注气过程中煤层裂纹扩展速度研究

Research on crack propagation speed in coal seam in the process of gas injection

陈德飞 康毅力 孟祥娟

CHEN Defei KANG Yili MENG Xiangjuan

中国石油塔里木油田分公司油气工程研究院，新疆库尔勒　841000 西南石油大学油气藏地质及开发工程国家重点实验室，四川成都　610500

Research Institute of Oil and Gas Engineering, Tarim Oilfield Company, CNPC, Korla 841000, China State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China

煤层中通常会通过注N2、CO2 提高煤层气采收率或实现CO2 埋存，注入的气体将使煤层中裂纹发生扩展，诱发煤层失稳。以宁武盆地9 号煤层为研究对象，开展等温吸附实验定量研究了煤岩对N2、CO2 的吸附量，并开展三轴压缩力学实验定量研究了N2、CO2 吸附对煤岩力学强度的影响；分析了煤岩中裂纹扩展机理，根据断裂力学理论推导出含气煤岩中的裂纹扩展速度方程，并根据方程计算出注不同气体时煤岩中的裂纹扩展速度。研究表明，在相同平衡压力下，煤岩对CO2 的吸附量是CH4 的6 倍；煤岩饱和CO2 后的力学强度明显低于饱和N2 的力学强度；煤层注CO2 比注N2 引起的裂纹扩展速度更大，并且注气压力越高扩展速度越大。该理论成果能够为优化注气的比例及注气压力提供理论指导，并且对防止煤层失稳，保障顺利注气具有重要意义。

N2 or CO2 is commonly injected into coal seam to enhance CBM recovery or realize CO2 storage. The injected gas will make crack growth occur in coal seam, which can induce instability. Taking the No. 9 coal seam in Ningwu basin as the research object, firstly, the isothermal adsorption experiment was carried out to study the adsorption capacity of coal rock for N2 and CO2 quantitatively and tri-axial compression mechanics experiment was carried out to study the influence of N2 and CO2 adsorption on the mechanical strength of coal rock. The mechanism of crack propagation was analyzed and the crack propagation speed equation of coal seam containing gas was deduced according to the theory of fracture mechanics and, according to the equation, the crack propagation speed was calculated when different gases were injected. Studies show that the adsorption of coal rock for CO2 is six times larger than CH4 under the same equilibrium pressure, the mechanical strength of coal rock saturated by CO2 is significantly lower than that of N2, The crack propagation speed of coal with CO2 injection is much bigger than that of N2, and the greater the gas injection pressure is, the higher the propagation speed will be. This theoretical result can not only provide a guidance for optimizing the proportion of gas injection and gas injection pressure, but also is of great significance to prevent the instability of coal bed and ensure gas injection smoothly.

10.13639/j.odpt.2015.03.018