A Method For Borehole Stability Prediction Of Discontinuous Coal Beds

As coal is divided into discrete elements by cleats,stress analysis is conducted on the element prone to collapse and an element analysis model for collapse pressure is constructed through the equilibrium condition of forces.It can be observed from the element analysis model that with the increase of borehole pressure,the wellbore stability can be increased or decreased,which depends on the hole size,cleat spacing,friction angle of cleat,plugging property of drilling fluid,cleat angle,and the angle between element and principal stress,all of which define the wall stability coefficient c.According to the three cases of coefficient c,methods to determine safe mud pressure are all presented.The analysis indicated that the sidewall stabilization can be reinforced by increasing liquid-column pressure when the sidewall stabilization coefficient is constant positive,otherwise,both the liquid-column pressure and plugging property of drilling fluid should be enhanced in order to keep the sidewall stabilized.The pressure transfer coefficient kp was obtained from the pressure transfer experiment of three kinds of drilling fluids,showing that the better the plugging property of the drilling fluid is,the smaller the pressure transfer coefficient will be,which leads to the obtaining of the larger density window of the safety drilling fluid.When the ratio of borehole size to cleat distance increases,the stability of wellbore will be reduced.