Determination of in-situ elastic properties and reservoir boundary conditions

Abstract

The elastic properties of reservoir formation are usually measured through laboratory triaxial tests or are estimated from empirical correlations obtained from well logs. Both approaches have similar limitation of the estimated value being representative of near-wellbore region. The former, however, has the further limitations of being costly and the unavailability of core samples for having a reliable estimate. In this paper, a new methodology is presented to determine the in-situ elastic properties (Poisson's ratio-PR- and Young's modulus) using pressure transient testing. Adopting the poroelasticity theory, a generalized diffusivity equation is formulated incorporating reservoir deformations into the fluid flow. By applying different boundary conditions, namely generalized plane stress, plane strain, and uniaxial strain, several analytical solutions were developed to obtain the PR from well testing. It shall be noted that the Young's modulus can be estimated given PR and bulk modulus. Two field data sets were used to implement and validate the proposed methodology for estimation of the PR (a build-up test data-set from a limestone oil reservoir and an interference drawdown well testing data-set from a sandstone aquifer). Results indicate that indeed the PR is a strong function of the reservoir deformational behavior. Moreover, estimation of PR under different boundary conditions provide a tool to identify the dominant reservoir deformational conditions. This was identified for the first data set by having the sonic well log data available and for the second data set by comparing the pressure-time measurements with their counterparts obtained from fully coupled fluid flow/geomechanics simulations. Moreover, once the validity of the output is confirmed by comparison with the available PR from well logging and/or coupled flow/geomechanics simulations, the proposed protocol can be re-used multiple times during reservoir life to estimate the elastic properties under the appraised BCs, without the necessity to run new well logs or lab tests.