水平井压裂过程中固井界面裂缝的扩展规律

Rule of Propagation of Fractures through the Bonding Interfaces of Cement Sheath in Horizontal Well Fracturing

页岩气水平井压裂过程中,高压流体的注入可能会引起压裂液沿水泥环一、二界面的窜流,对页岩气井井筒完整性提出了严峻挑战。基于CZM模型建立了定注入压力下水泥环一、二界面裂缝扩展模型,分析了水泥环、地层力学参数对固井界面裂缝扩展的影响规律。研究结果表明:提高固井二界面的胶结强度能同时抑制一、二界面的裂缝扩展;水泥环弹性模量由4 GPa增加至9 GPa,一、二界面扩展长度分别降低10 m、19 m,当弹性模量超过6 GPa时,其对二界面的影响减弱,泊松比由0.2增加至0.3,一、二界面裂缝长度分别降低10 m、1 m,表明泊松比主要会对一界面的裂缝扩展有所影响;地层弹性量从25 GPa增加到35 GPa,一、二界面裂缝长度分别增加3 m、9 m,表明弹性模量较低的地层有助于控制界面裂缝扩展。研究结果对水平井压裂过程中水泥环密封完整性的评价以及水泥浆体系的优选具有一定的指导意义。

In fracturing a horizontal shale gas well,the injection of a high pressure fluid into the well may cause flow of the fracturing fluid along the interface between the casing and the cement sheath and the interface between the cement sheath and the borehole wall,and this imposes a serious challenge to the borehole integrity.A model describing the extension of fractures on both sides of cement sheath under constant injection pressure was established based on the CZM model,and the effects of cement sheath,formation mechanical parameters on the extension of the fractures on the cement sheath were analyzed.The study shows that increasing the cementing strengths of the two interfaces can simultaneously inhibit the extension of the fractures on both sides of the cement sheath.When the elastic modulus of the cement sheath is increased from 4 GPa to 9 GPa,the lengths of the fracture extension on the side of the cement sheath in contact with casing string and on the side of the cement sheath in contact with borehole wall will be reduced by 10 m and 19 m,respectively.When the elastic modulus exceeds 6 MPa,its effect on fracture extension on the side of the cement sheath in contact with borehole wall is becoming weakened.When the Poisson’s ratio is increased from 0.2 to 0.3,the lengths of the fracture extension on the side of the cement sheath in contact with casing string and on the side of the cement sheath in contact with borehole wall will be reduced by 10 m and 1 m,respectively,meaning that the Poisson’s ratio mainly affects the fracture extension on the side of the cement sheath in contact with casing.On the other hand,when the elastic modulus of the formation is increased from 25 GPa to 35 GPa,the lengths of the fracture extension on the side of the cement sheath in contact with casing string and on the side of the cement sheath in contact with borehole wall will be increased by 3 m and 9 m,respectively,meaning that a formation with low elastic modulus is beneficial to the controlling of the fracture extension.The achievements made in the study is of a certain guiding importance to the evaluation of the sealing integrity of the cement sheath in fracturing a horizontal well,and to the optimization of a cement slurry system.