致密储层样品体积测量对孔隙度误差的影响

Influence of bulk volume measurement on porosity error in tight reservoir core plug analysis

非常规油气致密储层孔隙度较低，一般小于10％，页岩储层甚至小于5％，常规方法对这类岩石样品孔隙度测定误差近来受到关注。针对常用的孔隙度测量方法（氦气注入法），对致密储层和常规储层样品建立了孔隙度测量误差计算数学模型，统计样品体积测量的误差来源和误差限，结合数学模型和统计结果，确定孔隙度误差限。研究得出以下结论：①采用游标卡尺测量的致密储层孔隙度绝对偏差为0．59％～2．47％，高精度三维扫描方法孔隙度测量绝对偏差可以减小到0．38％～1．04％，与常规储层不同，致密储层需提高孔隙度测试精度并进行误差分析；②不同精度的测量方法均很难达到行业标准规定（孔隙度绝对偏差小于0．5％），建议将孔隙度误差质量控制标准降低，即孔隙度绝对偏差放宽到小于1％，并给出了相应的体积测量误差要求；③孔隙度测量误差与样品长度成反比，为了提高测量精度，样品长度应尽量大于3 cm。

Unconventional tight reservoir has a low porosity, which is usually less than 10%, while that of shale reservoir is even less than 5%. The porosity test error of helium injection method has been considered. A mathematical model is established for the error of porosity determination and the statistical data of source and error limit was obtained for volume measurement. （1） The error limit of porosity determination of tight reservoir using vernier caliper is 0.59%-2.47%, while that can be reduced to 0.38%-1.04% by using a highprecision 3D laser scanner. Different from conventional reservoirs, the porosity measurement accuracy of tight reservoir should be improved and analyzed. （2） The accuracy of all porosity determination methods cannot reach the industry standard （〈0.5%）, so a new standard is suggested that reducing the error limit standard （i.e. 〈1%）. The demand of volume measurement was made based on the new standard. （3） An inverse proportion is found between porosity error and core plug length, so sample length should be longer than 3 cm to reduce porosity error.