A New Technique for Production Prediction in Stress-Sensitive Reservoirs

Abstract

Abstract In a stress-sensitive reservoir, variation of the effective stress resulting from fluid production may induce deformation of the rocks. It is essential to quantify such deformation for optimum reservoir management since damage caused by rock compaction is irreversible. In practice, the permeability reduction caused by the rock deformation is characterized by using the deformation coefficient, which is generally assumed to be constant for production prediction in the stress-sensitive reservoir. This assumption may result in a significant error in interpreting the well response, especially at a large pressure drawdown. In this paper, a new technique is developed for predicting production performance in the stress-sensitive reservoir by considering the deformation coefficient as a function of the pressure drawdown. Experimentally, the deformation coefficients are measured for the core samples at the various pressure drawdowns. Theoretically, a mathematic model is formulated for production prediction to integrate effects of the pressure drawdown and the irreversible rock compaction in the stress-sensitive reservoir. It is shown from a field application that the newly developed technique provides a more accurate prediction of the oil production in comparison with conventional models. Introduction Fluid production from a hydrocarbon reservoir generally leads to a decrease of fluid pressure and an increase of effective overburden load on reservoir rock. As fluid production proceeds, the progressive increase in effective overburden load will, in turn, compact the reservoir rock and change the stress state in the reservoir. Reservoir rocks with fluid-flow characteristics (permeability) that are highly sensitive to the effective stress changes and/or if they are of weak mechanical strength causing large rock deformation, are considered to be stress-sensitive(1–4). Sensitivity to changing stresses is probably the most pronounced factor in tight, overpressured low permeability and naturally fractured reservoirs where variations of both elasticity of rock and pressure can significantly change the fracture apertures(5–8). For a stress-sensitive reservoir, the reduction in permeability caused by changes in the stress during production, if any, may greatly reduce the expected productivity(9–14). Systematic well testing results in the Qingxi Oilfield, China, show that well production decreases due to permeability reduction as the pressure drawdown exceeds a certain value. A similar phenomenon is also found in the Dagang, Jilin and Daqing Oilfields in China(15). Therefore, it is desirable to accurately predict the oil production at different pressure drawdowns in the stress-sensitive reservoir. Previously, several studies have been conducted for predicting the oil flow rates and permeability variations with stress(16–18). These models characterized the dependence of permeability on both the pore pressure and the effective stress(16–23). In particular, most of these studies assumed the deformation coefficient, defined as the relationship between the permeability and the compression stress of the skeleton (formation pressure), to be constant. In practice, it has been found that there exists significant error for production prediction with the conventional models, especially when the oil well is produced at a large pressure drawdown. In this paper, a new technique is developed for accurate production prediction in a stress-sensitive reservoir. More specifically, limitations of the conventional models are first analyzed.