压裂后焖井期间页岩吸水起裂扩展研究——以四川盆地长宁区块龙马溪组某平台井为例

Fracture initiation & propagation in shale due to imbibition during well shut-in after fracturing:A case study from one well platform in Longmaxi Fm of the Changning Block,Sichuan Basin

为了确定页岩气井压后的焖井时间,需要预测关井期间储层条件下页岩自吸压裂液对裂缝起裂扩展的影响。为此,基于裂缝闭合判断准则,针对四川盆地长宁区块三轴地应力测试结果,判定裂缝的闭合状态,然后基于张开裂缝的拉伸和剪切起裂准则,计算拉伸破坏和剪切破坏情况下的最大周向应力和最大有效剪切应力,并结合三轴及单轴岩石抗压力学实验和单轴巴西劈裂抗拉强度实验结果,获得裂缝起裂阈值压力,进而探讨了该区块下志留统龙马溪组页岩气井的合理焖井时间。研究结果表明:(1)该区块龙马溪组页岩气井压裂后,在焖井期间储层中的微裂缝总是保持开启的;(2)储层条件下裂缝所处的应力状态十分复杂,即使是张开裂缝,也有可能发生剪切破坏;(3)当裂缝倾角为0°时,裂缝尖端获得最大周向应力值,对应裂缝起裂角为0°,即裂缝会沿着最大水平主应力方向起裂延伸,当缝内流体压力低于70 MPa后,裂缝不会再产生拉伸起裂;(4)当裂缝倾角为0°时,对应最大有效剪切应力,此时裂缝起裂角为55°,当缝内流体压力低于60 MPa后,裂缝不再产生剪切起裂;(5)若要充分发挥页岩气井压裂后焖井期间压裂液的改造作用,该区块龙马溪组气井压裂后的合理焖井时间为5～10天。

In order to determine the soak time of shale gas wells after fracturing,it is necessary to predict the influence of shale's fracturing fluid imbibition capacity on the fracture initiation and propagation under reservoir conditions during well shut-in.In this paper,according to the fracture closure criterion,the closure state of fracture was analyzed based on the triaxial stress test results of Changning Block.Then,the maximum circumferential stress and the maximum effective shear stress under tensile failure and shear failure were calculated according to the tensile and shear initiation criteria of open fracture.And combined with the results of the triaxial and uniaxial compressive strength tests and the uniaxial Brazilian splitting tensile strength test,the threshold pressure of fracture initiation was obtained.Finally,the reasonable soak time of shale gas wells in the Longmaxi Fm of Lower Silurian in Changning Block,Sichuan Basin was discussed.And the following research results were obtained.First,after the fracturing of shale gas well in the Longmaxi Fm in Changning Block,the micro-fractures in the reservoir remain open during well shut in.Second,the stress state of the fractures under reservoir conditions is very complicated,and even the open fractures may suffer shear failure.Third,when the fracture dip is 0°,the maximum circumferential stress value is obtained at the fracture tip,and the corresponding fracture initiation angle is 0°,which means that fracture initiation and propagation will be along the direction of the maximum horizontal principal stress,and that when the fluid pressure inside the fracture is lower than 70 MPa,tensile fracturing will not happen any longer.Fourth,when the fracture dip is 0°,the corresponding maximum effective shear stress is,obtained and the fracture initiation angle is 55°.When the fluid pressure inside the fracture is lower than 60 MPa,shear fracturing will not occur any longer.Fifth,if full play should be given to the stimulation effect of fracturing fluid during well shut-in after fracturing,the reasonable soak time after fracturing in the Longmaxi Fm of the Changning Block shall be 5-10 days.