页岩过剩吸附量与绝对吸附量的差异及页岩气储量计算新方法

Difference between excess and absolute adsorption capacity of shale and a new shale gas reserve calculation method

页岩吸附气含量的准确测试对于储量评价及开发方案编制等具有重要的意义,但过去在计算地层压力条件下的吸附气含量时,未考虑过剩吸附量和绝对吸附量之间的差异。为此,基于重量法等温吸附实验,得出了以下认识:①当考虑吸附相体积的存在时,等温吸附实验并不能直接测得甲烷的实际吸附量(绝对吸附量),实验测得的应为过剩吸附量;②当压力在10 MPa左右时,过剩吸附量达到最大值,此后随着压力的增大而减小,这一现象是超临界甲烷过剩吸附量的本质特征。为了将过剩吸附量转换为绝对吸附量,提出了计算甲烷吸附相密度的改进方法。改进后的方法对吸附实验数据的拟合效果更好,吸附相密度的计算结果也更加合理。进而对绝对吸附量进行校正,发现绝对吸附量与过剩吸附量的差值随着压力的增大而增大,如果采用低压条件下的实验吸附曲线直接进行页岩吸附能力的评价,将严重低估页岩气储层的实际吸附能力,由此提出了应用过剩吸附量和游离气量计算页岩气地质储量的新方法。结论认为:过去用老方法计算得到的页岩含气量明显大于新方法,有可能高估了页岩气藏的地质储量。

The accurate measurement of adsorbed shale gas content is quite significant to reserve evaluation and development plan preparation. However, the difference between excess and absolute adsorption capacity was not taken into account before when the adsorbed gas content in reservoir conditions was calculated. In this paper, a series of analysis was carried out according to isothermal adsorption experiment principles based on gravimetric method. It is shown that, when adsorbed phase volume is taken into account, the absolute methane adsorption capacity cannot be measured directly through the isothermal adsorption experiment, and the measured value is actually the excess adsorption capacity; and that the excess adsorption capacity reaches the maximum value when the pressure is up to about 10 MPa, and then it decreases with the increase of pressure. This phenomenon is the essential characteristic of excess adsorption capacity of supercritical methane. An improved method for calculating the adsorbed phase density of methane was proposed to convert the excess adsorption into the absolute adsorption. The improved method is better fitted with the adsorption experiment data, and the calculated adsorbed phase density is more reasonable. Then, the absolute adsorption capacity was corrected. It is demonstrated that the difference between the absolute and excess adsorption capacity enlarges along with the pressure. If the experimental adsorption curve under low pressure is directly used for shale adsorption capacity evaluation, the actual adsorption capacity of a shale gas reservoir will be significantly underestimated. In view of this, a new method was proposed to calculate the original shale gas in place by using the excess adsorption capacity and the free gas volume. It is concluded that the shale gas content calculated by the old method is much higher than that of the new method, so the original shale gas in place may be previously overestimated.