Research On Dynamic Deformantion During Drilling And Completion In Shale Formation

The shale gas reserves in China are abundant.Accelerating the exploration and development of shale gas resources can guarantee the safety of oil and gas resources in our country.However,the instability of wellbore and deformation of casing are prone to occur in shale formation during drilling and completion.The liquid intrusion mechanism of shale formation and the distribution law of stress field are the basis of this paper,and study the dynamic deformation of shale formation during drilling and completion by numerical simulation and theoretical model,which can provide theoretical support for scientific analysis of compelx downhole accidents caused in shale formation during drilling and completion.Through the research in this paper,the main achievements and understandings are as follows:(1)Because of the difference between matrix,natural fracture and hydraulic fracture caused by fracturing,the mathematical model of liquid phase infiltrate into shale formation during drilling and completion is established.Through the calculation of the mathematical model,the change of the pressure in matrix and the fracture system with radial distance and time during the drilling and completion process were obtained,and analyzed the influencing factors of the liquid phase infiltrate into shale formation under different pressure.(2)There is a significant difference in the seepage law of the liquid phase invading the shale formation during the drilling and completion process,which leads to the difference of the stress field distribution.In this paper,the shale is regarded as a rock which is transverse isotropic,and the internal pressure of wellbore,well angle,seepage condition of the working fluid,temperature,additional stress,pore pressre,and the fracture distribution pattern were considered comprehensively.The mathematical model of the well stress distribution during drilling and completion process were established respectively through the stress field superposition principle.According to calculation of model,the distribution law of the well stress field during drilling and completion was obtained.(3)The finite element software was used to establish and analyze the volumetric strains in the two-dimensional plane and the three-dimensional structure through the shale rock mechanical parametrers.The study shows that the shale formation with stronger heterogeneity has smaller volumetric strain.The thermal expansion of rock caused by temperature or the intergranular expansion caused by the seepage will weaken the deformation of the formation caused by the stress.The difference of well pattern will lead to the difference of seepage and stress around wellbore,which cause the deformation in directional smaller than the straight section.The water-sensitive clay mineral has a significant effect on the volumetric strain in shale formation,and the formation volume strain which obtain the mudstone is much larger than the single shale layer.(4)The shale constitutive model which considered thermal-hydration damage was established,and the calculation results of volumetric strain with time in wellbore and circum-well during drilling and completion was obtained.The calculation results show the direction of the deformation is turning towards wellbore under the fluid-solid-temperature coupling.The volumetric strain after fracturing at the wellbore is about three times than the process of drilling,which is reason why casing collapse occurred during fracturing.The factors affecting the shale formation dynamics deformation were analyzed.The results show that if the bottom hole pressure,matrix permeability,fracture aperture,angle of bedding,porosity,crack initiation angle were larger,while the liquid viscosity,elastic modulus were smaller,the stronger the deformation ability would be caused.The effect of the angle of the well on the volumetric strain varies with the radial distance from the wellbore.