In-Situ Poisson's Ratio Determination From Interference Transient Well Test

Abstract

Summary Poisson's ratio is usually determined with well logging, fracturing data, and core samples. However, these methods provide us with a Poisson's ratio that is representative of only near-wellbore regions. In this paper, a technique is proposed by extending currently used pressure-transient-testing concepts to include reservoir stresses. More specifically, the interference well test is generalized to find not only conventional flow parameters such as reservoir transmissivity and storage capacity, but also the average in-situ Poisson's ratio. This is accomplished with the generalized diffusivity equation, which takes into account flow-induced stress changes. First, a generalized diffusivity equation is formulated by considering a deformable porous medium. The main goal of the generalized diffusivity equation is to extend current well-testing methods to include both fluid-flow and rock-mechanics aspects, and to present a way to determine the rock-mechanics-related property, Poisson's ratio, from the interference-well test. The line-source solution to the diffusivity equation is used to modify the current interference well-test technique. A synthetic example is presented to show the main steps of the proposed transient well-testing analysis technique. In addition, application of the proposed method is illustrated with interference-well-test field data. With a Monte Carlo simulation, effects of uncertainty in the input data on the prediction of Poisson's ratio are investigated, as well. In addition, a coupled fluid-flow/geomechanical simulation is performed to show the validity of the proposed formulation and corresponding improvement over the current analytical approach. One can put in practice an average in-situ value in different applications requiring accurate value of Poisson's ratio on the reservoir scale. Some examples of these include in-situ-stress-field determination, stress distribution and rock-mass deformation, and the next generation of coupled fluid-flow/geomechanical simulators. By use of Poisson's ratio that could capture flow-induced stress changes, we would be able to find the stress distribution caused by production/injection within the reservoir more precisely as well.