Study Quantifies Stress Sensitivity of Fractured Tight Reservoirs

Abstract

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper IPTC 20260, “Stress Sensitivity of Fractured Tight Reservoirs,” by Gang Han, SPE, Aramco, and Kirk Bartko, SPE, Consultant, prepared for the 2020 International Petroleum Technology Conference, Dhahran, Saudi Arabia, 13-15 January. The paper has not been peer reviewed. Copyright 2020 International Petroleum Technology Conference. Reproduced by permission. Wells in unconventional reservoirs can experience sharp rate declines in the early stage of production, especially when experiencing aggressive drawdown. One key factor affecting rate decline is rock sensitivity to increasing compressive stress. The complete paper describes and quantifies the stress-dependence of compaction and permeability for anisotropic rock matrices, natural fractures, and hydraulic fractures, based on comprehensive rock tests of a fractured tight reservoir. Stress Sensitivity and Drawdown Laboratory data show that rock permeability can be reduced by 10 to 99% with increasing confining stress. Controlling factors include rock characteristics such as authigenic cementation, pore structure, clay content, natural fractures, and pore volume compressibility. Additional factors include pore throat size and shape. Low-permeability rocks are more sensitive to stress changes than are high-permeability rocks. A direct link exists between stress-dependent permeability reduction and production decline, especially in unconventional reservoirs in which production declines rapidly during the first year. As production starts and bottomhole pressure is lowered, the effective stress on the rock near the wellbore, defined as the difference between total stresses and pore pressure, increases. Higher loading stress reduces the permeabilities (or conductivities) of induced fractures, natural fractures (or laminations), and the matrix. The increase in effective stress can lead to proppant embedment and crushing, formation spalling, fines migration, rock compaction, and closing of natural fractures. Many of these processes are irreversible and lead not only to rapid production decline but also to reduction of estimated ultimate recovery (EUR). In 2011, analysis of a shale consortia database found that wells with restricted rate (limited drawdown) have 2 to 3 bcf higher EUR than wells with open rate (unlimited drawdown). The analysis also observed that the decline of stress-dependent permeability in propped fractures (millidarcy) is not as severe as the reduction in unpropped fractures (microdarcy) and matrix (nanodarcy).