Research On Wellbore Stability Of Shale In Coal Measures In Qinshui Basin

Qinshui basin has a wide range of shale in coal measures formation,the resource of shale gas is abundant.During the process of exploration and development of shale gas,the shale wellbore instability seriously affected the drilling cycle and hydraulic fracturing operation.Because the shale developed a lot of bedding plane and the attitude of bedding plane is changeable,the strength of the shale rock has significant anisotropic characteristic.The horizontal section of shale gas well is generally very long and contacts with drilling fluid for a long time.They will produce interaction include mechanical,physical chemistry and etc.The rock strength,formation pore pressure and wellbore stress distribution will change with time and space which increase the complexity of wellbore stability of shale gas horizontal well.In this paper,for the wellbore instability problem of coal shale in Shizhuang north block in Qinshui basin,the main research contents include the following aspects:(1)Trough the logging data and laboratory experiments,the mechanical property,fabric and physic-chemical properties of the shale in coal measures were tested.The mechanism of wellbore instability in this region was analyzed.(2)The influence of multi-field coupled on the wellbore stability was analyzed.The stress solution around a borehole was obtained which was affected by in-situ stress,hydraulic pressure differential,chemical potential differential,temperature differential.Due to the developed bedding plane,the coordinate transformation model around the borehole in the bedding shale was established.In combination with Mohr-Coulomb weak plane strength theory and Mohr-Coulomb criterion,the mechanical-chemical-temperature coupled model which consider in-situ stress,hydraulic pressure differential,chemical potential differential,temperature differential,rock strength weaken was established.(3)Based on the coupled model,a computer program was coded by MATLAB.The influence of the attitude of bedding plane,hydraulic pressure differential,chemical potential differential,temperature differential on collapse pressure was analyzed.Finally we realized the prediction of borehole collapse cycle.The results show that the shear failure along the bedding plane is the main reason of coal shale wellbore instability in Qinshui basin.Under the condition of the in-situ stress state and bedding occurrence of Qinshui basin,when the angle of deviation is less than 45°,the collapse pressure is lowest when the drilling direction along the minimum horizontal stress orientation.When the angle of deviation is between 45° and 90°,the collapse pressure is lowest when the drilling direction along the up-dip orientation.When the angle of deviation is 90 °,the shear failure along the rock mass happens along the up-dip and down-dip orientation,the collapse pressure is equal and low.A cooler mud with low activity can reduce the formation pore pressure,enhance the wellbore stability and extend the safe drilling cycle.When the drilling fluid system and density is constant,the decrease of the drilling fluid temperature will extend the safe drilling cycle.Through experiments and theoretical analysis,considering the influence of the mechanical,chemical and temperature,a prediction method of bedding coal shale wellbore stability was established which provide effective guidance for drilling engineering design in coal shale in Qinshui basin.