钻井完井液对煤层气解吸—扩散—渗流过程的影响——以宁武盆地9号煤层为例

煤层气以吸附气为主,解吸-扩散-渗流过程共同控制着煤层气的产量,仅采用基于达西定律的渗透率的方法来评价煤层气储层损害有待完善。为此,基于煤岩储层微观结构特征和煤层气运移产出机理,以宁武盆地9号煤层和现场用钻井完井液为研究对象,开展了煤层气解吸、毛细管自吸和钻井完井液动—静态损害评价等实验,并采用微观手段分析了钻井完井液影响煤层气解吸—扩散—渗流过程的机理。结果表明:钻井完井液作用后煤样与平衡水煤样、饱和水煤样相比,煤层气解吸量和扩散系数降低;与地层水相比,煤岩对钻井完井液的自吸能力强且吸附滞留严重,导致气相返排率偏低;钻井完井液滤液损害是造成煤层渗透率下降的主要原因。结合红外光谱、润湿角测定和扫描电镜分析结果,得出认识:钻井完井液滤液通过改变煤的结构、润湿性和孔隙连通性,进而影响到了煤储层气体的运移行为。

Effect of drill in fluids on CBM desorption,diffusion and percolation:A case study of No. 9 Coal Seam of the Ningwu Basin

With absorbed gas as the main part of coalbed methane gas, its output highly depends on the desorption, diffusion and percolation process, however, it is not enough to evaluate the CBM formation damage only by the Darcy′s permeability. In view of this, according to the coal microscopic structure features and CBM migration mechanism, experiments of CBM desorption, capillary imbibition and dynamic/static damage evaluation were conducted with coal samples from No. 9 Coal Seam of the Ningwu Basin as well as the field drill in fluids. The effect of the drill in fluids on CBM desorption, diffusion and percolation was analyzed by the microscopic method. The results show that the actual desorbed gas volumes and diffusion coefficients from the coal samples under the interaction of the drill in fluids are both lower than those of the other coal samples under equilibrium or saturated conditions at a given temperature and pressure. Compared with the formation water, coal rocks have stronger imbibition of drill in fluids and serious adsorption retention, which results in lower gas flowback rates. The results of dynamic/static evaluation show that the damage of drill in fluid filtration is the main cause for coal seam permeability decline. Combined with the analysis results of Fourier transform infrared emission spectroscopy (FTIRES), the measurement of wetting angle, and scanning electron microscopy (SEM), it is believed that the coal structure, wettability and pore connectivity will affect the gas migration in coalbed methane reservoirs under the interaction of the drill in fluids.