Study On Wellbore Stability Of Slim Hole Horizontal Well

In the process of drilling,mud shale sections of Shiqianfeng Formation and Shihezi Formation are encountered with serious problems of borehole instability.The safe density window of drilling fluid is narrow,and the borehole wall is easy to collapse.The main reason is that mud shale is easy to be invaded by drilling fluid due to its strong hydration,brittleness and relatively developed bedding.As a result,the strength of the rock around the borehole becomes low,the stress state changes,and the borehole wall is prone to collapse,falling block and other problems.In view of the wellbore instability problem in this area,it is necessary to accurately predict the safe density window of drilling fluid,and controlling the equivalent density of drilling fluid circulation in slime hole horizontal Wells within the safe density window of drilling fluid can effectively solve the problem of wellbore instability in slime hole horizontal Wells.The main achievements of this study are as follows:(1)In this paper,the field returned core of Sulige gas field in Changqing oilfield was used for laboratory tests,and the mineral composition,microstructure,and physical and chemical properties of the rocks were analyzed.X-ray diffraction test,SEM,specific surface area test and cation capacity exchange test were completed.The test results show that the rocks in the collapsed well section in this area have a high content of clay minerals,and the rocks have developed micro-fractures.The rocks of Shuangshi Formation have large specific surface area and cation capacity,and the rocks have strong hydration ability.Among them,the rocks of Shiqianfeng Formation contain expansive clay,which is easy to expand after hydration.(2)Conduct rock mechanics tests on the returned core,and complete uniaxial and triaxial compressive strength tests on the rock.By linear fitting of static elastic parameters obtained from laboratory tests and dynamic elastic parameters obtained from logging data,an empirical formula for the transformation of dynamic and static elastic parameters of rock mechanics is obtained,and a model for calculating rock mechanical parameters from logging data is established.(3)Firstly,the normal compaction trend line was established through acoustic logging data,and the Eaton method was used to predict the formation pore pressure,and the formation pore pressure profile was established.The in-situ stress profile of the formation was established through acoustic logging data combined with the combined spring model.Based on the Mohr-Coulomb strength criterion,the borehole collapse pressure and fracture pressure were calculated.The mathematical model suitable for the borehole stability of mud shale in this area was verified by analyzing the data of four Wells,and the safe density window of drilling fluid in the small hole horizontal well in this area was obtained.(4)Aiming at the problem of high annular pressure loss in slime boreholes,the rheological model of drilling fluid was selected to predict ECD in horizontal Wells in small boreholes by using the Hehr-Barr rheological model.The model considered the influence of drill string eccentricity,drill string rotation and cuttings bed height on ECD.The effectiveness of the model is verified by example analysis,and the sensitivity of ECD parameters is analyzed.Some parameters such as drilling fluid displacement and density can be adjusted to control ECD within the safety density window and ensure the pressure balance in the wellbore during drilling,which plays a certain role in safe drilling.Through the above analysis and research content,it can provide a certain theoretical basis for the research of borehole stability in the drilling process in this area,and ensure the safety of drilling.