Research On Well Integrity Of Horizontal Shale Gas Well With Hydraulic Fracturing

The existing well integrity problems in shale gas wells seriously affect the safety and efficient development of shale gas.It also causes the environment problems like atmosphere pollution,soil pollution,water pollution and so on.The well integrity problems need to be solved immediately.Hence,the well integrity problem is treated as the mair research object in this paper and the failure mechanism of well integrity failure including sustained annular pressure and casing deforamtion in shale gas wells are studied.The effects of various factors on well integrity failure in shale gas wells are discussed in order to provide useful references for design and control of well integrity problems.The cement sheath in horizontal and vertical sections of shale gas wells is the object in my research.For the sustained casing pressure problems occur immediately after the well is finished,the effects of periodic change of fluid pressure and temperature in casing during hydraulic fracturing process on stress stated of cement sheath are considered to establish the mechanical model for casing-cement-formation system in horizontal and vertical sections respectively.The stress state of cement sheath in horizontal and vertical sections of shale gas wells during the well completion process are calculated.In addition,the Drucker-Prager criterion and tensile strength criterion are introduced to evaluate the service state of cement sheath.For the sealing problems caused by the casing eccentricity,the mechanic model for calculating the stress state of eccentric cement sheath has been established.Pointing at the interfacial cracks on the casing-cement sheath interface and cement sheath-formation interface,an analysis model for the mechanism of generation of interfacial cracks formation and a calculation model for the width of cracks are established.Considering the properties of expansive cement varied with solidification time,an analytical model for the stress state of expandable cement sheath in wells is established and the effect of expandable cement on improving sealing ability of cement sheath has been accurately calculated precisely.Considering the problems that the well completion tools cannot be sent to the predetermined position during the hydraulic fracturing process,the casing deformation in shale gas wells are studied.The detected results of casing deformation in field shows that the shear deformation is the main style of casing deformation in shale gas wells.Based on this phenomenon,fault reactivation and fault slip is treated as the main reason for casing deformation in shale gas wells.According to different reactivation types of faults caused by hydraulic fracturing treatment,three different fault slip models are established and the fault slippages in different model are calculated respectively.Considering the conditions that fault slip and poor cementing quality,a mechanical model for stress state of casing in local loads is established.The stress state of casing in parallel and radial local loads are calculated respectively.The deformation of casing under local loads is calculated based on numerical simulation method.Two ways including reducing fault slippage and improving shear strength of casing are proposed and discussed to control the casing deformation in shale gas wells.