Permeability refers to the ability of rock to allow fluid to pass through under a certain pressure difference.
This is a parameter representing the ability of soil or rock to conduct liquid. Its size is related to porosity, the geometric shape of pores in the direction of liquid penetration, particle size and arrangement direction, and has nothing to do with the nature of liquid movement in the medium. Permeability (k) is used to indicate the size of permeability.
When the pressure gradient is 1, the seepage velocity of liquid with dynamic viscosity coefficient of 1 in medium. The dimension is L2.
Extended data:
The quality of rock permeability is expressed by the numerical value of permeability, which can be expressed by absolute permeability, effective permeability and relative permeability.
I. Absolute permeability
When a single-phase fluid flows through a porous medium with a cross-sectional area of A, a length of L and a pressure difference of Δ P in layers, the viscosity of the fluid is μ, and the amount of fluid passing through the pore of the rock in unit time is Q = K Δ Pa/μ L. ..
When a single-phase fluid flows in layers in porous media, the liquid flow through the rock cross-sectional area per unit time is directly proportional to the pressure difference and cross-sectional area, but inversely proportional to the length of the liquid passing through the rock and the viscosity of the liquid.
Where: q-the flow rate of fluid passing through the rock per unit time, cm3/s; A—— the cross-sectional area of liquid passing through the rock, cm2；;
μ-viscosity of liquid, Pa·s；; ; L—— length of rock, cm; Δ p-pressure difference before and after the liquid passes through the rock, MPa；;
The absolute permeability of rock is the permeability measured when there is only one fluid (single phase) in the pore of rock, and the fluid does not react with the rock physically and chemically, and the fluid flow conforms to Darcy's linear seepage law.
Because gas is obviously affected by pressure, when the gas flows from (high pressure) to (low pressure) along the rock, the volume of gas will expand, and its volume flow will be variable when it passes through the cross-sectional area everywhere, so the volume flow in Darcy's formula should be the average flow through the rock.
Second, effective permeability.
English: Effective permeability
Definition: Permeability of porous media under unsaturated water flow.
When multiphase fluid flows through porous media, the permeability of a fluid is called the effective permeability of the fluid, which is also called the phase permeability.
Third, relative permeability.
English: relative permeability
Definition: When multiphase fluid flows through porous media, the ratio of the permeability coefficient of a fluid to the saturated permeability coefficient of the media is called relative permeability, which is a dimensionless quantity.
Permeability as a cardinal number can be:
1, absolute permeability measured by air;
2. Absolute permeability measured with water;
3. Oil permeability under water saturation of a certain reservoir.
Like effective permeability, relative permeability is related to liquid saturation. The sum of relative permeabilities of different fluids in the same porous medium at a certain saturation is always less than 1. According to the relative permeability values measured under different saturations, the relationship curve between relative permeability and saturation is called relative permeability curve.
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