applications. ^[13]

In terms of the horizontal well segmented fracturing technology, China has made certain breakthroughs, and formed double-single-single-card segmented fracturing for horizontal wells in low-permeability oil and gas fields, hydraulic sandblasting segmented fracturing, packer slip sleeve segmented fracturing, etc Three major technologies have improved four supporting processes, including self-steering high-efficiency acidification / acid fracturing of carbonate reservoirs, staged fracturing of chemical temporary plugging, hydraulic fracture monitoring and evaluation, and horizontal well workover operations. Casing fracturing fracture and well pattern optimization design method. China has already realized 7 sections of horizontal well fracturing technology in Changqing, and has introduced and absorbed 10 sections of horizontal well fracturing technology for long well sections in the southern area of Sichuan. ^[13]

Shale gas development

Although there exists both adsorption and free-phase natural gas, the development of shale gas does not require drainage and pressure reduction. Due to the production of free-phase natural gas in shale, the purpose of pressure reduction can be achieved naturally, so as to guide the natural gas dissociation in the adsorption phase and a small amount of dissolved phase, to further increase natural gas production capacity and achieve long-term stable production. Due to the low porosity and permeability of natural gas, the productivity and recovery of natural gas are also low. Therefore, the ultimate recovery of rock gas depends on effective fracturing measures. Both the fracturing technology and the mining process directly affect the economics of shale gas wells. benefit. ^[15]

Some experts mentioned the root marginal gas. JAMasters (1979) proposed the idea of deepbagas, established a model of gas-water inversion, and described the broad prospects of natural gas exploration and development. Due to the difficulty of identification, PRRose et al. (1986) proposed Basin-center gas. BELaw et al.'S research idea improved this type of gas reservoir identification method from "regional gas-water inversion" to "no edge water", thus The tedious identification process is simplified and the identification time of this type of gas reservoir is greatly advanced. As the borderless bottom water is still determined by more drilling geological data, it is still not the best solution. ^[15]

After experimental research, the root margin gas was identified as the natural gas accumulation in the tight sandstone that is directly connected to the gas source rock, and the gas-bearing characteristics of the sandstone bottom were emphasized. Due to the close proximity of gas source rocks (roots), natural gas and formation water form a direct transmission

China's First Shale Gas Horizontal Well Tested in Weiyuan Gas Field Successfully Continuum is regarded as root-edge gas. The main body of this type of gas reservoir is formed after the sandstone is generally densified, which roughly corresponds to the pyrolysis of coal measures and dark mudstones and the cracking gas generation stage of petroleum, so the general burial depth is relatively large. ^[15]

The root edge gas research method is based on the principle of natural gas accumulation dynamics, and the gas reservoir identification technology is advanced to whether a single well profile appears and develops the gas characteristics at the bottom of the sandstone. The earliest rapid identification of gas reservoir types can be performed on a wildcat well (rather than a prospecting well or even a development well). The gas content at the bottom of the sandstone alone can explain the continuity of natural gas accumulation dynamics, and further elaborate the characteristics of natural gas accumulation mechanisms, so as to determine the type of natural gas accumulation mechanism, that is, deep basin gas, basin central gas, marginal gas, and even The existence of oblique gas or full basin gas determines the characteristics of natural gas accumulation, enrichment and distribution. ^[15]

Shale gas development status

1. Foreign development status

The world's shale gas research and exploration and development first began in the United States. Currently, the United States and Canada are the two major countries for large-scale development of shale gas. ^[3]

Shale gas is distributed in the North American craton basin, foreland basin, Jurassic, Devonian, and the Mississippi system is rich in shale gas resources of various genesis and maturities. ^[7] In other countries in the world, with the deepening of shale gas exploration and development, the depth of shale gas exploration and development in North America has reached 2,000 to 4,000 meters, and the maximum is close to 6000 meters. At present, in addition to the United States and Canada, Australia, Germany, France, Sweden, Poland and other countries have also begun research and development of shale gas. Large oil and gas companies in many countries are looking for shale gas worldwide, such as Australia, China, South Africa and Europe. Europe now has about 30 to 50 onshore rigs, while the United States has at least 1,500. The difference in the number of rigs will produce a qualitative difference in the mining process, because once shale gas is extracted, it needs to maintain production continuously. The initial gas production of shale gas wells will be very high, and if the production is not carried out in time, 70% to 90% of production will be lost. ^[3]

The United States has entered the stage of large-scale production of shale gas, and shale gas asset trading is extremely active. From January 2009 to April 2010, the total amount of US-related shale gas mergers and acquisitions reached US $ 50.033 billion. ^[3]

With the tilt of relevant policies in the United States, the number of companies participating in shale gas exploration and development has increased from 23 and 64 in 2005 and 2007 to more than 100 in 2009. The output increased from 34 billion cubic meters in 2007 to 95 billion cubic meters in 2009, exceeding the annual output of China's conventional natural gas (83 billion cubic meters). In 2009, the United States surpassed Russia for the first time in natural gas production, ranking first in the world, because its shale gas production increased by 10% in total natural gas production. The International Energy Agency said in its 2009 World Energy Outlook report that the United States expects shale gas production to increase to 20% of its domestic natural gas production by 2020.

Development status (5 photos)

The US government's emphasis on shale gas development has provided a strong impetus for shale gas development. In addition to national policy support, the US government has provided substantial funding to encourage unconventional natural gas research and development, including grants, loans and loan guarantees, training funding, research funding, and direct investment in exploration. The 2004 US Energy Act requires that $ 45 million be invested annually in unconventional natural gas research for 10 years. According to the United States Energy Information Administration (EIA), shale gas production will increase significantly in the next 20 years, and by 2030 shale gas production will account for 24.3% of total US natural gas production. ^[3]

At the beginning of this century, American companies combined horizontal well and fault technologies to smash hard rock layers, extracted shale gas, and broke through the technical difficulties of shale gas. The fierce competition among U.S. oil companies has forced some innovative, small energy companies to move to high technology, and their technology has been purchased by large oil companies. As a result, shale gas has achieved true commercial exploitation. Currently, shale gas in the world The most advanced mining technology is the United States. ^[3]

The general process of shale gas development in the United States: obtaining mining rights obtaining permits drilling hydraulic fracturing construction production improving recovery and repair technology gas well abandonment and reclamation. ^[3]

The rapid development of unconventional gas in North America is mainly due to technological breakthroughs. Advances in horizontal drilling and hydraulic fracturing technology have improved reservoir contact, equipment technology advancements have reduced unit production costs, especially low-cost shallow wells have progressed into shale gas business. Chemical mining has made an important contribution. The shale gas technology in the United States and Canada has matured and has entered the stage of large-scale production. ^[3]

2. Domestic development status

Changning-Weiyuan shale gas demonstration area Changning block In October 2009, the Ministry of Land and Resources launched China's first shale gas exploration project in Oujiang, Chongqing. This indicates that after the United States and Canada, China has officially begun the exploration and development of this new energy shale gas resource. It will play a positive role in the demonstration of China's new energy construction, and it has a milestone significance in China's oil and gas field. On March 20, 2012, Shell has signed a product sharing contract with PetroChina to conduct shale gas exploration, development and production in the Fushun-Yongchuan block of the Sichuan Basin, China. The development of shale gas by the two companies is still in the exploration stage and has not yet entered into substantial exploitation. On September 24, 2012, China's first shale oil and gas production capacity construction project, Sinopec Liangping Shale Oil and Gas Exploration and Development and Capacity Construction Demonstration Zone, started drilling in eight drilling platforms. ^[4]

The four-day 2012 China International Shale Gas Conference was held in Chongqing from November 13th to 16th, 2012, with the theme "Promoting Investment and Cooperation and Promoting the Commercialization of China's Shale Gas Industry". More than 200 shale gas resource developers, buyers, traders, technical supporters, oilfield services and equipment suppliers, top shale gas industry research experts and government consultants from around the world attended the meeting. At the meeting, Li Yuxi, a researcher at the Oil and Gas Resources Strategic Research Center of the Ministry of Land and Resources, revealed that a number of policies to support the industrialization of shale gas are in the pipeline. This series of supporting policies mainly includes: First, the shale gas market-based pricing policy, and at the same time, shale gas utilization methods can be flexible; second, the supervision system emphasizes first-level management, and it is actually divided into second-level and third-level supervision, that is, each province Supervisory system with participation in management, which extends the whole supervision process down to shale gas wells. Third, the majority of profits and taxes are left to the local government for the benefit of both enterprises and local governments. ^[16]

Although the industry s interest in China s shale gas continues to rise, industry experts participating in the 2012 China Shale Gas Development Forum believe that there are still some issues that need to be resolved in China s shale gas development, including technical preparation and resource reserve evaluation. From policy research to development models, we must learn from the successful experience of the United States, and we must not rush to achieve success. ^[16]

Wang Yan, deputy director of the China Geological Survey, pointed out at the "2019 National Geological Survey Working Conference" that last year, the Yiyang Well 2 in Yichang, the middle reaches of the Yangtze River, obtained a high-yield industrial gas flow, realizing the exploration and development of Chinese shale gas from the upper reaches of the Yangtze River to the middle reaches. Strategic expansion. ^[17]

According to the China Geological Survey, high-yield shale gas flows were obtained in the three geological strata of the Sinian, Cambrian, and Silurian systems in the western Hubei. The first is the high-yield industrial gas stream with a production volume of 55,300 cubic meters per day and an unobstructed flow of 198,200 cubic meters per day in the Lower Sinian Eyang page 2 well. It is by far the world's oldest shale gas reservoir, and it is the direction of China's shale gas exploration. The deep march provided a new layer. The second is the high-yield industrial airflow of 60,200 cubic meters / day and an unobstructed flow of 121,300 cubic meters / day in Well Eyi, Central Cambrian, and 78,300 cubic meters / day of unobstructed flow, and 28.85 in Yiyang Page 1 The high-yield industrial airflow of 10,000 cubic meters per day is the first time outside the Sichuan Basin to obtain industrial airflow in this stratum, opening up a new exploration area. Thirdly, the 500-meter horizontal section of Well Yiyi Page 2 in the Upper Silurian System produced an industrial airflow of 31,500 m3 / day and an unobstructed flow of 57,600 m3 / day, realizing the current main shale gas development layer system in the Yangtze River. Expansion from upstream to midstream. ^[17]

The China Geological Survey pointed out that the investigation of shale gas in the western Hubei area has innovated the theory and exploration technology of shale gas accumulation. Aiming at world-class problems such as the old formation age, difficult reservoir reconstruction, low formation pressure, and difficult gas production, the three major technological systems of selection evaluation, drilling engineering, and fracturing gas testing were innovated. Among them, the evaluation of complex area selection, reservoir fracturing transformation and fracturing gas testing have reached internationally advanced levels. ^[17]

The evaluation of shale gas resource potential shows that the geological resources of shale gas in the western Hubei region of China reach 11.68 trillion cubic meters, with a resource base of 10 billion cubic meters of annual production capacity. Eleven experts such as Kang Yuzhu, academician of the Chinese Academy of Engineering believe that the results obtained in the shale gas survey in western Hubei are a strategic breakthrough and have a milestone leading role. The overall results have reached the international advanced level, and some of them have reached the international advanced level.

The Shale Gas Development Plan (2016-2020) issued by the National Energy Administration proposes to innovate institutional mechanisms to attract various types of social capital and expand shale gas investment. Through technical research, policy support, and market competition, shale gas production has been greatly increased, and shale gas has become an important part of China's natural gas supply. ^[18]

The plan states that, as China s economic growth shifts, and the prices of traditional fossil fuels such as oil and coal have fallen, the competitiveness of natural gas has declined, and consumption growth has slowed significantly. The domestic natural gas production has steadily increased, and a series of long-term natural gas import agreements such as China and Russia have been signed, which will greatly increase the natural gas supply capacity in the future. According to the current energy consumption structure, the natural gas supply during the 13th Five-Year Plan period is generally sufficient. Shale gas is more expensive to develop than conventional natural gas, making market development more difficult. ^[18]

In terms of development goals, the plan is clear, mature and mature shallow sea facies shale gas exploration and development technologies at 3500 meters, and breakthroughs in deep sea facies shale gas, terrestrial facies and sea-land transition facies shale gas exploration and development technologies at 3500 meters; With the smooth development of the market, we will strive to achieve 30 billion cubic meters of shale gas production in 2020. ^[18]

The plan also looks forward to the development goals for 2030: During the "14th Five-Year Plan" and "15th Five-Year Plan" period, China's shale gas industry will accelerate its development, and a number of large-scale shale gas fields will be discovered and effective development will be achieved by 2030 Shale gas production is 80 billion to 100 billion cubic meters. ^[18]

The plan is clear, encourage the combination of independent development and foreign cooperation, actively introduce advanced and applicable technologies, support joint ventures and cooperative development of multiple investment entities, and strive to expand investment in exploration and development. At the same time, improve the shale gas block access and exit mechanism, increase investment entities, strengthen market competition, promote engineering technology upgrades, accelerate cost reduction, and improve the economics of shale gas development. In addition, in response to problems such as immature engineering technology, high exploration and development costs, and low economic benefits in the initial stage of shale gas development, we will improve relevant support policies to ensure the sustainable development of the industry. ^[18]

Analysis of Shale Gas Industry Development in China

Shale gas development recommendations

The first is to formulate a medium- and long-term development plan for China's shale gas resources strategic survey and exploration and development. Based on a careful analysis of the world s shale gas exploration and development situation and China s current situation, scientifically evaluate and analyze the potential of China s shale gas resources, conduct research on the trend of shale gas proven reserves, and strategically investigate and explore China s shale gas resources Make scientific plans for development goals, priorities, and development stages, clarify development positioning, and formulate medium- and long-term development plans for shale gas resource strategic surveys and exploration and development. ^[19]

The second is to formulate policies that encourage strategic surveys and exploration and development of shale gas resources. Based on the research on the preferential policies given in the development of shale gas in the United States and other countries, and in accordance with China's actual conditions, and with reference to domestic preferential policies for coalbed methane exploration and development, preferential policies for shale gas exploration and development will be given. State finance increases investment in strategic investigations of shale gas resources, encourages social funds to invest in shale gas, and reduces use of shale gas exploration and mining rights

Shale gas development Implement shale gas mining companies' value-added tax policy and implement preferential policies for corporate income tax; key equipment for shale gas development is exempt from import value-added tax and customs duties; subsidies are given to shale gas exploration; key technology research and development and Promotion and application of preferential policies, etc., guide and promote the development of industrialization of shale gas. ^[19]

The third is to improve and innovate the management system of shale gas mining rights. According to the characteristics of wide distribution of shale gas and strong exploration and development flexibility, the in-depth study on the setting system of China's shale gas mining rights. Drawing on the experience of coalbed methane mining rights management, a special shale gas block registration system will be established to implement state-level management. Qualified local enterprises and private capital are allowed to participate in the exploration and development of shale gas through joint ventures, shareholding and other methods. They can also invest independently and engage directly in shale gas exploration and development. ^[19]

Fourth, speed up the development of technical standards and specifications for shale gas. Strengthen government guidance, rely on the implementation of major projects for strategic investigation of shale gas resources, and the implementation of pilot exploration and development pilot areas, accelerate the construction of technical standards and regulatory systems for strategic investigation of shale gas resources, and exploration and development, and promote the sharing of information and standardized management. ^[19]

In addition, we must pay close attention to the development trend of shale gas in the world, and establish and improve the international cooperation and exchange mechanism of shale gas. Strengthen the cooperative development with powerful foreign companies, introduce advanced concepts and development technologies, explore and innovate core technologies suitable for China's shale gas development by introducing and digesting shale gas development technologies, and lay the foundation for China's large-scale shale gas development. basis. ^[19]

Shale gas utilization value

11 countries with the world's largest shale gas basins and reserves 1.Shale gas is a clean and efficient energy source

With the development of society and economy, energy conservation and emission reduction have become a common concern of the world. This requires greater use of clean and efficient energy. Shale gas is natural gas in the form of unconventional natural gas with methane as its main component. It is a clean and efficient energy resource and chemical raw material. It can be used in the fields of residential gas, heating, power generation, automotive fuel and chemical production A wide range of applications. ^[5]

2.Shale gas reserves are particularly rich

The particularly rich reserves are one of the important reasons for the favorable use and development of shale gas. China's shale gas resources are very rich, the resources are in the forefront of the world, the economic value is huge, and the resource prospects are broad (see picture).

China's shale gas exploration and development is in its infancy as a whole. However, with the further maturity of mining technology, shale gas will occupy a very important position in China's energy supply structure. ^[5]

3. Helps improve China's energy consumption structure

For a long time, China has been using coal as its main energy source. The proportion of natural gas in primary energy is still too low. Now vigorously developing natural gas is an inevitable choice for China to ensure energy supply and develop a low-carbon economy. ^[5]

According to estimates, with the adjustment of China's energy structure and utilization methods, the proportion of natural gas in China's energy consumption structure will increase to more than 10% in 2020. Compared with coal, natural gas is cleaner, so the country will further promote the use of natural gas in the future. However, the growth potential of conventional natural gas is always limited, which requires the exploration and development of unconventional natural gas such as shale gas. ^[5]

Accelerating the development and utilization of shale gas has many aspects. Whether it is to meet the huge demand for clean energy from social and economic development, or to control greenhouse gas emissions, to build resource-saving, environmentally-friendly production methods and consumption models, it has an important role in improving residents' energy use environment and ecological civilization. Level also has important practical significance. ^[5]

4.Increase chips to ensure national energy security

China's dependence on crude oil and natural gas In recent years, China's energy consumption demand has been increasing, and the dependence of crude oil on foreign countries has increased year by year. At present and for a long period of time in the future, China's economic development will increasingly require energy resources. At the same time, the pressure on energy supply and demand and environmental degradation that China is already facing will increase. In order to ensure China's energy security and ease the pressure of natural gas supply, vigorously developing shale gas is an ideal and very realistic way, which has important strategic significance in the national energy distribution. ^[5]

In recent years, the growth rate of domestic natural gas consumption demand has exceeded coal and oil, the external dependence has increased rapidly, and the gap between supply and demand has also become larger and larger (pictured), especially the short-term imbalance between supply and demand has affected the safety of urban gas supply and Improvement of people's living environment. However, merely relying on the increase of conventional natural gas production cannot effectively alleviate the contradiction between supply and demand, and it is urgent to increase the intensity of shale gas exploration and production, increase China's shale gas production, and effectively supplement conventional natural gas. ^[5]

Problems with shale gas

The development of shale gas is not only to extract natural gas from shale formations, but also to maintain coordinated development with the surrounding environment, facilities and other resources while mining in the operating area. Shale gas development is a systems engineering and every technology is important. Due to its late start and lack of experience, domestic shale gas development faces many problems. ^[7]

1. Theoretical and development technical issues

First of all, China is still weak in basic shale gas theory and experimental research, especially in the early stage of experimental testing and selection evaluation. Many domestic studies are limited to the macro-comparison of geochemical and geological data of typical shale basins in China and the United States, and no simulation experiments or in-depth studies have been conducted on the microscopic accumulation mechanism of shale gas and the rock characteristics of shale. At the same time, shale is much more heterogeneous and anisotropic than sandstone, so methods suitable for domestic resource evaluation are still being explored. ^[7]

Secondly, shale gas reservoirs have poor physical properties, extremely low permeability, and very high technical requirements for later development. Exploration practices in the United States have shown that breakthroughs in development technologies, such as horizontal well drilling, hydraulic multi-stage fracturing, repeated fracturing, simultaneous fracturing, and comprehensive fracture monitoring, have played an extremely important role in the rapid development of the shale gas industry. Moreover, China's shale gas burials are generally deep, which further exacerbates the difficulty of its development. For example, the Lower Cambrian shale gas layer in the Sichuan Basin is generally buried at a depth of 2 to 3.5 kilometers, while the five major shale systems in the United States are buried at a depth of 0.8 to 2.6 kilometers. Therefore, China is facing difficulties in the development of deep shale gas. ^[7]

2. Risk and economic benefits

Shale gas is an unconventional, low-grade natural gas, with a long development cycle, large investment, and high risks. Unconventional shale gas is booming in the short term. Therefore, the possibility of large-scale investment in shale gas development in China is not high at present, and there is still a long way to go for the rapid development of shale gas. ^[7]

3. Resources and environmental protection issues

Although the exploitation of shale gas has brought huge economic benefits, its potential environmental impact is even more worrying. On the one hand, the exploitation of shale gas requires the consumption of a large amount of fresh water resources, which may threaten the sustainable use of local and regional water resources; on the other hand, the shale gas production process generates noise, waste water and waste gas containing harmful chemicals The environmental pollution caused by it and the major losses caused by its mining accidents. At the same time, the development of shale gas also requires related infrastructure, such as water conservancy and roads. These unfavorable factors restrict the rapid development of shale gas. ^[7]

Shale gas development trends

Because the United States has the earliest discovery and production of shale gas, its basic research work is relatively complete. In the future, the focus will be on the field of shale gas development technology, that is, increasing production capacity. In China, shale gas is in its infancy and development stage. It faces many challenges, such as: low degree of theoretical research and exploration investigation, non-breakthrough of development technology bottlenecks, high initial risk investment, and slow returns. In this context, overall There will be the following development trends. ^[7]

1. Shale gas geological theory and its resource evaluation system are the key research content of China in the future. The current research in China mainly focuses on explaining the geological characteristics of shale gas reservoirs, comparing the geological conditions of China and the United States shale gas basins, and predicting the distribution of favorable areas of shale gas resources in China. With the completion of early exploration of shale gas, a large amount of first-hand data will be obtained. How to use these data to correctly evaluate the shale gas exploration potential in the research area will be an urgent problem to be solved. In view of the fact that the current research relies on organic geochemical data, rock minerality and other data, and lacks experimental testing and micromechanical research, therefore, in the coming period, domestic research institutions will gradually pay attention to and solve these problems. Lay the foundation for later development. ^[7]

2. Shale gas related development technology is the bottleneck that China must break through. Exploration practice in the United States has proven that technological progress is the key to the successful development of shale gas. Although China has key technologies for shale gas production, such as horizontal well technology and hydraulic fracturing technology, due to the lack of in-depth understanding of the mechanical properties of shale gas rocks, there is still a gap between the United States and the U.S. in production stimulation and fracturing measures for increasing production . Therefore, with the deepening of research, continuous improvement of technical means is required to successfully extract shale gas resources. ^[7]

3. In the context of Sino-foreign cooperation, combining reality and persisting in innovation will be an inevitable trend for future development. At present, related research in China is still very weak. To develop and utilize shale gas resources as soon as possible, we must cooperate with foreign companies with mature theories, rich experience and advanced technology to jointly develop shale gas resources. At the same time, it must also combine the actual situation of domestic sedimentary strata and structural characteristics to form a set of exploration and development technologies suitable for China's geological characteristics to promote the development of China's shale gas. ^[7]

4. After 7 years of reconnaissance and repeated research, the scope covers Yunnan, Guizhou, Guizhou, Hunan, Hubei, Sichuan, Chongqing, Shaanxi and other provinces and regions. Finally, the dominant area was locked in the southeast of Chongqing, and an area with the Minjiang River was identified. The starting point is via the development routes of Wansheng, Nanchuan, Wulong, Pengshui, Qianjiang, Liyang, and Xiushan; that is, China's shale gas development will start from Minjiang and go nationwide. ^[7]

5. In view of the huge potential of shale gas resources and its important strategic position in unconventional energy sources, the country will gradually increase its support. The important role played by the successful exploitation of American shale gas in alleviating its domestic energy tensions has given China a new dawn in resource replacement. China's shale gas is rich in resources and has a bright future. Once the production capacity is formed, it will inevitably ease the pressure on domestic energy. ^[7]