煤层生物改造多尺度生化侵蚀渗流机理研究

Coalbed methane (CBM) reservoirs in China have the characteristics of small thickness, low permeability and high gas content. Improving the gas saturation of CBM reservoir is the key to achieve the large-scale exploitation of CBM. Our research team put forward a technical method by combining microbial enhancing CBM with hydraulic fracturing to stimulate CBM reservoir. CBM mining is a multi-scale seepage process, as biological stimulation affects the coal matrix pore, the hydraulic parameters of fractures, and complex fracture morphologies, the migration and production rules of CBM in the reservoir become more complicated. First, this project aims to carry out imbibition experiments of coal samples in nutrient solution, and the effects of biological stimulation on pore structure and adsorption characteristics of coal samples will be analyzed. The non-equilibrium adsorption diffusion model of coal matrix under biological stimulation will be established. Then, the experiments of gas seepage in biological stimulated fractures will be conducted. The dynamic output of pulverized coal and the dynamic permeability of fractures under biochemical erosion effects will be observed, and multiple gas-solid flow regularity of matrix-erosion layer-fracture will be studied. Finally, triaxial simulation experiments of biological transforming reservoir to increase production is to be conducted. Combining with numerical simulation, analysis on gas-solid two-phase path selection and flow rules in complex fractures will be made. The seepage and stress coupling model of biological transformation complex fractures will be established, and multi-scale seepage mechanism of biochemical erosion under biological transformation will be revealed. The research results can provide a theoretical guidance for biological transformation scheme optimization and production prediction in CBM field development.

我国煤层气储层普遍具有厚度薄、渗透率低、含气量高的特点，增加煤气藏饱和度是我国煤层气实现大规模开采的关键，研究团队提出采用微生物增产结合水力压裂体积改造煤层的技术思路。煤层气开采是一个多尺度渗流的过程，随着生物改造对煤层基质孔隙、裂缝水力参数、复杂裂缝形态的影响，煤层气在储层中的运移及产出规律更加复杂。为此，本项目首先进行营养液浸泡煤岩实验，分析生物刺激对煤基质孔隙结构及吸附特性的影响，建立生物刺激煤岩基质的非平衡吸附扩散模型；然后开展生物改造裂缝的甲烷渗流实验，研究煤粉的动态产出规律及裂缝生化侵蚀动态渗透率，得到基质-侵蚀层-裂缝的多重气-固流动规律；最后进行大型真三轴生物改造模拟增产实验，结合数值模拟方法分析复杂裂缝气-固两相路径选择及流动规律，建立生物改造复杂裂缝渗流-应力耦合模型，揭示煤层生物改造多尺度生化侵蚀渗流机理。研究成果可为现场煤层生物改造方案优化及产能预测提供理论支撑。

项目针对我国煤层气储层特点，提出采用微生物增产结合水力压裂体积改造增加煤气藏饱和度的技术思路。围绕“生物改造煤层气多尺度渗流”关键问题开展研究，主要进展及结果如下：1）开展营养液浸泡煤岩实验，结合电镜扫描、X射线衍射、压汞及液氮吸附实验，得到生物刺激对煤样孔隙结构及吸附特性的影响规律，揭示了生物刺激煤岩孔隙结构改造机理。生物转化后煤岩整体孔隙率增加，其中大孔数量增多，中、微孔数量减少，有利于提高煤样解吸能力。2）开展生物改造煤岩基质及裂缝长时甲烷渗流实验，结合裂缝表面三维轮廓扫描，分析生化侵蚀对裂缝表面形态的影响，得到了煤基质渗透率演化及裂缝生化侵蚀气-固流动规律。生物改造煤岩基质渗透率与孔隙度的关系符合KC模型，工作液可有效清除煤粉颗粒，并与裂缝表面发生矿化作用，从而降低了裂缝的应力敏感性。3）进行了大型真三轴煤层改造模拟增产实验，结合CT扫描，得到煤层体积改造复杂裂缝形态，建立了复杂裂缝渗流-应力耦合模型，结合前述基质及裂缝生化侵蚀渗流模型，形成煤岩生物改造复杂裂缝等效渗流模型。煤岩体积改造形成的复杂裂缝对应力的敏感性低，体积改造后的煤岩渗透率较改造前提高2~3个数量级。研究结果揭示了煤层生物改造增产机理，论证了技术可行性，可为煤层生物改造产能预测及现场施工提供理论支撑。