重复压裂气井热-固耦合应力场拟三维模型

A Pseudo-three-dimensional Model for Thermoelastic Stress Field around a Re-fractured Gas Well

初次人工裂缝周围的应力场变化是影响重复压裂气井裂缝转向的关键因素。为研究人工裂缝和低温液体热—固耦合作用下应力场的变化情况,采用位移不连续法(DDM)与热源函数理论,考虑远场地应力作用、缝内压力非均匀分布及热效应,建立了重复压裂气井热—固耦合拟三维模型并得到其半解析解,可用于重复压裂气井的裂缝扩展模拟,计算结果明确了压裂液注入后裂缝周围应力场的分布及参数的影响情况。模拟计算表明,垂直裂缝面方向最小水平主应力增加值与增加范围均大于最大水平主应力,使应力转向区主要分布在裂缝壁面附近,有利于应力场发生转向,但同时该区域的破裂压力也相应增加。压裂液的注入在裂缝周围产生了很小的张性应力,对新裂缝的起裂与转向帮助不大,应力重定向的关键影响因素是缝内净压力。因此,封堵初次裂缝尖端不仅能阻止新裂缝沿原裂缝方向延伸还能提高缝内净压力,为重复压裂气井的定向射孔及缝内暂堵技术提供了理论依据。

Stress field around a re-fractured gas well is the major factor that leads to the reorientation of hydraulic fractures.Considering in-situ stress,non-uniform distribution of net pressure and the thermal effect,apseudo-three-dimensional model is developed via displacement discontinuity method and heat source function to analyze the changes of thermoelastic stress field around a re-fractured well,which can be used to simulate the fracture propagation.This paper clarifies the thermoelastic stress distribution around the primary fracture and elaborates the major influence factors.It implies that the increment of the minimum horizontal principal stress and its controlled area are both larger than that of maximum horizontal principal stress,resulting in stress-reverse region that emerges around primary fracture surfaces which is beneficial to fracture reorientation while the failure pressure increases at the same time.Furthermore,the simulation results show that only very small stresses can be induced by the cold-water injection and it plays a limited role in fracture re-orientation and propagation,which means that the thermal stresses induced by the cold-water can be ignored in the study of refracturing technology.It is also found that the net pressure is critical for stress redistribution.Therefore,temporary plugging technology applied in the crack tips can not only prevent the secondary fracture propagation along the primary fracture,but also increase the net pressure in the fracture.This paper provides a theoretical foundation for oriented perforating and temporary plugging technology in re-fractured gas well.