深部煤岩防塌钻井液密度的计算

受煤阶和地应力的双重影响,深部地层煤岩井壁失稳可能与其自身岩石力学性质有关.在这种情况下,由于煤层气含量大小对煤岩变形破坏及力学响应的作用和弹性力学自身的缺陷,防塌钻井液密度的计算会产生误差.研究表明,这种误差跟游离气和吸附气的双重影响有关,往往受孔隙流体压力大小的控制,压力越大,影响越明显.在深部高孔隙流体压力地层环境下,这种影响不容忽略.文中采用经典混合物理论的本构建模方法,揭示了含气煤总应力与孔隙压力、孔隙率、气体的等温吸附特征、固相煤骨架弹性模量、泊松比之间的关系,用其可以定量求出孔隙流体压力对于深部煤岩岩石力学性质的影响,继而可精确计算出钻进深部煤层防塌钻井液密度.

Calculation of Caving Prevention Drilling Fluid Density in Deep Coal Formation

Affected by coal rank and earth stress,caving of coal rock may be associated with rock mechanics properties.Bymeans of the effect of CBM content to coal rock deformation and mechanical response and the defects of elasticity,it leadsan error in calculating caving prevention drilling fluid density.Researches show that this error is caused by both free andadsorbed gas,and it has proportional relationship with pore fluid pressure,which greater pore fluid pressure exists,comesmore error.In this paper,the classical mixture theory construction mode is applied to reveal the relationship between coalstress and pore pressure,porosity,isothermal adsorption characteristics,gas solid-phase coal skeleton elastic modulus andpoison’s ratio.By using this model,the influence of pore fluid pressure to coal rock mechanics properties in deep coalformation can also be quantitative obtained,then the accurate caving prevention drilling fluid density can be carried out.