What Is Sand Control?

Sand control refers to measures to properly select cementing and completion methods, formulate reasonable mining measures, control production pressure differences, and limit seepage speed to prevent sand blockage during oil production in accordance with the conditions of the formation and wells.

China's unconsolidated sandstone reservoirs have a large distribution range, large reserves and important output. Under normal mining conditions (except for heavy oil using the new technology of sand drainage and cold recovery), sand production from oil wells is extremely harmful. In order to prevent sand production from oil wells, on the one hand, the correct cementing and completion methods must be selected according to the conditions of the oil layer and oil wells. , Formulate reasonable mining measures, control production pressure difference, limit seepage speed, strengthen management of sand wells, and try to avoid strong pumping inducement measures; on the other hand, according to the requirements of the oil layer and mining process, corresponding sand control ( sand control) process technology to ensure the normal production of oil wells. ^[1]

Sand production from oil and gas wells is one of the important problems encountered in oil extraction. If sand damage cannot be treated, sand production from oil and gas wells will become more and more serious, which will result in the ineffective development of sand oil and gas wells. The harm of sand production is mainly manifested in the following three aspects:

Causes serious abrasion of the ground and downhole equipment, and even causes sand jams; the maintenance workload of sand flushing pumps, ground tank cleaning, etc. has increased dramatically; sand buried oil layers or wellbore sand plugging can cause oil wells to stop production; when sand production is serious, it can also cause well walls and even The collapse of the oil layer damages the casing and even causes the well to be scrapped. These hazards not only increase the production cost of crude oil, but also increase the difficulty of oilfield exploitation. ^[1]

There is no obvious depth limit for formation sand production. Generally speaking, sand formation may occur if the formation stress exceeds the formation strength. The strength of the formation is determined by the cementation of the formation cement, the adhesion of the fluid in the trap, the friction between the formation particles, and the gravity of the formation particles themselves. Strata stress includes stratum structural stress, overburden pressure, pushing force on stratum particles during fluid flow, as well as the forces formed by stratum void pressure and production pressure difference. It can be seen that the formation of sand is determined by many factors. It can be divided into congenital reasons and development reasons.

Congenital cause

Congenital causes refer to the geological conditions of the sandstone formation, that is,

Sand control machinery

Mechanical sand control methods can be divided into two categories. The first type is sand control methods that only run into mechanical pipe strings, such as wire-wrapped screens, slit liners, and various sand filter tubes. This method is simple and easy, and the construction cost is low. The disadvantage is that the sand control pipe string is easy to be blocked by the formation sand, which can only prevent the formation sand from being produced on the surface and cannot prevent the formation sand from entering the wellbore.

The second type of mechanical sand control method is to fill the gravel of the pipe string, that is, to insert mechanical pipe strings such as wire-wrapped screens or slit liners into the wellbore, and then fill the ring of the mechanical pipe string and casing with gravel or other similar materials. In the space, it is squeezed into the formation around the wellbore to form a multi-stage sand filter barrier to achieve the purpose of sand retention. This type of method has complicated design and construction and high cost. However, it has good sand retaining effect, long validity period, high success rate, and wide applicability. It can be used for fine, medium, and coarse sandstone formations, vertical wells, directional wells, and thermal recovery wells. condition. The shortcomings of gravel pack sand control are mainly complicated construction and high one-time investment. If the size of the gravel is not selected properly, the formation sand will invade the gravel layer and increase the resistance of oil flowing into the well, which will affect the oil well productivity after sand control. The research results show that the main pressure drop attached to the gravel-filled wellbore is in the perforated blasthole filled with gravel. Due to the long construction process, attention must be paid to reducing damage to the oil layer during operation.

1, sand filter sand control

Sand filter sand control refers to that the pipe string and auxiliary tools for the sand filter with a sand filter function made by a special process are directly suspended from the sand production layer in the well, as shown in the figure. This sand filter has high permeability, allowing formation fluids to pass but can block formation sand. The formation fluid must pass through the sand filter to enter the wellbore and flow to the surface. Currently, sand filters commonly used include wire-wrapped sieve tubes, slotted liners, double-layer pre-filled gravel wire-wrapped sieve tubes, various sand filter tubes, and other new sand filter tools. Various types of sand filters can not only prevent diameters larger than the sand filter gap width (or pore or mesh diameter), but also use the "bridge" effect to prevent part of the formation sand smaller than the sand filter gap width from flowing into the wellbore.

(1) Wire-wrapped screen tube

The fully welded stainless steel wire-wrapped screen tube consists of a base pipe (central tube with a hole), longitudinal ribs, and stainless steel wire, as shown in Figure (a). The base pipe is drilled with a circular hole of a certain density and bore diameter to provide a channel for the fluid to flow into the wellbore after passing through the wire-gap. There are longitudinal ribs on the base tube to support the winding wire. Domestic use of 1Cr ₁₈ Ni ₉ Ti stainless steel wire as the raw material for stainless steel wire winding, rolling into a certain size of triangular cross-section wire and longitudinal

Tendons. During the winding process, each cross-contact point of the wire and longitudinal ribs is welded together by resistance welding, so that the screen sleeve forms a whole with a certain strength. According to a certain length, the two sides and the three knife ends are cut flat, and the coupling is welded. Pass the perforated central tube through the screen sleeve. Then the two ends of the screen sleeve are welded to the central tube. The reason why the wire is pressed into a triangular or trapezoidal cross-section is that the gap made of steel wire wound in this shape has a "self-cleaning" effect on the formation sand particles, as shown in Figure (b). Once there are particles entering the wire-wound gap with the liquid flow, the sand particles will not stay and block in the gap due to the larger the gap outward.

In addition to its "self-cleaning" characteristics, stainless steel wire-wound sieve politicians have the advantages of corrosion resistance, long working life, continuous gaps, and large flow energy area. This is also the main reason why wire-wrapped sieve is widely used. The disadvantage is the high cost, which can often be 2 to 3 times that of the slit liner.

(2) Slit liner

As shown in the figure, the structure and processing of the slotted liner are relatively simple. Slotted liners are made by milling casing directly on a milling machine using a saw blade cutter. Slotted slits allow formation fluids to pass through while blocking formation sand. Slotted slits are arranged in parallel axial directions and staggered. The length of the slit eye of the parallel slotted liner is L = 50 ~ 300mm, the length of the vertical slotted liner is L = 20 ~ 50mm, the high value of the small-diameter high-strength liner is the high value, and the low value of the large-diameter low-strength liner is the low value.

The cross section of the slotted slit of the slotted liner is trapezoidal and the angle is about 12 ° C. The large trapezoidal bottom edge is the inner surface of the liner. This narrow outer shape and wide inner shape can prevent sand particles from jamming in the eye of the seam and blocking, which has a "self-cleaning" effect. There are two types of arrangement of the slit eyes in parallel axial direction and vertical axis direction. Due to the high strength of parallel slits, equal slits are usually used, and the slits are preferably arranged staggered.

Slotted liner sand control is a liner that is pre-slotted under the oil layer. Depending on the liner hanger on the top of the liner, the liner is suspended from the technical casing and the annular space between the liner and the casing is sealed. The sand control mechanism of the slotted liner is to allow small sand particles of a certain size that can be carried to the ground by crude oil to pass, while blocking larger sand particles outside the liner, and large sand particles forming a sand bridge outside the liner, and The formation sand is blocked in the annulus of the slit liner and the casing to achieve the purpose of sand control.

The slitting liner is simple in sand control technology, convenient to operate, and low in cost. However, due to the limitation of mechanical processing technology, the minimum slit width of the liner slit is about 0.3mm, and the range of applicable formation sand is smaller. With the advancement of processing technology, the minimum slit width of the slit liner is also getting smaller and smaller. The slotted liner has a short effective period of sand control, and the effect is poor, and the joints are easily blocked or worn by fine sand.

(3) Double-layer pre-filled gravel wound wire screen sand control

The double-layer pre-filled gravel wire sieve tube (referred to as the double-layer sieve tube) is composed of inner and outer wire sieve tubes, coated gravel and a central tube. Disperse the pre-coated gravel on the ground into the annulus between the inner and outer wire-wrapped sieve tubes. After the two ends are sealed, the coated gravel can be consolidated by heating. as the picture shows.

The gap size of the inner and outer sieve pipes of the double-layer sieve pipe is the same. Generally, the width of the gap is less than 1/2 of the diameter of the pre-coated gravel to prevent the stability of the gravel pack. The diameter of the outer tube should be as close as possible to the inner diameter of the casing, and the gap is about 10mm; the inner diameter of the inner tube is 2mm larger than the diameter of the central tube, and the thickness of the annulus clearance of the inner and outer tubes is maintained at 15mm ~ 25mm.

Double-layer screen sand control is pre-filled in the ground with gravel that meets the requirements of oil layer sand sieve analysis in the double-layer annular space with an inner and outer pre-made sand control screen. The sand control screen pipe is lowered into the casing, aligned with the oil layer, and fixed on the top of the oil layer with a hanger, or directly connected to the bottom of the oil pipe. The wire wound and the gravel layer in the interlayer can prevent the formation sand from passing through.

(4) Sand control of metal wool filter sand pipe

The structure of the metal cotton sand filter tube is shown in the figure. It consists of a perforated base pipe, metal wool and a protective pipe. The protective pipe of the base pipe is drilled to provide a fluid flow channel. The interlayer between the base pipe and the protective pipe is filled with stainless mesh fibers, that is, metal wool, as the filter medium of the sand filter pipe. The metal fibers are placed in a pre-made mold in a certain amount, and the broken and mixed wires are rolled and carded to form under a certain pressure. They are embedded in the hole of the wall of the tube or sleeved on the central tube, and fixed and added. The screen sleeve is protected for future use. The pore diameter and permeability of the metal fiber body can be adjusted by the amount of fiber and the molding pressure, so that it maintains a certain elasticity, and is selected by oil wells with sufficient strength and permeability for different situations. The thickness of the metal fiber is generally 10-20mm, and the compression coefficient and permeability can be adjusted as required. The protection tube is used to protect the metal cotton sleeve from damage. The number of holes is 300 / m and the hole diameter is 10mm.

The sand control principle of the metal cotton sand filter pipe is that a large number of metal fibers are compacted and stacked together to form a sand filter with a high gap density to prevent the formation of sand particles, and the size of the gap is related to the tightness of the fiber accumulation. By controlling the degree of fiber compaction, it can meet the sand control requirements of different oil layer sand diameters. Because the metal fiber layer is rich in elasticity, under a certain driving force, small sand particles can pass through the gap, thereby preventing the metal fiber from being blocked. After the sand particles pass, the fibers can return to their original state and play a self-cleaning role. A large amount of formation sand is blocked by the outer metal mesh to form a sand arch, which can block formation sand with a mesh diameter.

Metal wool filter sand tube is divided into single type (inlay) and integral type. The single-piece metal wool is composed of a porous cover (strainer), a stainless steel metal wool, a pressure plate, a weld, and a base pipe. Integral metal cotton sand filter tube is composed of stainless steel metal cotton wire, protective sleeve (perforated casing), central tube and centralizer.

(5) Ceramic sand filter tube

The ceramic sand filter pipe is composed of an outer pipe, a ceramic pipe and a sealing part. The ceramic tube is installed in the outer tube, and the two ends are sealed with copper-based seals with good oil resistance, water resistance and abrasion resistance. The lower end is fixed to the outer tube with screws, and the upper end is a free sliding end. The outer tube part mainly plays a protective role, ensuring a certain circulation area during design and having a certain tensile strength. The structure diagram is shown in the figure. The outer base pipe plays a role of connecting the pipe string and protecting the ceramic pipe. Its large circulation area is sufficient to meet the high production requirements of oil wells, and has a very high tensile strength to ensure that the sand control is not broken during the post-processing. The upper end between the ceramic tube and the outer base tube is free axial sliding to eliminate the thermal effect of the ceramic tube.

Ceramic tube is a kind of microporous filter material with good zigzag connectivity. Depending on the median size and permeability of the sand in the oil layer, it is preferred that the ceramsite sand with different particle sizes and a certain proportion of inorganic cement are bonded and shaped and sintered at high temperature Made. When the oil layer fluid brings out the formation sand and flows through the micropore communication channels of the ceramic pipe, the fluid enters the central pipe and is extracted, and the formation sand is blocked in the ceramic pipe and the casing annulus to play a sand control role. Ceramic pipe sand control column is mainly composed of ceramic pipe, centralizer, thermal expansion compensator, safety joint, heat-resistant packer and lost joint.

(6) Sand control of resin quartz sand filter sand pipe

Resin quartz sand filter sand pipe consists of sand filter pipe, guide shoe and central pipe. Use well-selected quartz sand and epoxy resin or phenolic resin, mix uniformly in a certain proportion, load it into a special mold, solidify and mold under certain conditions, and take it out after demolding to obtain a product with a certain shape and proper permeability. Sand filter tube and guide shoes.

(7) Porous metallurgy powder sand filter tube

The porous metallurgy powder sand filter tube is sintered with screened copper particles or iron powder, and most of them are sintered with iron powder. The cost is low. It is made of copper alloy coated metal powder with a unique production process. The sand filter tube is connected by sealing cement, with thin wall thickness and low connection strength, and it is easy to crack, so that the central tube is filled with formation sand and causes blockage.

The sand filter tube has a curved filtering channel, and the pore channels are criss-crossed. Its porosity and pore size can be controlled and regenerated. It has a high permeability, stable permeability and sufficient strength. It has the characteristics of high temperature resistance and vibration resistance. Can prevent formation sand with median particle size larger than. ^[1]

2.Sand screen gravel filling sand control

Gravel-filled sand control methods are earlier sand control methods. Due to the continuous improvement of theory, technology and equipment in recent years, it is considered to be one of the best sand control methods at present, accounting for nearly 90% of all sand control construction abroad. ^[2]

Screening gravel sand control method refers to lowering the wire-wrapped sieve or slot liner into the sand control section of the well, carrying a certain quality fluid to the gravel with a certain particle size selected on the ground, and filling the screen and the oil layer or casing. Between them, a gravel layer of a certain thickness is formed to prevent the sand particles of the oil layer from flowing into the well. The size of the gravel is selected according to the size of the sand of the oil layer. It is expected that the sand particles carried by the oil layer fluid will be blocked out of the gravel layer, and a coarse to fine sand arch will be formed outside the gravel layer by natural selection. Can effectively prevent the formation of sand. There are two commonly used gravel packing methods, namely the early open-hole wire-wrapped gravel packing for open-hole completion [Figure (a)] and the casing gravel packing for perforating completion [Figure (b)] Both.

The completion of the open-hole wire-wrapped sieve gravel pack sand control completion procedure is performed after the surface casing is completed and cemented.

or

(Shallow oil layer) The drill bit is drilled to the top of the expected oil layer, and a 7 "technical casing is drilled. The external well oil cement plus 30 quartz powder temperature-resistant cement is returned to the ground. Then use a high-quality completion fluid to drill for the third time, using 6 "The drill bit penetrated the oil layer. Generally, it is drilled 4 to 5 meters below the oil layer, and electrical measurement confirms that the oil layer has been drilled. Then use a reaming drill to expand the wellbore in the oil layer section below 0.5 1.0m to 12 ". The lower 7" casing pipe scraper scrapes the mud cake on the inner wall of the casing and rinses it out. Then enter the 5 "wire-wrapped sieve string, fill the screen with gravel, and then smash the lead seal to complete the well. The schematic diagram of the early open-hole gravel pack completion is shown in Figure (a) above.

The naked eye gravel filling has a large infiltration area, a thick gravel layer, a good sand control effect, and a small impact on the productivity of the oil layer. However, it is often used for early sand control in oil wells, the process is more complicated, and it requires a certain strength of the structure of the oil layer, and requires high conditions of the oil layer (such as a single oil layer, large thickness, airlessness, water interlayer, etc.).

This kind of sand control technology has already been adopted abroad. In China, in the early 1980s, deep well heavy oil steam injection thermal recovery tests began. Liaohe Gaosheng Oilfield, Shengli Shanjiasi Oilfield, and Karamay 9th Area have successively tested heavy oil reservoirs and applied this sand control technology.

(2) Sand control with casing gravel packing gravel

The sand control of casing gravel packing for gravel packing was developed on the basis of the prior open-hole wire-wrapped gravel packing method. It is suitable for cased completion wells. First, wire-wrapped sieve or slit liner is inserted into the wellbore, facing the oil layer. Then, the gravel is filled in the annular space of the screen and the casing with a sand carrying liquid to form a sand filter layer. In this way, the screen supports the gravel layer, and the gravel layer plays a sand retaining role. It not only controls sand production in the oil layer, but also maintains high percolation capacity, so that oil wells can produce normally. This method is usually called gravel packing in casing, as shown in Figure (b) above.

Corresponding to the gravel packing in the casing, there is another filling method called outer pipe filling. For some old wells that have produced sand, the wells outside the casing are attached. Due to the long-term production of sand, the formation deficit is serious. In this case, in order to improve the effect of sun sand, usually before the sandstone filling in the casing, the outer layer of the pipe is squeezed and filled, that is, the gravel is squeezed and filled into the void area outside the pipe through the perforation. This is called filling outside the tube. After filling outside the tube, filling inside the tube can get better sand control effect. ^[1]

Sand Control Chemical Sand Control

Chemical sand control is to squeeze a certain amount of chemical agent or a mixture of chemical agent and mortar into the formation to achieve the purpose of filling, consolidating the formation and improving the strength of the formation. Chemical sand control is mainly divided into two methods: artificial cemented formation and artificial well wall. Artificial cemented formation is to inject resin or other chemical sand-fixing agent into the formation to directly consolidate the formation sand; artificial well wall is to squeeze resin mortar fluid, pre-coated gravel, water dry lime sand, cement mortar, emulsified cement, etc. In the formation around the wellbore, an artificial well wall with a certain strength and permeability is formed after consolidation.

The chemical sand control method is suitable for short and thin well sections, and has a good sand control effect on fine sandstone formations. No mechanical device is left in the wellbore after construction, which is convenient for later processing. Disadvantages are high cost of organic chemical materials, poor adaptability to reservoir temperature, easy ageing, short validity period, formation permeability after consolidation, and significant loss of productivity.

1. Artificial cementation and sand consolidation method

(1) Phenolic resin cemented sand layer

The phenolic resin cemented sand layer is made of phenol and formaldehyde as the main materials, with alkaline substances as catalysts, mixed in proportion, and heated to prepare a first-stage resin (viscosity controlled at about 300 mPa · s). This resin solution is squeezed into Sandstone oil layer is thickened by diesel oil, and then squeezed into hydrochloric acid as a curing agent, which reacts and solidifies at the temperature of the oil layer, cements the loose sandstone, and prevents oiling and sand production from water wells. This method is suitable for early sand control of oil and water wells. After the cementation, the sandstone has a flexural strength of about 0.8 MPa, a permeability of about 50%, a temperature resistance of 100 ° C, water, oil, hydrochloric acid, and other media, and no resistance to soil acid erosion. It is easy to grasp, but the cost is high and the construction operation time is long.

(2) Underground synthetic sand control with phenolic solution

Underground synthesis of phenolic solution for sand control is to mix phenol and formaldehyde with catalyst, mix them in proportion according to the proportion, and use diesel as a pore-enhancing agent. After the phenolic solution is squeezed into the sand layer, a resin is gradually formed at the oil layer temperature and deposited on the surface of the sand particles. After curing, the oil layer sand is cemented firmly, while the diesel does not participate in the reaction as a continuous phase filled with pores, so that the cemented sandstone maintains good permeability In this way, a method of improving the cementation strength of sandstone and preventing sand production in the oil and gas layer is played.

This method is an early and early sand control method for oil wells, and is suitable for medium and fine sandstone reservoirs with a temperature higher than 60 ° C and a lower clay content. The average validity period is more than two years, the construction is relatively simple, and the sand control effect on the oil layer has been a lot, or the sand control effect is poor, so it is not suitable. ^[3]

2. Sand control method for artificial well wall

(1) Artificial coated gravel wall

The pre-coated gravel artificial well wall refers to the outer surface of the quartz sand, which can be evenly coated with a layer of resin by physical and chemical methods, and dried at normal temperature to form stable particles without adhesion. This pre-coated gravel is carried to the sand production layer of the oil well using sand carrying fluid. Under certain conditions (extrusion of curing agent and temperature), the resin on the surface of the gravel softens and consolidates to form a good penetration. And strength of the artificial well wall to prevent sand production in the oil and gas layer. This method is suitable for sand control in oil layers with large absorption capacity and temperature higher than 60 . The construction is simple and the success rate is high. The flexural strength of the gravel after cementation can reach about 5MPa, and the permeability can be maintained above 90% of the original value. Currently a better chemical sand control method.

(2) Artificial mortar wall of cement mortar

The cement mortar artificial well wall is cemented with cement as the cement and quartz sand as the support. It is mixed uniformly in proportion, mixed with an appropriate amount of water, carried with oil to the well, squeezed out of the casing, and accumulated in the sand production site. A method for preventing sand production in oil and gas layers of artificial well walls with certain strength and permeability. This method is a late sand control method for oil wells. It has a high permeability, a wide range of raw materials, and simple construction. However, it uses a large amount of oil and has a flexural strength of less than 1 MPa after cementation and a shorter effective period.

(3) Resin walnut shell artificial well wall

Resin walnut shell artificial well wall is made of phenolic resin as cement, walnut shell crushed into certain particles as proppant, and mixed uniformly according to a certain proportion, so that each walnut shell particle surface is coated with a layer of resin, and a small amount of diesel oil is used to infiltrate Then, it is carried downhole with oil or active water, squeezed out of the casing section of the incident hole, and piled up in the sand production layer. It is consolidated by the reaction resin for a certain period of time under the action of the curing agent to form an artificial with a certain strength and permeability. Method for preventing sand production in oil and gas layers of the well wall. This method is suitable for early sand control of oil and water wells. After cementation, the artificial well wall has higher permeability, higher strength, and better sand control effect, but the source of raw materials is difficult.

(4) Artificial mortar wall of resin mortar

Resin mortar artificial well wall is made of resin as cement and quartz sand as proppant. They are mixed uniformly in proportion, so that a uniform resin film is coated on the surface of quartz sand, and a small amount of diesel oil is added for wetting. The method of preventing sand out of the oil and gas layer is to form an artificial well wall with a certain strength and permeability after solidification to form a layer of sand outside the casing. This method is a late sand control method for oil and water wells. It is suitable for oil and water layers with high absorption capacity. It has strong adaptability and is not limited by the depth of the well. However, the amount of mixing on site during construction is large and it is difficult to carry sand fluid. ^[1]

Sand control composite sand control

The composite sand control uses the advantages of mechanical sand control and chemical sand control to complement each other. On the one hand, it can form an artificial well wall with good permeability in the near-well zone; on the other hand, it uses the mechanical sand control pipe string to form a secondary sand blocking barrier, which plays a good role. Sand control effect. The composite sand control effect is good and the validity period is long. The composite sand control commonly used mechanical sand control pipe columns are sand filter tubes and wire-wrapped screen tubes. The chemical methods used in combination with them are often chemical agents and coating sand.

Composite sand control has wide adaptability and can be used for sand control measures under almost any complicated conditions. However, the complex sand control technology is complicated and costly. Therefore, it is generally used when a single sand control method is not effective, especially for fine sandstone and poor permeability formations, and also for old well sand control with serious formation deficits.

1. Conventional mechanical-chemical compound method

The most common composite sand control method is the combination of the above-mentioned mechanical sand control pipe string and chemical sand fixing agent.

Figure (a) is a schematic diagram of composite sand control with wire-wrapped screen tube + coated gravel (coated sand) sand. Firstly, the pipe is filled with sand and squeezed out of the pipe. After consolidation, the plug is drilled, and then the wire-wrapped sieve is inserted. Figure (b) is a composite sand control schematic diagram of chemical agent sand fixation + double-layer pre-filled wire-wrapped sieve pipe. After the formation is consolidated with chemical agent, the double-layer pre-filled wire-wrapped sieve pipe is lowered.

2.High permeability fracturing filling sand control

High permeability fracturing filling sand control is a new composite sand control technology developed rapidly in the 1990s. Hydraulic fracturing and gravel filling are applied to high permeability and loose sandstone formations. The two processes are organically combined to achieve the traditional The best effect that can not be achieved by the process is to make the oil well high production and control sand production.

The main technical principles of high-permeability fracturing filling sand control can increase production and effectively control sand as follows:

(1) Flow characteristics of formation fluids after compression

Before fracturing, the flow of the homogeneous formation fluid into the wellbore is radial flow; after fracturing, the flow of the formation fluid is in two modes, first the linear flow flowing inside the formation to the fracture surface, and then the fluid directly enters the wellbore along the fracture, such as As shown in the figure, a bilinear mode is formed.

(2) Hydraulic cracks can avoid and mitigate rock damage

Fracturing fractures with extremely high diversion capabilities transform formation fluids from original radial flow to bilinear flow, which reduces the production pressure differential and greatly reduces the flow pressure gradient. So as to alleviate or avoid the destruction of the rock skeleton, the tendency and degree of sand production are also alleviated.

(3) Cracks can reduce the ability of flow scour to carry sand particles

The fluid's ability to scour and carry particles mainly depends on its flow velocity. The greater the flow velocity, the stronger the scour effect on the formation and the more serious the sand production. The bilinear flow pattern generated by the fracture and the huge fracture surface area can play a good role in shunting, greatly reducing the flow velocity after pressure, thereby reducing the scouring and carrying effect of formation particles, and greatly reducing the degree of sand production.

(4) The gravel filled in the fracture has a blocking effect on the formation sand grains

The principle of action is similar to that of conventional gravel filling. The gravel filled in the fracture has a blocking effect on the formation sand. Sometimes it is possible to use resin-coated sand as a proppant or seal it at the bottom of the well with the film-coated sand to improve the barrier to formation sand. ^[1]