Study On The Casing Deformation And The Cement Sheath Sealing Failure Mechanism During The Shale Well Volume Fracturing Process

Shale gas reservoirs are characterized by low porosity and low matrix permeability.Volume fracturing technique is usually used to increase shale production.During the shale gas volume fracturing process in China,the well integrity failure occurred in many wells.First,the casing was damaged and deformed.As a result,the fracturing tools could not enter.After the completion of the stimulation,the bridge plug could not be drilled.Second,the failure of the sealing of the cement sheath leads to sustained annular pressure.The shale gas well integrity failure has seriously affected the development of shale gas in China.In this paper,the shale gas horizontal well casing-cement sheath-formation system(CCFS)is taken as the research object.The law of the mechanical behavior of the CCFS during the volume fracturing process is analyzed,the shale gas well integrity failure mechanism is defined.The research results can provide references for the improvement of well integrity control methods during the shale gas volume fracturing.The following achievements have been achieved:Aiming at the dynamic changes of formation mechanics properties,well temperature field and stress field during the shale gas volume fracturing process,based on sequential coupling method,the temperature stress generated during the transient heat transfer process and the stress generated during the fracturing process are superimposed,and a new mechanical model of the shale gas volume fracturing well system is established.This model takes into account the initial stress and strain of the cement sheath before hydraulic fracturing,and overcomes the inadequacy of the conventional model without considering the change of the initial state of stress in the cement sheath.Based on the new mechanics model of CCFS,the influence of the formation properties and cementing quality on the casing stress during the fracturing process is analyzed,and the results are compared with the conventional model.The results show that during the fracturing process,the combined effect of cement sheath defect and the decrease of formation elastic modulus will increase the risk of casing deformation;the area where the elastic modulus of the formation is reduced less,the high-pressure bound fluid in the voids of the cement sheath can easily cause the casing to collapse.Aiming at the characteristics of shale reservoir natural fractures/fault development and large stages of volume fracturing,a 3D formation slip model of shale gas volume fracturing is established.The influence of factors such as the trend of fault sliding interface,the number of stimulation stages,the degree of stimulation,and the asymmetrical stimulation of reservoirs on the stress distribution of the casing in the long horizontal section was analyzed.The results show that increasing the distance between the fracturing position and the sliding interface and reducing the stimulation volume of the reservoir at the sliding interface can effectively reduce the probability of shearing failure of the casing.The mechanism of cement sheath seal failure during volume fracturing was studied.First,based on the new mechanics model of CCFS,the influence of the formation properties and the reduction of the initial stress in the cement sheath during the fracturing process is analyzed.Then,Using the non thickness contact element with the penalty stiffness Mohr-Coulomb model,a new numerical model for the deformation of the CCFS considering the presence of micro-gap at the cementing interface is established,and the evolution process of the micro gap is analyzed,which overcomes the shortcomings of the conventional model which can not analyze the formation properties and the temperature change on the micro gap.Finally,based on the cohesive pore pressure element,a finite element calculation model for fracture growth at the cement interface is established.The study shows that the temperature drop in the well,the decrease of the geo stress and the reduction of the initial stress in the cement sheath are the three main factors that lead to the micro gap during the fracturing.