ABSTRACT
This paper provides a quick method to determine subsidence, compaction, and in-situ stress conditions resulting from pore pressure changes. The method is useful for formations showing large Young’s modulus contrasts compared to the surrounding rocks. The induced in-situ stress is usually calculated assuming that the rock deforms uniaxially without inducing strain in any perpendicular directions. Previously, the amount of subsidence was computed using Geertsma’s strain nucei method, but it required to assume a thin reservoir at reasonably great depth with a rigidity close to that of the confining formations. However, field measurements have statistically shown that many hydrocarbon reservoirs are thick or shallow with elastic moduli that are significantly different from the confining formations. When a hydraulic fracture, a sand-control process, a subsidence-control operation, or an evaluation of formation damage (resulting from Permeability reduction) is conducted in such a reservoir, accurate information in in-situ stress, reservoir compaction, or subsidence induced by pore-pressure changes helps in designing such operations.
A parameter study was conducted to delineate groups of parameters controlling the in-situ stress, the subsidence, and the compaction. These groups were then used to perform three-dimensional (3-D), general, non-linear, finite element (FEM) simulations. Example problems are also included to prevent confusion on sign convention and units.
This work does not use or develop new mathematical techniques, but emphasizes two important issues. First, the common practice in the oil industry to calculate PV compressibility, reservoir compaction, and in-situ stress changes from reservoir-rock property data, is shown to be dangerous as some reservoirs also require knowledge of the caprock property data to evaluate these quantities. Second, it provides design curves to quickly evaluate PV compressibility, reservoir compaction, in-situ stress change, and subsidence. Although techniques to calculate these values were already available, they were usually tedious and required access to sophisticated simulators.
A rough approximation is often sufficient during the reservoir development stage since accurate descriptions are usually not available 6yet. Such a crude estimation is however essential because the decisions on downhole and surface facility designs are made during such early development stages. The model can also handle various complexities such as multilayers with heterogeneous rock properties, inclined reservoirs, irregular reservoirs, nonuniform pore pressure distributions, nonlinear rock properties, and hysteresis effect of cyclic loading.
