考虑井筒加载历史的压裂过程中套管剪切变形数值模拟研究

Numerical study of shear deformation of casings during hydraulic fracturing considering wellbore loading history

体积压裂技术作为致密油气开发的重要技术手段,在实现储层有效改造的同时,也可能带来套管变形失效的问题,其主要失效原因之一是水力压裂诱发的断层激活和界面滑移造成套管剪切变形和破坏。本文首先通过研究压裂液进入断层的不同模式,阐释了体积压裂诱发断层激活理论,为分析断层滑移量对套管变形的影响提供了理论依据。其次,本文建立了考虑水力压裂过程中断层滑动引起套管剪切变形的新模型,模型创新性地考虑了井筒全生命周期加载历史(钻井、下套管、注水泥、水泥浆固化、水力压裂)对套管变形的影响。最后,利用建立的模型,量化分析了不同工程和地质条件下(压裂液压力、断层倾角、裂缝长度、套管壁厚和水泥环弹性模量),断层滑移后套管内径的变化规律。结果表明,断层的滑动会对套管产生剪切作用从而产生套管缩径,导致套管发生屈服破坏。随着断层长度的增加,套管剪切变形程度也随之增大,当断层长度为80 m时,套管的缩径量达到11.95 mm。随着压裂液压力的不断增大,套管剪切变形程度也随之增大,当流体压力为80 MPa时,套管缩径量最大达到了9.94 mm。断层倾角在30°左右时,套管缩径量最大可达9.53 mm。增大套管的壁厚和水泥环的弹性模量,对套管缩径量的改善效果并不明显。因此,在井眼轨迹设计的过程中应尽量避免井眼穿越断层,压裂设计时应避开大尺寸断层区域;同时,应该合理设计压裂液的泵入速度,尽可能使其保持在较低的值,以降低流体压力。该研究所建立的考虑井筒加载历史的数值模型可以更加全面地解释页岩气井多阶段压裂过程中断层滑动后套管剪切变形情况,为工程作业提供指导意义。

The volume fracturing technique is one of the key methods of shale gas development.It,can cause the problems of casing deformation while facilitating shale gas development.The main reasons for the casing deformation are fault activation and interface slippage induced by hydraulic fracturing.Firstly,this paper explains the mechanism of fault activation induced by hydraulic fracturing by analyzing the different modes of fracturing fluid entering the fault,which provides a theoretical basis for analyzing the influence of fault activation on casing deformation.Secondly,a new model considering the shear deformation of casing caused by fault activation and interrupted layer slippage during the hydraulic fracturing is established.The model innovatively considers casing loading history within the full life cycle of wells,including drilling,casing running,cementing,cement slurry hardening,hydraulic fracturing,production,and injection.Finally,a parametric study is performed to determine the effects of fracturing fluid pressure,fault dip,fracture length,casing thickness,and cement sheath property on shear deformation of casing.The results show that fault sliding will produce a shear force acting on the casing system which will result in a reduction in the casing diameter.Eventually,it may lead to the shear failure of the casing.As the length of the fault increases,the degree of casing shear deformation also increases.When the length of the fault is 80m,the shrinkage of the casing reaches 11.95 mm.With a continuous increase of fracturing fluid pressure,the degree of shear deformation of the casing also increases.When the fluid pressure is 80 MPa,the maximum casing diameter reduction reaches 9.94mm.When the fault dip is about 30°,the casing diameter shrinks the most,up to 9.53 mm.However,increasing the casing thickness and the elastic modulus of the cement sheath does not significantly improve the shrinkage of the casing.Therefore,well trajectory should be prevented from crossing large faults during well structure designing,and large-scale fault areas should be avoided during hydraulic fracturing design.Meanwhile,the pumping speed of fracturing fluid should be reasonably designed to keep it as low as possible to reduce fluid pressure.This work is helpful for understanding the mechanism of shear deformation of casings induced by fault activation and interrupted layer slippage during multi-stage hydraulic fracturing in shale gas,which also provides guidance for engineering operations.