Research On Casing Load Analysis And Casing Deformation Mechanism Of Shale Gas Wells During Multistage Fracturing

As a clean energy source,shale gas plays an important strategic role in alleviating China's natural gas supply and improving environmental problems.With the exploration and development of shale gas,it is becoming a new growth point in China's petroleum industry.Reccently,some serious wellbore integrity problems,especially for casing deformation,ouucred during multi-stage fracturing for many shale gas wells in China and the other countries.Casing deformation wells even account for up to 40% in some shale blocks.As a result,the fracturing operation cannot be carried out successfully and the single well production is not high,which greatly hinders the normal development of shale gas field.At present,the deformation mechanism of casing is unclear,the prevention and control measures are not targeted.Therefore,there are of great significance for the efficient development of shale gas in China to carry on the research on casing deformation mechanism during multi-stage fracturing,and clarify the main control factors.Under this condition,scientific and reasonable prevention measures can be put forward to gurrantteing the casing safety and improve the development of shale gas field.Based on the casing deformation problem at home and abroad,this paper mainly carried out the following works:Eliminating the influence of intial stress and strain in the shale formation on casing load,a new analytical model has been established to calculate the wellbore stress under the codition of thermal-pressure coupling,which overcomes the shortcomings of the existing model that can not meet the actual far field displacement boundary conditions and is in good agreement with the numerical results.The effects of cement sheath parameters,geo-stress,fracturing parameters and casing size on casing stress have been studied.The different of casing stress prediction between these two models are compared.The results show that the existing model will underestimate the casing stress in the formation with high elastic modulus,low Poisson's ratio and non-uniformity geo-stress.Considering the characteristics of the dynamic change of the bottom hole temperature caused by the large pump rate during the fracturing process,the temperature difference calculation model is established,and the influence of the fracturing construction displacement on the transient temperature field of the wellbore is studied.Based on the dynamic law of fluid properties of annulus with temperature,combined with shale anisotropy and water immersion experiments,a new stage finite element method model of anisotropic formation-cement sheath-casing combination is established considering the transient thermal-pressure coupling and the initial stress-strain state in the formation.The dynamic variation of casing stress during multi-stage fracturing is studied based on this new FEM model.The results show that the wellbore temperature will decrease rapidly in a short time during fracturing operation with a large pump rate.The larger the pump rate,the greater the decrease of the bottom hole temperature.The high construction pressure,the formation lithology interface,and the shale stiffness degradation will significantly increase the casing stress.When there is the cement channeling around the annulus,the decrease of the wellbore temperature results in a sharp pressure drop in the annulus void fluid,casuing a more complicated load condition at casing string.As the number of fracturing stages increases,the risk of casing deformation increases greatly.According to the spatio-temporal distribution characteristics and magnitudefrequency relationship of microseismic signals during fracturing,a method for identifying fault slip at a casing deformation point is proposed.Based on the principle of focal mechanism,the relationship model between source and fault parameters and casing deformation is established.The numerical simulation of casing deformation under the condition of fault slippage is carried out.The effects of parameters such as the angle between the fault and the wellbore,the parameter of the fracturing construction,the propertiy of the cement sheath and the casing size on the casing deformation during multistage fracturing are studied.The results show that when the microseismic signal is abnormal and oblique with the wellbore and the b value is less than 1,it indicates that the fault is reactivated to slip.The casing stress and deformation increase exponentially with the increase of the magnitude of the microseismic.When the fault stiffness is small and the stress drop is large,the fault slip distance increases dramatically,so do the casing stress and deformation.A small angle between the wellbore and the fault,the cement sheath with a low elastic modulus,and the casing with a small diameter and a low ratio of diameter and thickness can effectively reduce the risk of casing deformation.The shale core saturation experiment in high-pressure is carried out to study the dynamix change of mechanical propertiy.Based on Weibull statistical distribution,the statistical damage constitutive model of shale is established considering time effect.Based on the cusp catastrophe theory,a fault-surrounding rock system catastrophe model is established.The critical condition and displacement mutation as the system mutate are derived.The spatio-temporal distribution characteristics of fracturing construction parameters and shallow-source seismic signals during multi-stage fracturing of shale gas wells in China and Canada are compared and analyzed to clarify the relationship between fault slip and casing deformation during fracturing construction.The research shows that the elastic modulus decreases and the Poisson's ratio increases when the shale is immersed in water,which increases the risk of instability of the fault-surrounding rock system.If the ratio of the stiffness of the surrounding rock to the equivalent stiffness of the weakened fault plane is less than m during the fracturing process,the mutation ocures and the fault is reactivated to slip.When the wellbore passes through the sliding area,the casing is prone to deformate during fracturing operation.Through the research of this paper,the casing load evaluation model under various complicated conditions in the fracturing process of shale gas wells is established,and the main control factors leading to casing deformation are sorted out,and corresponding control measures are proposed.The results can provide reference for the solution of the integrity control problem of shale gas wells in China.