大倾角巨厚煤层气储层排采有效应力变化及其对渗透率的控制机理

Coalbed methane reservoir in ultra-thick coal seams with large dip angle is characterized by great span of the burial depth, strong heterogeneity in the vertical direction, and varying levels of geostress, reservoir pressure and effective stress in a limited scope along the dip direction. As a result, stress propagation during methane drainage in ultra-thick coal seams with large dip angle shows a distinct dynamic characteristic in comparison to that in subhorizontal coal seams, gently inclined coal seams and medium-thick or thick coal seams. Therefore, this study takes the methane reservoir in an ultra-thick coal seam with large dip angle as a research case so as to reveal the distinct characteristic of the stress propagation. Field investigation and lab tests are carried out to obtain the essential parameters of the reservoir, such as in-situ reservoir stress, physical and mechanical properties. With the acquired parameters, a multi porosity media seepage-stress coupling model in consideration of the stress gradient is constructed, which is later integrated into COMSOL Multiphysics. In addition, MATLAB was also employed to perform relevant calculations. Finally, a numerical model serving for methane drainage in ultra-thick coal seams with large dip angle is established. This numerical model could not only advance understandings of the dynamic changes in reservoir pressure, effective stress and permeability, but also help uncover the dynamic characteristics of stress propagation in the upper and lower part of the inclined reservoir in different types of wells, coal seams with different dip angles and coal seams with different thicknesses. The findings combined together could contribute to a good understanding of the role of the effective stress in permeability control during methane drainage in ultra-thick coal seams with large dip angle. This study is of great theoretical and engineering significance for the efficient exploitation of CBM in complex reservoirs in Zhunnan coalfield in Xinjiang.

大倾角巨厚煤层气储层埋深跨度大，垂向上非均质性强，倾向上有限范围内地应力、储层压力和有效应力变化剧烈，煤层气井排采过程中压力的动态传播特征与近水平或缓倾斜、中厚或厚煤储层显著不同。以大倾角巨厚低强度煤储层为研究对象，在对煤储层压力及物性参数现场调研和室内测试的基础上，通过构建考虑应力梯度的多孔介质渗流-应力耦合数学模型，将数学模型与有限元软件COMSOL Multiphysics对接，结合MATLAB编程求解，建立大倾角巨厚煤储层煤层气开发数值模型。通过数值模拟研究排采过程中储层压力、有效应力和渗透率的动态变化规律，揭示不同开发井型、不同煤层倾角及厚度条件下储层上倾及下倾方向压降范围的动态扩展规律，进而揭示大倾角巨厚煤层气储层排采有效应力对储层渗透率的控制机理。研究对于新疆准南煤田复杂煤储层条件下煤层气的高效开发具有理论和工程意义。

新疆准南盆地阜康等矿区煤层气储层具有倾角大、厚度大、层数多、力学强度低等特点，储层呈现埋深跨度大、垂向上非均质性强、倾向上有限范围内压力参数变化剧烈等复杂特征，严重有别于中东部矿区的近水平或缓倾斜、中厚及厚煤层气储层条件。针对此，通过理论推导、数值模拟、实验室测试等手段围绕以下五方面内容展开研究：1）应力梯度与储层压力、有效应力及渗透率之间的耦合关系；2）大倾角巨厚煤储层压力参数对渗透率的动态影响；3）不同开发井型条件下压降范围的动态扩展规律；4）煤储层水分对渗透率演化的影响；5）基于储层恒定体积的渗透率模型。研究表明：1）大倾角煤储层渗透率沿走向的变化受基质收缩影响更大，有效应力主要控制倾斜方向，导致渗透率沿倾向关于排采井非对称；2）深部和浅部储层的初始地质特征（地应力、初始储层压力和渗透率）的差异导致了储层压力、渗透率、流体饱和度的非同步演化和产气量的“双峰”特征；3）将煤层气井布置在储层上倾方向有利于煤层气产出，顺煤层水平井比穿煤层水平井拥有更大的煤层气解吸面积和更高的煤层气解吸程度，更有利于煤层气开发；4）不同层理倾角试样具有不同的应力敏感性，孔隙压缩率随层理倾角增加而先增大后减小，在45°角时增加到最大值，渗透率对应力的敏感性最强，渗透率降幅最大；5）水对煤体弹性模量有显著弱化作用，随储层水饱和度增大，煤样弹模逐渐减小，煤样在干燥与饱水状态下的弹模相差达29%~59%，可能严重影响储层渗透率的演化行为，即随排采进行煤体硬度增大，抵抗由于储层压力降低、有效应力增大而导致孔隙裂隙压缩的能力增强，有利于渗透率的提高。项目揭示了大倾角巨厚煤层气储层排采有效应力对储层渗透率的控制机理，并新增了部分研究内容。研究对于新疆准南煤田复杂煤储层条件下煤层气的高效开发具有理论和工程意义。