煤层气在基质孔隙中解吸传递机理及产能预测

Coal seams are dual porosity media containing two systems of fractures and matrix pores. The size of matrix pores is very small while specific surface area is large; a large amount of natural gas is adsorbed on the pore surface. It is generally believed that after the adsorbed gas desorbs, the desorbed gas in matrix pores first diffuses into cleats (fractures) under the driving of concentration difference, and then flows to the wellbore in the form of gas-water tow-phase flow. However, it is found that, predicted gas production based on the diffusion mechanisms of desorbed gas through matrix pores greatly deviate the actual production data. Through the preliminary research it is found that the diffusion theory ignores the existence of water in matrix pores, which makes the diffusion model inapplicable, and the gas production model based on the molecular diffusion has some problems. Therefore, we intend to start our study on the initial distribution of gas and water in matrix pores, and then construct a more accurate transport model by studying the dynamics and phase characteristic of gas in water bearing matrix pores. Based on the more accurate transport model, a corresponding gas production prediction model is established, and the gas production rule of coal-bed methane (CBM) is obtained. Our study will provide a more accurate technique for gas production forecasting of CBM wells, and moreover, it will provide scientific evidence to the rational development of CBM reservoirs.

煤岩发育有裂隙和基质孔隙双重孔隙，其中基质孔隙尺寸很小，但比表面积极大，孔隙表面吸附着大量的甲烷气。目前普遍认为，基质孔隙中的吸附气解吸后，首先在浓度差的作用下扩散进入割理（裂隙），然后以气水两相渗流的方式流向井筒。然而，基于扩散-渗流模型进行气井产能预测时，与实际生产曲线存在较大的偏差。通过研究发现，目前煤层气扩散理论忽视了基质孔隙中水的存在，使得扩散模型选择不妥，进而基于分子扩散而建立的产气模型存在一定的问题。因此，本课题拟从基质孔隙内的原始气水分布入手，研究含水基质孔隙中解吸气的相态特征及动力学行为，建立一个更准确的传递模型，进而建立相应的产能预测模型，揭示煤层气产气规律。本课题的研究将为煤层气界提供一套更准确的产能预测方法，为煤层气的合理开发提供科学依据。

在对煤储层基质孔隙分类、煤储层中气水变化特征、基质孔隙中甲烷的动力学行为、煤层气解吸模型、扩散模型、渗流模型和产气模型综述和评论的基础上，揭示了煤储层孔隙特征及煤层气成藏过程和生产过程气水分布演变特征；评价了气水界面压降对煤层气产出的影响；开展了饱和水煤样的吸附解吸实验并建立了煤层气基质孔隙吸附/解吸模型；开展了大尺寸不规则原煤渗流实验；揭示了煤层气解吸扩散渗流机理并建立了基质孔隙中气体的非线性渗流模型；建立了煤层气井数值模拟模型，进行了煤层气井的产气预测。.研究表明：储层条件下煤层气的吸附解吸过程均在水环境下进行的，由于吸附气甲烷的相变过程和毛管力“封堵”作用导致甲烷在饱和水煤岩的解吸过程存在严重滞后现象；同时，由于基质孔隙水的存在，大量的解吸气聚集成泡和成柱并在煤基质孔隙中发生非达西渗流，而并非通常意义的“气相扩散”过程，仅有少量的解吸气溶解并在储层孔隙水中扩散；建立的煤层气井数值模型模拟了解吸气“成核成泡”和在煤基质孔隙中非达西渗流、自由气从煤基质孔隙至割理-裂隙系统的窜流、自由气从煤基质孔隙至井筒的非达西渗流（仅限低煤阶）、及自由气从割理-裂隙系统至井筒的达西渗流过程，结果表明整个生产过程经历产气量上升期、产气量长时间相对平稳期和后期的产气量递减期的三个阶段。.本基金研究成果可为煤层气开发提供理论依据，对煤层气产气规律的认识具有指导意义。