Study On The Mechanism Of Casing Damage During The Backflow Of Fracturing Fluid For Shale Gas Wells

Shale beddings are not only highly developed but also features a high heterogeneity.Many fractures with diverse patterns are generated near the wellbore during hydraulic backflow.The induced stress by these fractures leads to changes of crustal stress within shale formations in both its direction and the magnitude.The changed crustal stress are totally posed on the casing and the cement mantle around,which causes plastic deformation and consequently casing damage.Casing damage is a long-lasting issue during the backflow of fracturing fluid in shale formations.A new method needs to be proposed to accurately calculate the stress distribution and predict casing damage for large scale fracturing operations of shale reservoir.Many factors including the seepage of fracturing fluid,rock temperature around the wellbore and the induced stress of hydraulic fractures are considered to analyze the stress distribution of casing during hydraulic backflow.By the superposition theorem,a calculation model is established to calculate the stress distribution around the wellbore for a fractured shale gas well.Based on the stress calculation model around the wellbore and Elastic Mechanics,the mechanical state around the perforations are analyzed to obtain the stress calculation model for a single perforated hole on the casing during the backflow of fracturing fluid.Factors including the cement mantle and casing ellipticity are taken into consideration to establish the theoretical calculation model of the collapsing stress of casing during hydraulic backflow based on the principle of surface strain in Elastic Mechanics.Core rock mechanic experiments are firstly conducted to analyze the stress imposed on casing during hydraulic backflow.Rock properties including uniaxial and triaxial tensile strength,elastic modulus and Poisson's ratio are obtained under different coring directions.The heterogeneity of rock mechanical properties is also analyzed.A finite element calculation model of the combination of shale-cement-casing for a fracture-type shale formation during hydraulic backflow is established based on the experiment results.The simulation results give that casing stress is noticeably affected by the difference in Young's modulus and Poisson's ratio between the fractured and the adjacent shale formations: when the Young's modulus and Poisson's ratio of the fractured section increase,the Mises stress around the perforations decreases;when the Young's modulus and Poisson's ratio of the adjacent sections decrease,the Mises stress around the perforations increase.The research of this paper provides a new train of thoughts to calculate casing stress for large scale hydraulic backflow of shale and a reference for the prediction of casing instability and the formulation of reliable fracturing scheme.