非连续介质力学模型预测煤层井眼稳定性

A discontinuous medium mechanical model for the sidewall stability prediction of coal beds

煤层为割理、裂缝发育的非连续介质,井壁失稳是煤层气钻井难题之一,目前利用连续介质力学模型预测钻井液安全密度窗口,实际应用效果较差。为此,针对煤层为非连续介质的特点,基于非连续介质力学方法建立了煤层坍塌压力的离散构元模型,进而推导出用地应力、割理间距、割理倾角、井眼尺寸、面割理黏聚力和面割理摩擦角等弱面参数表达的坍塌压力、破裂压力非连续煤层数学方程,并定义了方程中与井眼液柱压力、坍塌压力相关的井壁稳定系数。分析表明,井壁稳定系数为正时,可以采用提高液柱压力稳定井壁;否则,不可单一采用提高液柱压力稳定井壁,还需要提高地层的承压能力。另外,坍塌位置对井眼尺寸较敏感,而坍塌压力受井眼尺寸影响较小。

Coal beds are discontinuous media with well-developed cleats and fractures. The prediction of safe-density window of drill- ing fluid with the continuous medium mechanical model currently used can not achieve good effects in coal beds, and the sidewall de- stabilization is one of the difficulties in coalbed methane drilling. For this reason, we established a divergent elementary model for coalbed collapse pressure based on the discontinuous medium mechanical theory, and a discontinuous combed mathematical equa- tion for collapse pressure and breakdown pressure was deduced via ground stress, interval and dip angle of cleat, size of well- bore, cohesive force and friction angle of face cleat. Moreover, sidewall stabilization coefficients related to wellbore liquid-column pressure and collapse pressure in the equation were defined. The analysis indicated that sidewall stabilization can be reinforced by increasing liquid-column pressure when the sidewall stabilization coefficient is positive, otherwise, both the liquid-column pres- sure and the formation pressurization capacity should be enhanced in order to keep sidewall stabilized. The size effect analysis showed that collapse location is sensitive to the size of wellbores, however, collapse pressure changes a little with various sizes of wellbores.