Determining the Effect of Geological and Geomechanical Parameters on Reservoir Stress path through Numerical Simulation

Abstract

Abstract Knowledge of the stress path (ratio of the change of effective horizontal stress to the change of effective vertical stress due to the reservoir pore pressure drawdown) that reservoir rock will follow during production and its effect on reservoir properties are critical for reservoir management decisions to maximize productivity. However, in-situ stress measurements needed to determine reservoir stress path are difficult and expensive to conduct, and the measurement may take several years to collect. In situ stress measurements in carbonate and clastic reservoirs indicate that the reservoir stress path is not isotropic loading (equal to 1.0) and can range from 0.14 to 0.761–2. In the laboratory usually reservoir stress path is calculated using uniaxial strain condition. The calculated uniaxial strain stress path can be significantly different from the measured stress path (Table 1). This paper presents a finite element numerical model to predict the reservoir stress path due to the reservoir drawdown. The model incorporates essential geologic and geomechanical factors that may control reservoir stress path during production. The effect of size and geometry of the reservoir, contrasts in elastic properties between the reservoir and the bounding formations rock, transverse isotropy in elastic properties, plasticity, far-field boundary conditions, anisotropy in the reservoir initial stress states and stratigraphy and structure of the reservoir was studied. For isotropic reservoir properties, stress path becomes lower as the aspect ratio of length to thickness increase. Lenticular sandstone reservoirs have a higher stress path than blanket sandstone reservoirs that are continuous across the basin. Transverse isotropy with lower vertical elasticity than horizontal of reservoir rock increases the stress path, compared to isotropic conditions. Plasticity of the reservoir rock decreases the stress path significantly at the lower aspect ratio of reservoir, but increases with the increase of aspect ratio. However, plasticity of the bounding formations reverses the process. Validation of the simulated results was done by simulating and matching the stress path of an existing reservoir. Reservoir initial conditions were obtained from published literature2 and considering that initial conditions simulated reservoir stress path was matched with the actual field measurements by adjusting the magnitude and orientation of the geological and geomechanical parameters.