Principle of mercury injection method: Mercury is a non wetting phase for most rock cores. If the pressure applied to mercury is greater than or equal to the capillary pressure of the pore throat, Mercury will overcome the resistance of the pore throat and enter the pore. According to the pore volume fraction and the corresponding pressure, the relationship between capillary pressure and mercury saturation can be obtained.
Because the surface tension and wetting contact angle of mercury are relatively stable, the pore size and distribution are usually converted from the capillary pressure curve measured by an injection mercury porosimeter.Assuming that the pore system is composed of cylindrical capillary bundles of different thickness, the relationship between capillary pressure and pore diameter is as follows:
Where:
ρc — Capillary pressure (absolute pressure in MPa)；
σ — surface tension in N/m；
θ — wetting contact angle（in °）
rc — Capillary radius（in μm）
Under laboratory conditions, σ=0.48N/m, θ=140°, there is:
According to equation (2), the capillary pressure curve is converted into pore throat size and distribution curve.