人工裂缝宽度对页岩气渗流规律的影响

针对人工裂缝宽度是水力压裂后产生的评价气田开发效果的重要参数,为研究人工裂缝宽度对页岩气渗流规律的影响,建立了基质—裂缝系统双重介质气体渗流数学模型,采用有限元方法对页岩气在储层渗流过程中的压力进行求解,揭示在定产量条件下定性和定量的描述人工裂缝宽度对储层压力及井底压力的影响及其渗流规律,并优化人工裂缝宽度。结果表明:人工裂缝宽度对储层压力影响较小,增加人工裂缝宽度,基质和裂缝系统的压降未产生明显差异,但提高了压力波及范围。当人工裂缝宽度为7 mm时,页岩气的泄气面积最大,基质系统吸附气解吸量最大,压力波及范围最广且传播到井底的速度最快,气体渗流效果最佳;人工裂缝宽度增加对井底压力影响较小,但井底压力有小幅上升趋势且下降速度逐渐减慢。当人工裂缝宽度为7 mm时,井底压力最高,基质向裂缝方向压力波传播速度最快,基质系统吸附气解吸量最大,裂缝与基质沟通效果最优。综上,在裂缝宽度研究范围内,人工裂缝宽度为7 mm时为最优裂缝宽度。因此,压裂时应充分考虑人工裂缝宽度对页岩气渗流规律的影响,通过储层压力展布及井底压力变化有效指导压裂开采。

Effect of Artificial Fracture Width on Seepage Flow of Shale Gas

Artificial fracture width is an important parameter to evaluate the development effect of gas field after hydraulic fracturing. In order to study the influence of artificial fracture width on seepage flow of shale gas. The double-medium model of gas seepage flow in the matrix and fracture system is established. The pressure of shale gas in the process of reservoir seepage flow is solved by finite element method. The variation law of artificial fracture width on reservoir pressure and bottom hole pressure under fixed production is revealed, and the artificial fracture width is optimized. The results show that the artificial fracture width has little impact on the reservoir pressure. With artificial fracture width increasing, the pressure drop of the matrix and the fracture system does not have significant difference, but increases the pressure sweep range. When the width of the artificial fracture is 7mm, the shale gas has the largest gas drainage area, adsorption capacity in the matrix system has the largest desorption capacity, the widest pressure sweep range and the fastest speed to the bottom of the well, and it has the best gas seepage effect, firstly. Secondly, the increase of fracture width has little impact on the bottom hole pressure, but the bottom hole pressure has a slight upward trend, and the decline speed gradually slows down. When the width of the artificial fracture is 7mm, the bottom pressure is the highest, the propagation speed pressure wave from the matrix to the fracture system is the fastest, the adsorption gas in the matrix system has the largest desorption capacity, and the communication effect between the fracture and the matrix system is optimal. In conclusion, within the range of crack width, the optimal artificial fracture width is 7mm. Therefore, the influence of artificial fracture width on shale gas seepage flow should be fully taken into account during fracturing and effectively guide fracturing.