Study On Casing Failure Mechanism In Volume Fracturing Process

Stimulated reservoir volume (SRV) technique has effectively helped develop unconventional oil and gas reservoirs in recent years. This technique successfully breaks the traditional fracturing seepage theory model and greatly shortens the effective seepage distance and is especially applicable to the reconstruction of high brittleness rock layer. Meanwhile, a new problem of casing deformation failure has occurred during multi-stage hydraulic fracturing of SRV. However, studies on casing failure in hydraulic fracturing of SRV have not been lucubrated. The form of casing failure is not clear. Therefore, to carry out the casing failure mechanism researches in hydraulic fracturing of SRV is very meaningful for solving the casing failure problem and is fairly difficult.Massive related literature materials were collected included casing failure during multi-stage hydraulic fracturing of SRV. Through analyzing these materials, research technique and clue on casing failure was mastered. Next, based on analyzing31cases of casing failure in12wells in Weiyuan, Changning, Qiulin, Jinhua and Longqian oil and gas fields, carried out simulation study on SRV including vertical and horizontal well, on the basis of micro-seismic monitoring data. And put forward some measures and suggestion to preventing the failure of casing in hydraulic fracturing of SRV. The main innovation and results of this dissertation are as follows:(1) The constructed horizontal well multi-cluster SRV finite element model is based on the rock damage mechanics and fracture area data from micro-seismic monitoring, and the FE model can be applied to effectively and quantitatively study the axial bending deflection and radial ellipse deflection of the casing in SRV process.(2) After FE numerical study of the third-to-sixth staged fracturing, it can be known that there will be the "tensile stress" areas and "zero stress" areas in the fracture areas after sixth staged volume fracturing. Although the existence of tensile stress areas is favorable for increasing fracture width and flow conductivity, the "stress deficit" areas will make casing remain in suspending state and lead to radial deformation.(3) Some areas receives refracturing treatment, leading the formation rock mechanical properties to decrease, the in-situ stress field non-uniform degree and stress deficit areas to increase, the shear effect to increase, and the radial ellipse deformation and axial S-shaped deformation of casing to increase at the same time. All these changes will result in casing deformation failure.(4) From the great deal of the FE numerical simulating studies, the style of casing failure in vertical well is oval deformation, and the horizontal well mainly resulted from axial S-shaped deformation of casing that prevents the access of running tools.(5) By contrast with the field investigation, the FE results showing in this paper are the same to the actual working condition. Therefore, the current method and FEM are correct.(6) The methods and achievements in the paper provide theoretical supports for the popularization and application of shale stimulated reservoir volume and controlling the casing in hydraulic fracturing of SRV.