注液速率及压裂液黏度对煤层水力裂缝形态的影响

注液速率及压裂液黏度是煤层气井压裂设计中两个重要的可控参数,其不仅影响水力裂缝起裂压力及压裂施工压力,而且控制水力裂缝形态。采用鄂尔多斯盆地东南缘大宁-吉县地区天然煤岩,基于试验室物理模拟试验研究注液速率及压裂液黏度对水力裂缝形态及施工压力的影响。结果表明:注液速率及压裂液黏度较小时,主裂缝与分支缝连通形成沿最大水平主应力方向的复杂裂缝网络系统;随着注液速率及压裂液黏度的增加,水力裂缝复杂程度降低,形成平直单裂缝。提高注液速率或压裂液黏度会增大施工压力。对注液速率及压裂液黏度进行合理控制,可先在井筒附近生成平直裂缝,后在远离井筒处生成复杂裂缝网络,有利于增大煤层气单井排采体积。

Influence of injection rate and fracturing fluid viscosity on hydraulic fracture geometry in coal

Injection rate and fracturing fluid viscosity are two important controllable parameters in the fracturing design of coalbed methane (CBM) well. They not only are the factors that affect the breakdown pressure of the borehole and treatment pressure during the fracturing treatment, but also can control the geometry of hydraulic fracture. Through several groups of large scale true tri-axial tests, influences of injection rate and fracturing fluid viscosity on hydraulic fracture geometry and treatment pressure in coal specimens collected from Daning-Jixian coal field, southeast of Ordos Basin were studied. The results show that low injection rate and low fracturing fluid viscosity reopen the coal cleats, and branch joints connect with the main fractures to form fracture networks with a dominating direction along the direction of maximum horizontal stress. With injection rate or fracturing fluid viscosity increasing, hydraulic fracture complexity is greatly reduced and a planar fracture tends to form. Increasing injection rate or fracturing fluid viscosity will also increase the treatment pressure. Optimal design of the injection rate and fracturing fluid viscosity can enhance the drainage volume of single CBM well by generating a complex fracture network at a certain distance from the wellbore after the creation of a planar fracture near the wellbore.