Research On The Characteristics Of Fault Slip And Its Influence On Casing Shear Deformation During Shale Gas Fracturing

The transformation of the global energy structure has entered a critical period.As a clean and low-carbon fossil energy,natural gas will gradually replace traditional highcarbon fossil energy,of which shale gas is expected to become the core growth point in our country.Our country is rich in shale gas resources,its development prospect is broad.However,due to the complex occurrence environment of shale gas in our country,hydraulic fracturing often induces fault slippage,resulting in a particularly prominent problem of casing shear deformation,which seriously restricts the efficient development of shale gas.Therefore,this paper analyzes the characteristics of fault slip during the fracturing,and based on this,the risk assessment mechanism of fault instability is studied.Finally,the calculation model of the fault slip and the casing shear model are established,the influence law of the fault slip on the casing shear deformation is clarified,and prevention and control measures are proposed.The main research results are as follows:(1)Influence of hydraulic fracturing parameters on fault slip stateThe basic characteristics of fault slip are analyzed,and it is clarified that the fault stick-slip needs to meet the strength condition,velocity friction state condition and stiffness condition.On this basis,a two-dimensional model of fault slip that only considers the stress field triggered by the hydraulic fracture tip is established,and the effects of fracturing parameters on fault slip under different slip states are studied.The results show that proper reduction of pump pressure can help to reduce the scale of fault slip,thereby alleviating the shearing effect of the fault on the casing.(2)Risk assessment mechanism of fault instability during fracturingBased on the Moore-Coulomb criterion,a slip risk assessment model suitable for any fault is established,which considers the in-situ stress changes caused by fracturing and the paths the fracturing fluid enters the fault.Furthermore,deterministic parameters are used to predict the accurate probability of fault slip,and Monte Carlo method is used to calculate the probability accumulation curve of fault slip,thus establishing a complete fault instability risk assessment mechanism.(3)Fault slip calculation model and casing shear modelBased on the multi-triggering mechanism of faults,a three-dimensional fault slip calculation model is established using the Moore-Coulomb criterion.Furthermore,a finite element model of casing shear deformation is established,and the influence of different fault parameters on casing deformation is indirectly studied by taking the fault slip as the boundary condition.The results show that the designed wellbore trajectory should try to pass through the formation with low and uniform stress field,and try not to pass through large faults and faults that are vertically parallel to the maximum horizontal stress field.