Laboratory Study of Shale Matrix Permeability and the Affecting Factors for Yanchang Shale Gas Production

Qu, Hongyan (China University of Petroleum) | Zhou, Fujian (China University of Petroleum) | Jiang, Zhenxue (China University of Petroleum) | Pan, Zhejun (CSIRO Earth Science and Resource Engineering) | Peng, Yan (University of Western Australia)

OnePetro 

Abstract

Matrix permeability, a key factor controlling gas production in shale gas reservoir, is difficult to measure at laboratory and affected by the geochemistry and geology settings including the Total Organic Carbon (TOC) contents, mineral compositions, pore structure, and deposit environment. In this work, the pressure decay method was modified before applied to measure the matrix permeability of the continental shale with extensive TOC variation from Yanchang formation, Ordos Basin, China. Results indicate that matrix permeability varies significantly with both TOC and formation depth, while TOC is the predominant factor controlling the matrix permeability in Yanchang formation shale. Even at the same depth, porosity and permeability are different due to the variation of TOC resulted from geological anisotropy.

1 Introduction

Yanchang formation shale in the Ordos basin is the largest and typical continental shale formation with high gas content in China. However, the shale gas production remains very low even after hydraulic fracturing in recent years, impeding the economical industrial development. Among all the factors controlling the long-term gas productivity, matrix permeability is very critical but seldom studied (Chalmers et al. 2012). Unlike the enormous acknowledgement of the marine shales in the USA (Ghanizadeh et al. 2013, 2014), the micro gas flow mechanism in the continental shale is still poorly understood, and there have been few attempts to characterize the petrophysical properties of Yanchang formation shale.

There are multiple methods for conventional laboratory permeability measurement (Clarkson et al. 2012, Handwerger et al. 2011, Sinha et al. 2012, Suarez-Rivera et al. 2012), based on different physical principles and samples at different scales are utilized. However, only the non-steady-state gas flow method is feasible for the unconventional gas reservoir, due to the low porosity and permeability (Cui et al. 2009, Luffel et al. 1993, Tinni et al. 2012). The permeability measurements with pressure decay profile, pulse-decay and pressure-decay methods are primarily applied for the tight gas shale (Ghanizadeh et al. 2015).

In this work, laboratory study was carried out to investigate the controlling factors of the fluid transport properties in the matrix system of Yanchang formation shale. The pressure-decay method with crushed samples was modified before applied to measure the shale matrix permeability, and the effect of different geological and geochemical factors including TOC and porosity on matrix permeability was analyzed. The experimental data, combined with the Back Scattered Electron Microscopy (BSEM) observation extend our understanding of this continental shale reservoir in China and provide some basic insights of the micro gas flow mechanism behind the low gas production in Yanchang formation shale.