Evaluation Of Reservoir Properties From Geomechanical Tests

Abstract

Abstract Over the past decade an increasing awareness of the geomechanical behaviour of a reservoir has resulted in a greater number of geomechanical tests being conducted on reservoir core specimens. These tests are primarily aimed at measuring compressibility, thermal expansion and stress-strain-strength properties. Traditionally two types of geomechanical tests are performed an oedometer test, and a triaxial test. The interpretation of geomechanical test results, however, must be done with a full understanding of how the tests were conducted. A brief summary of the test procedures and the results obtained from each type of test are presented. Test results for oil sands will be used to evaluate reservoir properties from typical geomechanical tests. Several definitions of compressibility are used, such as, rock compressibility, bulk compressibility, pore compressibility, grain (solid) compressibility and fluid compressibility. Similar definitions are used for thermal volume change. Equations are developed for these compressibilities in terms of effective stress. For sands and many rocks, these properties are not constants but vary with the magnitude of the effective stresses in the formation and the stress path that is followed. To determine the in situ values of these properties, laboratory equipment and test procedures must be used which can model the in situ stress paths. Analysis Of conventional thermal volume change equations reveal an error in a basic assumption and corrected equations are given. Introduction Reservoir simulators for thermally enhanced oil recovery processes require mechanical properties of the geologic materials for the numerical analysis of the reservoir performance. The most common material properties used in these numerical simulators are the compressibility and the thermal volume change as these properties affect permeability, storage and other factors in the recovery process. Several definitions of compressibility are used, such as, rock compressibility, bulk compressibility, pore compressibility, grain (solid) compressibility and fluid compressibility. Similar definitons are used for thermal volume change. For sands and many rocks, these material properties are not constants but vary with the magnitude of the effective stresses in the formation. In addition, these properties will vary depending on the stress path followed in the formation. To determine the in situ values of these properties, laboratory equipment and test procedures must be used which can model the in situ stress paths. Two common geomechanical tests, the oedometer test and the triaxial test, and the stress paths that are followed in these tests are now frequently being used to determine the relevant properties. The calculations of compressibility and thermal volume change from these test procedures are usually made in terms of effective stress in contrast to conventional reservoir analyses which employ total stress and pore pressure. The test procedures and stress paths followed during the test must be taken into account when developing equations, showing the relationship betwefI1 the different stress compressibility and thermal compressibility parameters, for use in reservoir numerical simulators. Geomechanical Principles The concepts presented in this paper are based on the principle of effective stress. The behaviour of all porous media is controlled by effective stresses. Equation (1) defines effective stress: