Study On The Capillarity Of CO₂-CH₄–H₂O In Low-permeability Sandstone Under The Condition Of Formation Temperature And Pressure

Study on the capillarity of CO₂-CH₄-H₂O in low-permeability sandstone under the condition of formation temperature and pressure,as well as the gas flow behavior and breaktrough process are of great importance to the development of oil and gas reservoirs,the exploitation of shale gas resources and the greenhouse gas geological sequestration.Therefore in this phD dissertation,we carried out breakthrough pressure experiments of CO₂ and CO₂-CH₄ gas-mixture at core-scal at temperature of40?and pressure of 8.5 MPa.Low permeability sandstone cores from Ordos Basin were chosen for the gas breakthrough experiments under six different degrees of water saturation.The porosity of low permeability sandstone core is in the range of6.45%-16.03%,and the permeability is distributed in the range of4.03×10^-3-4.27×10^-1md.The main mineral components are quartz and feldspar,followed by a small amount of calcite and clay minerals.The macropore volume accounts for the largest proportion,followed by the mesopores and micropores.The breakthrough pressure of CO₂ decreases with the increase of average pore radius and most probable pore radius in the form of power law function.When the average pore radius and most probable pore radius is less than 0.151?m and 0.713?m respectively,the CO₂ breakthrough pressure increases exponentially with the increase of water saturation P_bt=a?e^{b S}w.When the average pore radius is greater than 0.151?m,the degree of water saturation will has a little effect on the breakthrough pressure.On this foundation,if the most probable pore radius of the core sample reaches 1.760?m,the breakthrough pressure will not be impacted by the increasing water saturation.However,the presence of CH₄ increases the interfacial tension between gas and water,resulting in the fact that the breakthrough pressure of gas mixture is higher than that of pure CO₂.The higher the CH₄ concentration is,the greater the breakthrough pressure of gas mixture is.It is also found that the breakthough pressure of gas mixture increases exponentially with the increasing water saturation,which is not limited by the pore structure.This illustrates that under the same water saturation,the influence of gas-liquid interfacial tension on the gas breakthrough pressure is higher than that of the pore structure.It is evident that the scCO₂ permeability is strongly affected by the pore structure:higher scCO₂ absolute permeability and effective permeability are found in the samples with larger average pore radius or most probable pore radius under the same water saturation conditions.Higher water saturation in the core sample resultes in a considerable decrease in the scCO₂ effective permeability,and their relationship could be described as exponential function form K_eff=??e^-?Sw.Under the same temperature and pressure conditions,the viscosity and adsorption capacity of gas mixture is smaller than that of the pure CO₂ because the presence of CH₄,which allows gas mixture to flow more easily in the porous medium and leads to the larger permeability of the gas mixture.The higher the CH₄ concentration is,the greater the permeability of gas mixture is.Chromatographic partitioning results show that a gas mixture chromatographically partitions after advancing through a completely or partially water-saturated porous medium because of the different solubilities and adsorption capacities of two gases.Since CO₂ is more soluble in water compared to CH₄ under the experimental conditions and more favorable than CH₄ during competitive adsorption.Thus,CH₄ gathers at the leading edge of the breakthrough gas,facilitating the advancement and breakthrough of CH₄ through the sample.