煤层注液态CO_2压裂增透过程及裂隙扩展特征试验

针对我国煤层"高储低渗"的赋存特征,基于液态CO_2黏度低、阻力小、酸化解堵及相变增压的特点,采用液态CO_2进行煤层压裂增透现场试验,研究煤层压裂过程裂隙扩展规律。结果表明:压裂过程流量随着压力的增长呈现出"波动"特性,钻孔周围煤体受CO_2压力作用产生裂隙并向前延伸,压裂初期裂隙扩展速度较快,随后逐渐减小;压裂裂隙同时沿压裂孔轴向和径向扩展;压力及流量曲线在1.8,2.2 MPa处出现拐点,压裂过程分为3个阶段,各阶段煤体破坏形式依次为钻孔破碎区裂隙起裂—弱面扩展—微孔隙破坏。现场压裂试验结果表明压裂半径可达10~20 m,且与水力压裂相比在压裂安全性、时间和效果方面存在技术优势。

Experiment on the liquid CO2 fracturing process for increasing permeability and the characteristics of crack propagation in coal seam

In terms of low permeability in China coal seams,based on the fact that liquid CO2 has the characteristics of low viscosity,low resistance,acidification-dispel and phase supercharge,the authors performed a field experiment of hydraulic CO2 fracturing for increasing coal seam permeability,and studied the crack developing process.The results show that the flow curve presents the characteristics of fluctuation with the pressure increase.Under the action of CO2 pressure,some cracks appear and extend forward gradually around the borehole.The speed of cracks extension is fast at the initial phase of the fracturing,and then decreases gradually.Cracks extend both in axial and radial directions along the fracture hole.There are two turning points of pressure curve at 1.8 MPa and 2.2 MPa.The fracturing process is divided into three stages,and the coal damage forms at each stage,including initial rupture in broken zone around borehole,weak plane extending and micro pore damage during fracturing process respectively.The results of testing pressure and temperature show that the fracture radius can reach 10 meters,so it is possible to increase coal seam permeability by applying liquid CO2 fracturing.