Variation Of Stress Field During Hydraulic Fracturing And Its Influence On Casing Deformation

Casing deformation frequently occurs during unconventional oil and gas development in China.Taking the shale gas development zone in Sichuan Basin as an example,by April 2019 the overall casing deformation rate of the Weiyuan block,Changning block,and Zhaotong block is 57.7%,31.9%,and 22.5%,respectively.Casing deformation has seriously affected the fracturing operation and subsequent production operation.At present,some studies have been carried out,but the problem of casing deformation is still unsolved.The mechanism of casing deformation and the main controlling factors affecting casing deformation are not fully understood.In particular,the following three problems cannot be well explained:(1)The mechanism of casing deformation risk differences in different blocks and wellbore locations is not clear;(2)The influence of slipping fracture system crossing wellbore on casing deformation is insufficiently understood,especially the slip mechanism of artificial fractures;(3)The mechanism of casing deformation caused by hydraulic fracturing is not completely clear.Based on the analysis of casing deformation characteristics and existing casing deformation literature,the casing deformation mechanism and effective control measures were studied from the perspective of the variation of stress field during hydraulic fracturing.The main research results are as follows:(1)The combination of regional tectonic stress,local small structure,and high intensity hydraulic fracturing leads to casing deformation.The Weiyuan,Changning and Zhaotong blocks in Sichuan Basin have strong tectonic stress,large horizontal stress difference(approximately 20MPa),developed natural fractures,high hydraulic fracturing intensity,and therefore high casing deformation rate.The casing deformation is mainly shear and extrusion.Based on the focal mechanism of natural earthquakes in shale gas production areas,the regional stress field was inverted,which was used to establish the regional casing deformation risk assessment model.The model classifies the casing deformation risk into five levels: extremely high,high,medium,low,and extremely low.The model evaluation results show that the casing deformation risk is high level in W202 and W204 regions of the Weiyuan block,and medium level in N201 and N209 regions of the Changning block.(2)The variation of stress field induced by hydraulic fracturing was studied by numerical simulation,and the method for characterizing the non-uniform effect of stress field was established by using the change of Coulomb failure stress.In addition,the mechanism of stress accumulation-release along the wellbore was revealed during hydraulic fracturing.The results show that the uniformity of stress field is increased in the larger area around the hydraulic fracture,and the interactive fracturing of platform well significantly increases the uniformity of stress field between wells.Hydraulic fracturing causes stress to accumulate-release continuously along the wellbore,and the stress accumulates a large value in the heel and middle of the horizontal well.Stress accumulation leads to extrusion deformation of casing under large non-uniform external extrusion load,and makes casing deformation risk in the middle and heel of horizontal wellbore higher under the same conditions.(3)The slip behavior of fracture system was analyzed by using the Coulomb criterion and fracture mechanics theory.The slip mechanism of artificial fracture was studied,and the slip distance of finite scale fracture was calculated.The results show that the slip of the fracture system crossing the wellbore is an important reason for the shear deformation of casing.The increase of the Coulomb failure stress of the fracture leads to the slip,and the slip distance is closely related to the length and height of the fracture,the fluid pressure in the fracture,and the fracture angle.There is a "stable slip distance"(about 70mm)in the fracture.The induced stress field of hydraulic fracturing and the decrease of friction coefficient of propped fracture lead to artificial fracture slip.The larger the length and height of artificial fracture is and the larger the stress deflection angle is,the larger the artificial fracture slip becomes.(4)The coupling effect of "induced stress-fracture-casing" is likely the core mechanism of casing deformation,therefore casing deformation can be controlled by adjusting hydraulic fracturing intensity and operation mode.For the casing deformation risk points such as faults,natural fractures and bedding,the risk of casing deformation can be reduced by optimizing the perforation position and number of perforation clusters,as well as reducing the fracturing fluid volume,sand volume,construction displacement and net pressure,and taking temporary plugging measures.For platform wells,it is recommended to adopt the fracturing method of "inter-well interlaced fracture distribution mode,zipper interactive fracturing,and overall front parallel fracturing" to actively control the stress field through inter-fracture and inter-well stress interference,so as to achieve "balanced stress fracturing".This can improve the uniformity of stress field and reduce the risk of casing deformation.In addition,the casing deformation can be evaluated by the magnitude and b value of microseismic events.Through this study,the role of induced stress field of hydraulic fracturing and fracture slip in casing deformation is recognized,and various mechanisms of casing deformation caused by horizontal well staged fracturing of high intensity are determined.The corresponding casing deformation control measures are put forward,which can provide some guidance for casing deformation prevention in the field.