Elastic wave velocities in rocks vary with effective stress due to the presence of stress-sensitive discontinuities within the rock such as grain boundaries, microcracks, fractures, etc. Because of the low ratio of the thickness to lateral extent of hydrocarbon reservoirs, production of hydrocarbons leads to anisotropic changes in the stress field in the reservoir and surrounding rocks. Since the response of any discontinuities depends on their orientation relative to the principal stress axes, the anisotropic changes in the total stress field resulting from production can lead to significant elastic wave anisotropy that may be used to monitor the stress changes which occur. The elastic anisotropy resulting from anisotropic changes in the stress field can be written in terms of a second-rank and fourth-rank fabric tensor which quantify the effect of the stress-sensitive discontinuities on the elastic wave velocities in the rock. This allows elastic wave velocity measurements, amplitude versus offset, etc., to be inverted to obtain the components of these tensors. The theory allows the ratio of the normal to shear compliance of the discontinuities to be determined from seismic measurements. This ratio is of importance in determining the failure mechanisms involved in sanding, fault reactivation, etc.
1. INTRODUCTION
Production of hydrocarbons usually leads to changes in pore pressure which give rise to changes in stress acting on the reservoir and surrounding rocks [1]. A decrease in pore pressure due to depletion leads to an increase in the effective stress acting on the reservoir which may be accompanied by reservoir compaction, surface subsidence, casing deformation, reactivation of faults and bedding parallel slip. Strong evidence for stress changes in and around reservoirs undergoing depletion is provided by seismic events resulting from production [2, 3]. The purpose of this paper is to examine the possibility of using time-lapse seismic measurements to monitor changes in stress in the reservoir and surrounding rocks.
Elastic wave velocities in rocks vary with changes in effective stress due to the presence of stress-sensitive discontinuities within the rock such as grain boundaries, microcracks, fractures, etc. Since the lateral dimensions of hydrocarbon reservoirs are usually larger than their thickness, these changes in pore pressure often result in changes in the total horizontal stress which are significantly larger than the accompanying changes in total vertical stress. Since the response of any discontinuities depends on their orientation relative to the principal stress axes, the anisotropic changes in the total stress field resulting from production can lead to significant changes in elastic wave anisotropy that may be used to monitor the stress changes which occur.
In this paper it is shown that the elastic anisotropy that results from anisotropic changes in the stress field acting on the rock can be written in terms of a second-rank and fourth-rank fabric tensor which quantify the effect of the stress-sensitive discontinuities on the elastic wave velocities in the rock. This allows elastic wave velocity measurements, AVO (Amplitude Versus Offset), etc., obtained using time-lapse seismic measurements to be inverted to obtain the components of these tensors, and the sensitivity of such measurements to changes in the vertical and horizontal stress field is discussed.
