What Is a Porous Medium?

Porous media is a common space occupied by heterogeneous materials, and it is also a combination of coexisting heterogeneous materials. The part of the space without a solid skeleton is called pores, and is occupied by two phases: liquid or gas or gas-liquid. In terms of phases, the other phases are diffused therein, and the solid phase is used as a solid skeleton, and some of the voids constituting the void space communicate with each other.

Introduction to porous media

A substance composed of a skeleton composed of solid matter and minute voids separated into a large number of densely packed groups by the skeleton. The fluid in porous media moves in a percolation manner. The theory of studying the physical-mechanical properties of porous media involved in percolation mechanics becomes the basic component of percolation mechanics. The main physical characteristics of porous media are extremely small void sizes and large specific surface area values. Tiny voids in porous media may be mutual

Porous media Connected, it may be partially connected and partially disconnected.

Classification of porous media

There are many types of porous media. Divided by the cause, it can be divided into natural porous media and artificial porous media. Natural porous media is further divided into underground porous media and biological porous media, the former such as rocks and soil; the latter such as the microvascular network and interstitial spaces in humans and animals, and the roots, stems, branches, and leaves of plants. There are many types of artificial porous media, such as filters in filter equipment, foundry sand molds, ceramics, bricks, wood and other building materials, activated carbon, catalysts, saddle-shaped fillers, and glass fiber deposits. According to the morphology and structure of tiny voids, it can be roughly divided into porous porous media, fractured porous media, and multiple porous media. Porous porous media can be further divided into two categories: pores communicate with each other in all directions, and there is no obvious hierarchical relationship, such as the accumulation of sandstone, soil, and artificial granular materials; pores are like dendritic distribution, with obvious Membership in hierarchical relationships, such as a general microvascular network. The voids in the fractured porous medium are mainly micro-cracks, such as fractured limestone and dolomite. When the porous medium has multiple micro-voids, it is called multiple porous medium. For example, the carbonate layer of the fracture-pore system is a dual porous medium or dual medium for short.

Morphology of porous media

The voids inside the porous medium are extremely small. The pore diameter of sandstone formations that store oil and gas is mostly between 1 micrometer and 500 micrometers; the inner diameter of capillaries is generally 5-15 micrometers; the pore diameter of the alveolar-microbronchial system is generally about 200 micrometers or less; The diameter of the pores that transport water and sugar in the body is generally not greater than 40 microns.

The total surface area of all minute voids per unit volume or unit mass of a porous medium is called the specific surface area. The specific surface area of porous media is large. For example, the specific surface area of sandstone that stores underground fluid resources and energy such as petroleum, natural gas, geothermal, and groundwater is generally on the order of 105 m2 / m3. The specific surface area of the porous media of the vascular system of kidney, lung, liver, and heart About 104

On the order of magnitude, the specific surface area of some porous microsphere chromatography supports is 840

/ g, the specific surface area of powder metallurgy powder can reach 5 × 103

/ g. Specific surface area is an indicator of the degree of dispersion of porous media. Its numerical value has an effect on the surface molecular forces during fluid percolation, and has an important effect on the processes of adsorption, filtration, heat transfer, and diffusion of porous media.

Several important concepts of porous media

In porous media theory of percolation mechanics, there are several important concepts:

Porosity of porous media

The ratio of the total volume of the microvoids in the porous medium to the external volume of the porous medium. There are two kinds of porosity: the ratio of the total volume of interconnected microvoids in the porous medium to the external volume of the porous medium is called the effective porosity; the total volume of all the interlinked and non-interconnected microvoids in the porous medium and the porous medium The ratio of the apparent volume is called absolute porosity or total porosity. In common non-biological porous media, the porosity of saddle-shaped fillers and glass fibers is up to 83% to 93%; the porosity of coal, concrete, limestone and dolomite is the smallest, which can be as low as 2% to 4%; The porosity of sandstones related to energy and resources such as underground fluid resources is mostly 12% to 30%, the porosity of soil is 43% to 54%, the porosity of bricks is 12% to 34%, and the porosity of leather is 56. % 59%, all are medium values; the porosity of the vascular system of the kidneys, lungs, livers and other organs of animals is also a medium value. Porosity is an important parameter that affects fluid capacity and fluid percolation in porous media.

Porous media wettability

A physical property of a solid surface wetted by a fluid that appears on the three-phase contact surface of a solid and two fluids (two non-mutual solutions or liquid and gas). The wetting phenomenon is the result of the energy balance of the surface molecular layers of the three phases. The energy of the surface layer is usually expressed by polarity, and the wettability can also be expressed by the difference in polarity between solid liquids. The smaller the polarity difference, the more easily wetted. For example, the polarity of metal surfaces is smaller, and the polarity of water is greater than that of grease. Metal surfaces are often easily wetted by oil and not by water. Therefore, metals can be said to be lipophilic or hydrophobic; the surfaces of glass and quartz The polarity is large, and it is easy to be wetted by water and not easily wetted by grease. Therefore, it can be said that glass and quartz are hydrophilic or oil-repellent.

Under certain conditions, wettability is related to factors such as temperature and pressure. Factors such as fluid properties may also affect the wettability of solid surfaces. For example, a surface-active substance-containing fluid may change its wettability when it comes in contact with a solid surface. The wettability of some solid surfaces is complicated. For example, due to the contact with different liquids, there may be a phenomenon that both the lipophilic surface and the hydrophilic surface exist on the same oil storage rock.

Wettability has an important influence on the law of fluid movement in porous media and related production processes. For example, if the infiltration properties of oil storage rocks are different, the calculation methods of percolation mechanics, oilfield development principles, and production control measures are different.

Capillary pressure in porous media

The pressure difference existing on both sides of the interface between any two non-miscible fluids in the tiny voids of the porous medium, that is, the difference between the pressure of the non-wetting phase and the pressure of the wetting phase. Capillary pressure depends on the surface tension of the fluid, the wetting angle, and the curvature of the interface. During fluid displacement, capillary pressure can be either the driving force or the resistance to flow. Under the action of capillary pressure, the infiltrating phase can spontaneously displace the non-infiltrating phase, that is, the infiltration effect. The existence of capillary pressure affects the law of fluid movement in porous media, so it is an issue that must be considered in percolation mechanics and related engineering technologies. For example, in oilfield development, capillary pressure affects the effective permeability of the oil layer and the recovery of the oil layer; the capillary pressure curve can be used to determine the pore distribution and fluid distribution in the porous medium, and calculate the phase permeability of the porous medium and the recovery of the oil layer Rate etc.

Porous media permeability

Indicates the amount of permeability of a porous medium. The property of a porous medium that allows a fluid to flow through interconnected tiny voids is called permeability. Common porous media have a certain permeability. There is no fixed functional relationship between permeability and another physical property of porous media, porosity, but directly related to factors such as pore size and its distribution. The permeability value is determined by Darcy's law of percolation. The measurement unit of the permeability of the physical system is square centimeter, and Darcy and Thousand Darcy are commonly used in engineering, that is, one thousandth Darcy. One Darcy is equal to 1.02 x 10 cm. The absolute permeability value of sandstone reservoirs with industrial value ranges from a few to 3,000 thousandths of Darcy. The permeability of most sandstone reservoirs is 200 ~ 1000ths of Darcy; the permeability of bricks is 5 ~ 220ths of Darcy; Soil permeability is generally 0.29 to 14 Darcy.

Permeability can be divided into three categories: absolute permeability, which is the permeability value usually measured by air through porous media; effective permeability, which is the permeability taking into account the properties of fluids and their motion characteristics, such as two-phase or multi-phase fluid percolation When the permeability of porous media to each phase fluid is always smaller than the absolute permeability, it is called phase permeability; the relative permeability is the ratio of phase permeability to absolute permeability. Phase permeability is calculated by Darcy's formula for multiphase percolation. Experiments have shown that the value of phase permeability is related to factors such as the volume percentage of the phase fluid in the void, that is, the saturation of the phase. The relationship between relative permeability and saturation is called the relative permeability curve of porous media.

Permeability is one of the basic physical-mechanical properties of porous media. Permeability is an important basic data of seepage mechanics and related engineering technology, which characterizes the characteristics of the seepage process. Taking underground fluid resources and energy as examples, the greater the formation permeability, the greater the production capacity and recovery factor.