裂隙煤岩各向异性渗透率模型和煤层注气THM耦合行为

Anisotropy permeability model for highly fractured coal seams associated with coupled THM analysis of CO_2-ECBM

CO2煤层地质封存可以减少温室气体排放,同时可提高煤层气的采收率。注气开采过程涉及到温度场(T),多相多成分流场(H)和应力场(M)之间的相互耦合。煤层割理裂隙渗透率是影响CO2地质封存和煤层气开采率的重要参数。煤岩渗透性的关键性因素裂隙张开度同时受控于法向应力和剪胀效应。考虑基质和割理的共同作用,提出基于组合裂隙三向平板简化的各向异性渗透率模型。在此基础上,建立了注入CO2提高煤层气采收率的三维数值模型并利用耦合软件TOUGH-FLAC进行求解。模拟结果表明,气体注进采出的孔压作用会引起煤层膨胀或收缩。与孔压,吸附应力/应变以及温度相比,剪胀对裂隙渗透率的影响不明显。在注采过程中,渗透率表现出明显的各向异性,注入井附近的异向渗透率甚至可达30倍差异。此外,注气初期应适当控压,井口附近的高压损伤将带来不必要的裂隙气体泄漏,导致注气失败。

CO_2 injection into coal seams is recognized as an effective approaching to reduce CO_2 emission and enhance coalbed methane recovery.The injection-production process is a coupled process involving multiphase flow,heat transfer and mechanical deformation.The permeability of fractures within the cleat system is one of the key factors controlling the efficiency of injection and extraction,which relates to the fracture aperture and matrix size.The aperture is controlled by both of the effective normal stress and shear dilation.The matrix deformation is calculated by the total strain and aperture change.Then,based on the parallel plate fracture flow model,an anisotropy permeability model for the highly fractured coal is developed to predict the evolution of permeability on the different directions. A coupled THM model that employs the new anisotropy permeability model was applied into TOUGH-FLAC to study the CO_2 injection and methane extraction.The results of the numerical simulations indicate that the variation of pore pressure as a resultof CO_2 injection and methane production causes the coal seams uplift and subsidence around the area of injection and production well.The mechanical behavior has a significant effect on the evolution of permeability,especially the changes of pore pressure and temperature,and swelling/shrinkage strain induced by CO_2 adsorption/methane absorption.The permeability variations show the marked differences in the different directions.The greatest permeability anisotropy ratio reaches as high as about 30,which is sensitive to the effective normal stress and curvature parameter.Moreover,at the initial stage of CO_2 injection,the region near the injection well has the potential to occur the coal damage that provides a new path for CO_2 and methane leakage,so more particular attention should be given.