Research On The Evaluation Method Of Vertical Sealing Ability Of Faults In Mudstone Cap-rock Of Different Brittleness

Oil and gas preservation condition is one of the important factors for oil and gas enrichment.The key of preservation condition lies in the sealing ability of the cap-rock and the configuration relationship between fault and cap-rock.However,there are some existing challenges in the research of cap-rock and fault's vertical sealing ability,such as difficulty in sample drilling of mudstone cap-rock,as well as difficulty in obtaining mechanics data of rock that is under high confining pressure conditions.Taking the regional mudstone cap-rock in the Southern Junggar Basin and Kuqa Depression as an example,this research proposed a numerical simulation technology with the application of particle flow in order to obtain rock mechanical parameters under high confining pressure conditions,based on the laboratory rock mechanics testing of field mudstone cap-rock.The proposed numerical simulation lays the foundation for the evaluation of the cap-rock brittle-ductile and guides the prediction of the vertical sealing of the fault.Because capillary sealing predominates in the mudstone cap-rock in the Southern Junggar Basin,the Displacement Pressure Method(“DPM”)is effective in determining the sealing ability of the cap-rock.The research shows that the cap-rock has a strong sealing ability,which indicates that the capping ability of cap-rock is not the dominant factor of trap failure.Therefore,the key to determining the enrichment of the oil and gas layer stack is the configuration relationship between fault and cap-rock.Practice shows that under different regions and depth conditions,there are differences in the deformation mechanisms and vertical sealing evaluation methods of faults in cap-rock segments of different brittle-ductile stage.Therefore,the primary task of fault vertical sealing evaluation is to determine the brittle-ductile transition process of mudstone cap-rock.In this paper,the macroscopic mechanical parameters of mudstone obtained from the indoor triaxial compression test are used in the numerical simulation research by discrete element method with particle flow software PFC.Combined with the trial and error method and previous experience,this paper proposes the qualitative relationship between the macro mechanical parameters and the meso parameters of the mudstone cap-rock in the study,area as well as a quantitative relationship between three major meso parameters and the confining pressure.This provides a basis for the numerical simulation of mudstone cap-rock in the Tugulu Group,and then predicts the critical value of brittle-ductile transition.Combining the rock mechanics and numerical simulation results,it is revealed that the critical conditions for brittle-ductile transition of mudstonecap-rock are generally consistent: the transition threshold from brittle to brittle-ductile is 70 MPa,the transition threshold from brittle-ductile to ductile is 220 MPa.On this basis,combining the results of field analysis and the actual reservoir analysis,it is recognized that the fault is dominated by the brittle deformation mechanism in the brittle cap-rock segment,where fracture thickness method should be adopted to quantitatively predict the vertical sealing of the fault,whereas the fault is dominated by mudstone smearing in the brittle-ductile cap-rock segment,where the mudstone smearing coefficient method should be used to quantitatively predict the vertical sealing of the fault.Additionally,the fault hardly break through the ductile segment of cap-rock,and generally shows a sealing effect.