Study On Propagation Mechanism Of Cement Sheath Crack During Hydraulic Fracturing

Frequent perforating completion operations are required during volume fracturing,Perforation operation produce huge impact,this will destroy wellbore integrity.Cement sheath is the weakest part in wellbore,so cement sheath will be damaged during perforating operations,includes:(1)some initial micro-cracks occur in the cement sheath near the perforation hole,(2)the debonding of the cement sheath interfaces occur by the impact of perforation,resulting in the initial interfacial micro-cracks.The injection of high pressure fracturing fluid can make further propagation of these cracks,which may not only communicate the adjcent fracturing sections,but also aggravating the failure of cement sheath integrity.Therefore,it is important and necessary to study the propagation of cement cracks in order to keep the safe production of hydraulic fracturing wells.First of all,the physical experiment of cement sheath crack propagation is carried out in this paper,then the experimental results confirm that the initial cracks of cement sheath will further initiate and propagate under the driving of fracturing fluid.Limited by the experimental conditions,it is impossible to quantify behavior of the the crack propagation of cement sheath.So we use numerical simulation method to study the crack propagation mechanism of cement sheath during hydraulic fracturing.The three-dimensional finite-element models are built to simulate cracks propagation in cement sheath.These models are based on basic theory of fluidsolid coupling in porous medium and cohesive zone method.By this model,the impacts of key parameters on the development of cement sheath cracks are studied,including: geological parameters,mechanical properties of casing-cement-formation and fracturing construction parameters.The result indicates that:(1)Compared with the crack in cement sheath,the interface cracks are easier to initiate,and have longer extending distance during the same time.(2)The first interface crack is easier to initiate and has longer length than the second interface crack.(3)The interface cracks tend to propagate along the axis of wellbore,rather than circumferentially at the interface.(4)The proper decrease of elastic modulus of formation and cement can effectively control the propagation of the cement sheath and cement interfaces cracks,as well as the proper reduction of fracturing pump rate and fracturing time.The proper increase of cement sheath tensile strength and the interfaces tensile strength can effectively control the propagation of these cracks and realize the long-term effective sealing of the cement sheath.Therefore this study can provide guidances on the optimization of fracture section spacing,the selection of cement and the design of the hydraulic fracturing.