页岩气井压裂液返排对储层裂缝的损害机理

Mechanism of fracture damage induced by fracturing fluid flowback in shale gas reservoirs

为了弄清压裂液返排过程中对页岩气储层裂缝的损害机理,选取四川盆地长宁区块下志留统龙马溪组页岩和压裂返排液,利用压裂返排液对造缝岩样开展压裂液返排和气驱压裂液实验,监测压裂液返排流动阶段的岩样液相渗透率、返排液固相粒度分布和浊度变化,对比压裂液气驱前后的气测渗透率,分析压裂返排液对页岩气储层中裂缝的损害机理与损害程度。研究结果表明:(1)压裂返排液作用后,页岩渗透率损害率介于53.1%～97.6%,返排液固相粒度区间显著缩小,液相滞留所造成的相圈闭损害、固相残渣堵塞、气相携液诱发微粒运移和盐结晶是其主要的损害方式;(2)气相流阶段,渗透率损害率降至23.1%～80.2%,滞留液相损害有所缓解,但固相残渣堵塞和返排液在裂缝面的盐结晶损害仍然难以避免;(3)基于页岩气井压裂液返排过程中对裂缝的损害机理,考虑到返排液的处理难度及其对储层裂缝的损害,建议应积极发挥压裂液的造缝能力,优化压裂液性质与用量,尽量做到不返排或少返排压裂液。

In this paper,the Lower Silurian Longmaxi shale samples and the backflow fracking fluid in the Changning Block of the Sichuan Basin were selected to investigate the damage mechanism of retained fracking fluid to fractures in shale gas reservoirs.Thus,experiments were conducted on fracking fluid backflow and gas-driving fracking fluids.The changes of liquid permeability of shale samples,solid particle size distribution and turbidity of the backflow fraking fluid were monitored.The gas permeability before and after fracking fluid gas drive was compared,and the damage degree and mechanism of the backflow fracking fluid to the fractures in shale samples were analyzed.And the following research results were obtained.First,the damage rate of shale permeability after the fracking fluid backflow is between53.1%and97.6%,and the range of the solid particle size of the flowback fluid is significantly reduced.The main reservoir damage modes include phase trapping damage caused by liquid phase retention,blockage caused by the solid phase residue,particle migration induced by gas-carrying liquid and salt precipitation.Second,in the stage of gas phase flow,the damage rate of permeability drops to23.1–80.2%,and the damage caused by liquid phase retention is relieved,but the damage caused by the blockage of solid phase residue and the salt precipitation of flowback on the facture surface is inevitable.Third,based on the damage mechanism of fracking fluid backflow in shale gas wells to fractures,considering the treatment difficulty of the flowback and its damage to reservoir fractures,it is recommended to give a full play to the fracturing capacity of fracking fluid and optimize the properties and dosages of fracking fluid so as to reduce the flowback of fracking fluid as much as possible.