Research On Failure Mechanism And Control Method Of Casing Shear Deformation In Deep Shale Gas Wells

Deep shale gas reserves are abundant in China,and its safe and efficient development is of great strategic significance for improving the energy supply pattern and optimizing the energy industry structure.However,the shear deformation of casing in the process of large-scale volume fracturing of deep shale gas horizontal wells seriously threatens wellbore safety and hinders the efficient development of shale gas.The fault slip induced by fracturing is an important factor leading to casing deformation,but the connection mechanism between the three is still unclear,and the casing deformation control method needs to be further improved.Therefore,this paper deeply studies the casing shear deformation mechanism in the fracturing process of deep shale gas wells,and provides theoretical support for the casing deformation control method.The main research results are as follows:1.Characteristics of deep shale reservoirs and stress analysis of wellbore assemblyBased on triaxial rock mechanics experiments,the mechanical properties of deep shale with high plasticity and brittleness were determined.Considering the extrusion effect of the expansive cement on the casing,an experiment was carried out to monitor the casing stress during the setting process of the cement slurry,and the initial additional stress of the casing after the cement was cured was quantified.Combined with the lithological characteristics of deep shale,a casing-cement sheath composite body mechanics model suitable for deep shale gas wells is established,which makes up for the lack of considering the additional stress of casing.The calculation results show that the additional value of casing stress in deep shale gas wells is higher than that in conventional wells.After further considering the characteristics of deep shale,the radial stress and circumferential stress of the casing outer wall increase significantly,and the risk of casing deformation and failure increases.2.Analysis on the mechanism of fault slip during fracturing of deep shale gas wellsBased on the boundary element method,a calculation model of hydraulic fracture propagation under deep shale conditions was established,and the propagation law of hydraulic fractures in deep shale gas wells was revealed.Hydraulic fractures directly communicate with faults,or do not communicate but cause changes in formation pressure,which can trigger fault activation.Based on the theory of fracture mechanics,and considering the high plasticity characteristics of deep shale,a calculation model of fault slip when hydraulic fractures trigger fault activation is established.The calculation results show that the slippage of the fault increases with the increase of the net hydraulic pressure and the length of the hydraulic fracture.Reasonable control of the fracturing pump displacement and the length of the hydraulic fracture is an important way to reduce the risk of fault slippage.Based on this,taking Weirong deep shale gas area as an example,the fracturing parameter control method is proposed from the perspective of casing protection,and the optimization chart of pump pressure and displacement under different total liquid volumes in the section is drawn.3.Research on casing deformation law under fault slip conditionBased on the full-scale casing shear test,the radial deformation characteristic curve of casing under shear load was clarified,and the calculation formula of casing shear bearing capacity was established.Under the pure shear state,the casing is most prone to shear failure,and increasing the steel grade and wall thickness can improve its shear resistance.Furthermore,a numerical model of coupled casing shear stress-strain characteristic is established,and the casing deformation under fault slip is calculated.The research shows that there is a positive correlation between the slip of the fault and the deformation of the casing;when the angle between the fault and the axial direction of the wellbore is 90°,the deformation of the casing reaches the maximum value.Under the condition of the same slip,the casing stress and deformation of the low-diameterthickness ratio are smaller.Under the premise that the fracturing tool is feasible,it is recommended to use a small-sized and large-wall thick casing to alleviate the casing deformation problem.4.Casing deformation prevention and control methods for deep shale gas wellsBased on the cohesive cohesive element method,a finite element model of in-situ stress distribution near the wellbore during fracture propagation is established.The calculation results show that the multi-fracture propagation process of sequential fracturing will produce stress interference,the in-situ stress near the wellbore increases non-uniformly,and it is easy to induce fault activation.Based on this,a truncated shale gas reservoir fracturing method is proposed,that is,through the periodic release of in-situ stress,the stress accumulation is slowed down and the risk of casing deformation is reduced.At the same time,the idea of using the multi-cluster fracturing method in the section to alleviate the casing deformation problem is proposed.The calculation results show that the method can improve the stress field near the wellbore,so that the casing can bear a more uniform load and reduce the risk of casing deformation.Through the research of this paper,the mechanism of fault slippage and casing deformation in the multi-stage fracturing process of deep shale gas horizontal wells is clarified,and the corresponding preventive control methods are proposed from the perspective of casing itself and fracturing parameter process optimization,which can be used for fracturing.It provides theoretical guidance for the prevention of casing deformation at the cracking site,which has important theoretical and engineering significance.