Study On Fault Slip Law And Casing Deformation Characteristics Induced By Hydraulic Fracturing In Shale Reservoir

The development of unconventional shale oil and gas is of great significance to ensure China’s energy security.In the process of shale oil and gas hydraulic fracturing production,casing deformation is very prominent,which seriously restricts the efficient development of shale oil and gas in China.In recent years,the academic community has carried out a lot of research on this major technical problem,from the aspects of fluid communication activating fault shear casing,stress concentration caused by cementing quality defects,additional bending stress on casing caused by borehole dogleg degree,and casing load concentration at lithologic interface.The deformation mechanism of shale oil and gas hydraulic fracturing casing is deeply discussed,and relevant technical countermeasures are taken to reduce the casing deformation rate of unconventional oil and gas blocks.However,so far,the casing deformation rate of some shale oil and gas blocks in China is still about 30%.Therefore,it is of great significance for the efficient development of shale oil and gas in China to continue to understand the deep-seated causes of casing deformation induced by hydraulic fracturing in shale reservoirs and put forward more effective technical countermeasures for casing damage prevention and control.The previous studies,which are mainly based on the static stress field conditions and the assumption that fracturing fluid directly communicates with the fault,this paper studies the dynamic change law of the stress field in the fracture propagation area and reveals the mechanism of fault slip under the condition that fracturing fluid does not communicate with the fault.Firstly,the hydraulic fracturing fracture propagation model with distal fault is established by the extended finite element method(XFEM),and the variation law of the shear stress field in the fault area during the hydraulic fracture propagation is analyzed.The results show that the propagation of hydraulic fracture will disturb the original stress field of local formation,even if the fracturing fluid does not communicate with the distal fault.The shear stress in the local area of the fault may still exceed the friction stress in the same area,resulting in relative slip in the local fault.The influence of large-scale hydraulic fracturing operation on the relative slip of faults is related to the distance between the fracturing operation stage and the fault,the fault length,the original in-situ stress,the friction coefficient of formation rocks,and the volume of fracturing fluid.For the fault under the condition of no communication,the change of fracturing fluid volume caused by adjusting the fracturing pump rate is the main factor affecting the fault slip,while the change of fracturing pump rate has little effect on the fault slip under the condition of constant fracturing fluid volume.Furthermore,this paper establishes a multi-stage zipper fracturing fracture propagation model with remote faults and analyzes the influence of interstage interference and well interference on fault slip in the process of the fracturing operation.The results show that there is a synergistic effect of single well two-stage fracturing operation on fault slip,and the sequence of inter-well zipper fracturing operation has a great impact on the fault slip.Priority fracturing of the well stage n close to the fault can effectively reduce the fault slip(the maximum reduction is more than 50%).Therefore,reasonably designing the sequence of cross well fracturing operations is an effective means to reduce fault slip and prevent casing deformation.Finally,based on the newly constructed quantitative characterization method of casing deformation cross-section characteristics,the influence of fault strike and dip on casing section deformation morphology is systematically studied.The results show that in the block controlled by strike-slip faults,the casing sheared by low dip faults is easy to form concave casing deformation characteristics.Under the same slip amount,the influence of fault strike on casing deformation is greater than that of fault dip angle.Therefore,in the strike-slip fault block,reducing the angle between the wellbore trajectory and the fault strike is another way to control the deformation of shale oil fracturing casing.Through this study,a fracturing fracture propagation model with remote faults is established,which reveals the mechanism of local fault slip under the condition that the fracturing fluid does not communicate with the fault,combs the main control factors affecting the fault slip,and summarizes the influence of casing section deformation characteristic under different fault conditions,which has important guiding significance for further improving the unconventional oil and gas hydraulic fracturing casing deformation theory and putting forward effective prevention and control countermeasures.