In stress sensitive reservoirs, the variation of the effective stress resulting from fluid production may induce the deformation of the rocks, which should be considered in reservoir modeling. The damage dominated by rock compactions is irreversible; therefore its better description is essential for optimum reservoir management. The most commonly used methods to predict the productivity of the stress sensitive reservoirs are based on the assumption of constant deformation coefficient, which could result in significant error in interpreting the well response. This paper presents a discussion on the issues related to the interaction between the deformation coefficient and the effective stress. Results of the laboratory experiments show that the deformation coefficient of rock compaction changes greatly with the variation of effective stress. A new mathematic model for production prediction in stress sensitive reservoirs, which takes the deformation coefficient as a function of the effective stress as well as the irreversible rock compaction, is developed and field applied. A field application example is also given to show the difference between the results of the new model and the conventional ones. Comparisons between the results of the new model and test data indicate that results of the new model appear to be a good approximation of the oil well production.
Changes in the permeability caused by changes in the reservoir stress environment, which are in turn caused by changes in pore pressure, are well-documented at both laboratory and production levels [1–3]. This mechanism is commonly called the stress sensitivity of a reservoir. Sensitivity to changing stresses is probably most pronounced in tight, overpressured, naturally fractured reservoirs where the elasticity of rock and large pressure changes can cause significant changes in fracture apertures [4–5]. For stress-sensitive reservoirs, the reduction in permeability caused by changes in the stress state during production, if any, many significantly reduce the expected productivity [6]. Conventional model to predict production for stress-sensitive reservoirs is based on the assumption of constant deformation coefficient, which is used to describe the relationship between the permeability and the compression stress of the skeleton (formation pressure) [7–8]. However, significant error may be made when using the conventional model for production prediction, especially when the oil well is produced with a high drawdown pressure.
The purpose of this paper is to present a new mathematic model for production prediction in stress-sensitive reservoirs to enable engineers to precisely evaluate and predict the loss of productivity. We start with an analysis of the problems that the conventional model meets, then the effect of the variation of the deformation coefficient on the permeability and the oil well productivity is presented and analyzed. A new mathematic model of the production prediction is developed next on the basis of a new calculation method of the deformation coefficient and the drawdown pressure. Comparisons between the results of the new model and the conventional one are finally discussed. Although we discuss only field data from Qingxi oilfield, we think that our conclusion can be generalized and may be of value of other studies aimed at the production prediction of the stress-sensitive reservoirs.
