Modeling of capillary displacement and invasion percolation of immiscible fluids during CO2 sequestration and enhanced methane recovery

LIU Cao; YANG Shuo; WEI Jianping

doi:10.13247/j.cnki.jcumt.20250221

Abstract：Sequestration of CO2 in deep coal seam is one of the most important ways for coal industry to energy saving and emission reduction.However,coal seams are heterogeneous porous media with pore size distributions ranging from nanometers to centimeters.In particular,the complex system of coal rock pores coexisting with multiple phases such as CO2,CH4,and H2O can generate various magnitude of capillary pressures,which will inevitably affect the injection of CO2 displacement and mass transfer.And therefore,the key issue in coalseam geological storage is how much pore and fracture space can be invaded and occupied by injecting CO2?In this regard,a pore network dynamic model was first established for CO2 invasion percolation in coal seam;Then,the pore network consis-ting of 90 pores on a coal sample scanned by an electron microscope(SEM)was taken as an example,a quantitative study was conducted regarding on the two continuous fluid dynamics processes of CO2 injection and reaching mechanical equilibrium after injection cessation.The results indicate that fluids such as CO2,CH4,and H2O in coal seam environments generate immiscible interfacial capillary re-sistance due to their immiscibility,the injected CO2 always chooses the path with the lowest resistance to move forward,resulting in CO2 only entering the pore and fracture spaces with larger pore diameter and lower resistance.The invasion percolation process of CO2 injection in coal seams is essentially a competition between the displacement pressure difference ΔP and the capillary resistance at the im-miscible interface,the larger the displacement pressure difference ΔP,the smaller the pore that can be entered,and the larger the effective space for CO2 sequestration.The flow process of CO2 injection in coal seam does not completely follow the commonly accepted Darcy flow,but exhibits multiple sta-ges of flow characteristics,in the initial stage of injection,CO2 can only enter the wetting pores and compete with CH4 in these pores for adsorption and desorption before reaching the non-wetting pore capillary entry pressure;Thereafter,CO2 follows the invasion percolation movement in non-wetting pores,and conforms to the Haynes jump unstable flow at wetting pores or pore intersection nodes;Later on,CO2 breaks through the coal seam pore and fracture network when reaching the threshold capillary breakthrough pressure,which means that injected CO2 has formed a continuous flow path in the coal seam and reached the coal seam outlet such as the production well,at this point,the flow mechanism begins to follow Darcy flow;Finally,if CO2 injection is stopped,wetting fluids such as water will undergo spontaneously imbibition,cutting off and closing the injected CO2 into two parts:structural storage and residual storage.This study has important implications for explaining the mechanisms behind engineering phenomena such as coal and gas outbursts,CO2 injection failure in porous media reservoirs such as shale and coal,and low efficiency of negative pressure gas extraction in coal mines.