热应力影响下干热岩水压致裂数值模拟

Numerical simulation of hydraulic fracturing in hot dry rocks under the influence of thermal stress

干热岩水压致裂过程中低温诱导热应力与注入水压共同影响裂缝的萌生与扩展。首先通过THM耦合分析了低温压裂液注入过程中注入水压与热应力的相互作用及其对裂缝萌生的影响,随后建立描述岩石细观结构的THMD耦合模型对热应力影响下高温岩石水压致裂过程进行初探。结果表明:低温压裂液注入高温岩石产生的热应力包括岩石自身温度梯度形成的热应力与岩石颗粒非均匀膨胀导致的热应力,并在井筒周围呈现为拉应力。高注入压力将抑制热应力导致的多裂缝萌生,井筒附近热应力的存在对注入压力也具有削弱作用。基岩温度升高,裂缝萌生阶段更多裂缝在井筒附近起裂,缝网沿最大地应力方向的扩展速度减慢,但改造规模增加,同时多裂缝的存在也使得裂缝延伸压力增加。

Crack initiation and propagation in hydraulic fracturing process of hot dry rocks is affected by thermal stress induced by the low temperature of injected cryogenic fracturing fluid as well as its injected water pressure. Firstly, the interaction between injected water pressure and thermal stress during the injection of cryogenic fracturing fluid and its effect on crack initiation are studied by THM coupling analysis. Subsequently, a THM-D coupling simulation considering the meso-structure of rock is conducted to investigate the hydraulic fracturing process of hot dry rocks under different thermal conditions. The results show that the thermal stress which is generated by both temperature gradient in the rock itself and the non-uniform expansion of rock particles, appears as tensile stress around the wellbore. The high injection pressure will prohibit the initiation of multiple cracks induced by thermal stress and the existence of thermal stress around the wellbore will weaken the injection pressure. As the rise of rock temperature, more cracks emerge near the wellbore at the crack initiation phase and the propagation velocity of fracture network along the maximum geo-stress direction decreases while the reconstruction scale enlarges. Simultaneously, the generated multiple cracks also increase the crack extension pressure.