Wellbore Stability Investigation Of Shale Gas Well In HM Basin During Drilling

There is an increasing concern in the petroleum industry regarding stability problems and the need to improve wellbore operations become more imperative as the operators move towards more challenging and harsher environments such as ultra – deep waters as well as high pressures and temperature fields.To maintain a stable wellbore is therefore of primary importance during drilling and production of oil and gas wells.The shape and direction of the hole must be controlled whilst influx of solid materials must be prevented.To keep a wellbore stable,earth stresses,rock strength and pore pressure must be in a proper balance as well as fluid pressure and drilling fluid composition.This study investigates the HM Basin,Gas Shale instability mechanism.It was discovered during the research that,the drilling cycle and hydraulic fracturing was very much affected by the bedding and anisotropic nature of the shale when it got into contact with the drilling fluid.The study involved SEM analysis,analysis of physical and chemical properties in the region through the development of the HM gas shale mineral components and the borehole collapse mechanism.The content of clay minerals in HM basin shale formation is higher,mainly in illite,illite interlayer and kaolinite as well as montmorillonite mixed layer,electric difference between layers containing joint and easy hydration.A couple of tests were conducted using water based mud and an expansion rate from 12.6% to 16.5% was recorded,indicating a higher hydration expansion capacity.Cation exchange capacity between 116 ~ 149 mmol / kg was discovered and when in contact with water,the recovery rate was more than 60% with a weak dispersion capacity,which shows that the shale hydration dispersion ability is weak.This paper further discussed and compared the various failure criteria which included Mohr Coulomb,Mogi Coulomb,Dracker-Prager,Hoek-Brown and Modified Lade.Mohr Coulomb failure criteria was used to calculate shear failure along the bedding plane,whilst Mogi Coulomb was used in calculating shear failure along the intrinsic rock matrix.Based on this model,a computer program was coded using MATLAB which was employed in the prediction and estimation of breakout(collapse),taking into consideration azimuth,wellinclination and bedding plane.The results show that the collapse pressure of shale is related to its wellbore inclination and wellbore azimuth.It changes along the orientation of the minimum horizontal in-situ stress under different bedding.For horizontal wells drilling under different drilling bedding occurrence,collapse is always on the same symmetrical orientation.The required mud density increases with increasing dip angle.