Water weakening of chalk - Mechanical effects of water-glycol mixtures

Chalk petroleum reservoirs are characterized as fragmentary parts of calcite skeletons produced by plankton algae, known as coccolithophorids. As water injection into chalk reservoirs is an important method for improved oil recovery, chalk-water interactions have received considerable attention during the decades due to reservoir compaction. In the present study the mechanical properties of high porosity outcrop chalk (>40%) were studied with mixtures of water and glycol. The objective was to test the hypothesis that the activity of water is the key parameter in water-weakening of chalk. This hypothesis is based on experiments with brine-saturated chalk of different salinities. The fact that water and glycol are fully miscible fluids allows variation of water activity from unit to zero. Four types of tests were used: (1) triaxial tests at low confining pressure to assure shear failure conditions, (2) triaxial tests at higher confining pressure to assure a stress path passing through the end-cap, (3) hydrostatic tests to determine the hydrostatic yield stress and (4) "Brazilian" tests. The tests showed increasing strength with decreasing water activity. Compared with the results from brines, there was good agreement, confirming the hypothesis. Previously results using water-methanol mixtures resulted in the same conclusion. A water weakening mechanism is proposed which involves an additional pressure exerted on the grains by attraction of water molecules to the chalk surface. This adsorption pressure acts like an increase in pore pressure, decreasing the cohesion of the chalk. Evaluation of different mechanisms in chalk-water interactions, indicate that pressure solution and adsorption pressure may be the two most important ones. (C) 2005 Elsevier B.V. All rights reserved.