页岩柱塞脉冲衰减测试的渗流机理再认识

Recognitions on the flow mechanism of shale during the plug pulse decay measurement

在采用柱塞脉冲衰减法测试页岩渗透率时,对影响测试和分析精度的各种因素进行综合分析,有助于提高测试结果的准确性。由于页岩中发育大量的纳米级孔隙,测试压力越低,越容易出现非达西渗流。为了确保测试过程中气体呈达西渗流,根据克鲁得森数的定义,提出了孔隙中达西渗流的压力下限;针对柱塞脉冲衰减测试中的气体渗流过程,考虑气体在有机孔隙中吸附作用的影响,建立了气体解吸—渗流耦合模型;完成偏微分方程组的求解推导后,提出了相应的渗透率计算方法 ;柱塞脉冲衰减测试后,对页岩样品进行了配套的氮气等温吸附测试和甲烷等温吸附测试。实例分析结果表明:①在柱塞样脉冲衰减测试中,气体在岩样中作一维线性渗流,在构建渗流方程时,可以采用气体压力代替气体拟压力,简化计算难度;②氮气在页岩中的吸附性远弱于甲烷,上下游压差的变化在初始孔隙压力的5%范围之内时,氮气解吸对渗流的影响可以忽略。结论认为,现有的行业标准虽然没有考虑气体吸附的影响,但所得到的页岩渗透率仍然可以满足精度要求。

When using the plug pulse decay method to measure shale permeability,a comprehensive analysis of various factors influencing the measurement and analysis precision is conducive to improving the accuracy of the test result.A great number of nano-scale pores are developed in shale,so the lower the test pressure is,the more easily the non-Darcy flow tends to emerge.In order to ensure the gas is in the form of Darcy flow in the process of measurement,we put forward the lower pressure limit of Darcy flow in pores according to the definition of Knudsen number.Then,the gas desorption-flow coupling model for the gas flow process during the plug pulse decay measurement was established by considering the effects of gas on the adsorption in organic pores.After the partial differential equation set was solved and derived,the corresponding permeability calculation method was proposed.Finally,after the plug pulse decay measurement,the supporting nitrogen isothermal adsorption test and methane isothermal adsorption test were conducted on shale samples.And the following research results were obtained.First,during the plug pulse decay measurement,the flow of gas in rock samples is one-dimensional linear,so when establishing the flow equation,the gas pseudopressure can be replaced with the gas pressure to simplify the calculation.Second,the adsorption of nitrogen in shale is much less than that of methane in shale,so the influence of nitrogen desorption on the flow is negligible when the pressure difference(i.e.<5%)of upstream and downstream changes within 5%of the initial pore pressure.In conclusion,the available industrial standards don’t take into consideration the influence of gas adsorption,but the shale permeability measurement can still satisfy the accuracy requirement.