Total porosity measurement in gas shales by the water immersion porosimetry (WIP) method

Abstract

Over the past decade interest in shale properties has increased due to the commercial success of gas shale plays. Despite their commercial importance, porosity measurement from gas shale samples is still challenging due to their extremely low permeability and complex pore structure. This leads to a significant uncertainty in the economic assessment of these plays. The current energy industry standard technique for measuring porosity in gas shales is based on methodology developed by the Gas Research Institute (GRI) that involves crushing a rock and aggressive pretreatment. The objective of this study is to develop an alternative method of measuring total porosity in gas shales. A porosity measurement using a liquid saturation and immersion technique with deionized water was adopted and modified for such applications. The water immersion porosimetry (WIP) technique was used to measure total porosity of shale samples from an Eastern Europe Silurian gas shale play and the Haynesville Shale from East Texas, USA. The samples were characterized for whole rock quantitative mineral and elemental composition, along with cation exchange capacity (CEC) and organic matter. The results from the WIP measurements are compared with other standard techniques including the GRI method and mercury intrusion porosimetry (MIP). An assessment of the advantages, potential errors, pitfalls and reproducibility of this method are also presented.The experimental results indicate that WIP provides (i) highly reproducible porosity, grain density, and bulk density measurements for gas shales, (ii) the average absolute experimental uncertainty is ±0.22 porosity unit (p.u.), compared to the reported uncertainty level of 0.5p.u. for GRI measurements, (iii) standard MIP techniques systematically underestimate the porosity and grain density compared to WIP, because mercury cannot access the entire pore structure in shales, and (iv) grain density values obtained by the GRI method in samples with high organic matter content are higher compared to WIP measurements, probably because of dissolution of solid organic matter during solvent extraction pretreatment.