Abstract

Successful production performances from shale resources in North America have generated broad interests in several intriguing properties, such as organic-matter pore network, wettability, connate-water saturation, geopressure gradient, and brittleness. Although poorly understood, unique characteristics of these properties can have profound impacts on storage capacity, fluid flow, and production. Objectives of this study were to investigate potential effects of organic-matter pore network, wettability, low connate-water saturation, geopressure gradient, and effective stress on properties of organic-rich shales as well as fluid flow through shale reservoirs.

Wettability of organic-rich shale is a complex function of thermal maturity, total organic carbon (TOC), adsorption, content of polar components, and other parameters. Although results at low temperatures and pressures indicate that the hydrophobicity of organics seems to decrease with thermal maturity, the adsorption of gas at reservoir conditions tends to make organic matter hydrophobic.

Porosity in organic matter can be several times higher than that in the nonorganic matrix. Because of high porosity and predominantly single-phase flow, gas permeability in organic matter, significantly higher than that in the nonorganic matrix, tends to enhance gas permeability in shale gas reservoirs. In addition, the pore volume in the organic-pore network of some gas shales is estimated to be larger than that in fractures, and organic-pore networks can be pathways to high gas production.

Connate-water saturations in high-quality gas shales, varying from 15% to 40%, are low. Low connate-water saturations, most likely caused by hydrocarbon generation and expulsion at high temperatures and pressures, are closely associated with connate-water saturations at maximum burial depths and not a function of present-day depth. Effects of low connate-water saturations are to decrease the likelihood of water production from shale matrices and to decrease frack-water flowback efficiency.

Laboratory compression tests on mechanical properties of specimens from Barnett and Haynesville shales showed that splitting-failure and splitting-shear-mixed-failure modes are the main failure modes for shale samples under low effective stresses, whereas shear-failure modes predominate under higher effective stresses. Failure at less than 1% strain in most tests suggests that these shales are highly brittle, according to Griggs and Handin’s criteria.