页岩储层体积压裂的地应力变化研究

页岩储层属于致密超低渗透储层,需改造形成复杂缝网才有经济产能.体积压裂是页岩储层增产改造的主要措施,而地应力场特别是水平主应力差值是体积压裂的关键控制因素. 理论研究表明:(1)当初始两向水平主应力差较小时,容易形成缝网,反之不易产生缝网;(2)人工裂缝的形成能够改变地层初始应力场. 因此应在前人研究的基础上优化设计压裂方式,以克服和翻转初始水平主应力差值,产生体积缝网.基于此,建立了页岩气藏水平井体积压裂数值模型,模型中采用多孔介质流固耦合单元模拟页岩基质的行为,采用带有孔压的"cohesive"单元描述水力裂缝的性质,模型对"Texas Two-Step" 压裂方法进行了数值模拟,模拟结果得到了压裂过程中地层应力场的分布及其变化,模拟结果和解析公式计算结果吻合良好.模拟结果表明:(1)裂缝的产生减弱了地层应力场的各向异性;(2 对于低水平应力差页岩储层,采用"Texas Two-Step"压裂方法可以产生缝网. 对于采用"Texas Two-Step"压裂方法无法产生缝网的高应力差页岩储层,提出了三次应力"共振" 和四次应力"共振" 压裂方法并进行了数值模拟,模拟结果得到了压裂过程中页岩储层应力场的分布及其变化,得到了缝网形成的区域,模拟结果表明:(1)对于高应力差页岩储层,采用"Texas Two-Step" 压裂方法无法产生缝网;(2)对于高应力差页岩储层,三次应力"共振" 和四次应力"共振"压裂方法是有效的体积压裂缝网形成的方法.

RESEARCH ON THE GEOSTRESS CHANGE OF SHALE RESERVOIR VOLUME FRACTURING

Shale reservoirs have ultra-low porosity and permeability. Without creating complex fracture network, there will be no economical production. Volume fracturing is the primary treatment for shale reservoirs stimulation. The geostress and the di erential stress between the two horizontal principal stresses control the creation of fracture networks. Previous researches have shown that: (1) The small di erential stress between the two horizontal principal stresses eases the development of fracture network and vice versa; (2) The reservoirs geostress can be changed by the creation of hydraulic fractures. The numerical model about volume fracturing in shale gas reservoirs was constructed in which the pore pressure elements were used to simulate the behavior of porous media and the pore pressure cohesive elements were adopted to catch the characters of hydraulic fractures. The Texas Two-Step fracturing method was simulated in the model. The reservoirs stress distribution and change during the process of fracturing were obtained. The simulation results and the theoretical calculation are fit very well. The results of the simulation show that: (1) The generation of the hydraulic fractures reduces the stresses anisotropy; (2) The complex fracture networks could be created by means of the Texas Two-Step fracturing method. For the situation that the initial difference of the two horizontal principal stresses is large and the fracture networks could not be created by adopting the Texas Two-Step fracturing method, the triple and quartic stress "resonance" fracturing model were proposed. The reservoirs stress distribution and change during the process of fracturing were obtained. The area of fracture network was marked. The simulation results show that the triple and quartic stress "resonance" fracturing methods are e ective methods for the high stresses difference shale gas reservoirs.