Study On Wellbore Stability Of Shale Gas Development By Supercritical Carbon Dioxide

The high clay,hydration and swelling of shale oil and gas reservoir lead its exploration and development are very difficult especically in the process of drilling and completion.Because of its high density,low viscosity,and strong flow penetration ability,supercritical carbon dioxide is very suitable for the exploration and development of shale oil and gas reservoirs and other unconventional oil and gas resources.There will be no hydration expansion,damage to the reservoir and so on in the process of using supercritical carbon dioxide as drilling fluid.Therefore,the study of the wellbore stability in the shale formation of supercritical carbon dioxide drilling has a very important role.In this paper,the experiment of supercritical carbon dioxide soaking shale was designed and carried out.The change of shale strain in soaking and the change of mechanical properties after soaking were obtained:shale expands lightly at the beginning of soaking in SC-CO₂ but shrinks afterwards and tends to be smooth at last.After soaking,the uniaxial compression test shows that the uniaxial compressive strength of shale decreases while the Elastic Modulus?Young's modulus?and Poisson's ratio increases;the mechanical properties of shale changes rapidly when the pressure of CO₂ reaches the supercritical state.As the pressure keeps increasing,the change of mechanical properties of shale core is relatively slow and within a narrow range,the average reduction of uniaxial compressive strength is 18.4%,the average increase in modulus of elasticity and Poisson's ratio is 43.4%and 36.6%,respectively;With the increase of soaking temperature,the uniaxial compressive strength is gradually reduced,the average reduction is 17.7%,and the modulus of elasticity and Poisson's ratio are gradually increased,the average increase is 43.4%and 36.6%,respectively.It was found that the change of mechanical properties of shale is not sensitive to the change of CO₂ pressure through experiment.Therefore,the influence of CO₂ temperature was mainly considered in wellbore stability analysis in this paper.In order to study the influence mechanism of CO₂ between the wall shale of the wellbore,firstly,according to the actual drilling and completion conditions,the numerical model of heat conduction between carbon dioxide and shale formation is established and solved,and the temperature change process of shale formation in well area is obtained.Then,The transient temperature field obtained by numerical simulation was taken as a predefined field,the quantitative correlation between the mechanical parameters of shale,the volume modulus of supercritical carbon dioxide and the permeability coefficient in the shale with CO₂ temperature are established,then the quantitative correlation was taken as input of field variables,and seepage pressure coupling calculation was performed to get pore pressure and void ratio variation course and rule in wellbore.A wellbore stability analysis model was established,which considered the influence mechanism of carbon dioxide and shale,temperature stress and seepage stress coupling.The distribution of wellbore stress,collapse pressure and collapse index,the collapse pressure and collapse index section of the whole well section were calculated.Analysis reveals:With the increase of time,the collapse pressure of the bottom well reduced and the collapse index increased,which indicated that the collapse stability was enhanced.With the increase of well depth,the total well collapse pressure gradually increased,but it was always lower than the wellbore pressure.The collapse index of the whole well decreased gradually,but it was always greater than one,which shows that the well wall of the supercritical carbon dioxide drilling in shale formation is stable under certain conditions.In this paper,by means of a combination of experimental and theoretical calculations,according to the actual working conditions,the stability of the wellbore in the shale formation with supercritical carbon dioxide drilling was accurately analyzed and predicted.The research in this paper provides a theoretical basis and technical support for the development of shale gas development in drilling wellbore stability.