Incorporating Geomechanics into the Decline-Curve Analysis of Naturally Fractured Reservoirs

Abstract

Abstract This paper presents a new production decline model for naturally fractured reservoirs (NFRs), which takes the geomechanical behavior of the formation into account. Since the pressure transient data can be costly and may not be available for most wells, decline curve analysis that uses readily available well production data would be beneficial. Decline-curve analysis methods, in a variety of forms, have been used in the industry to analyze production data and forecast reserves for many years. However, the impact of geomechanics is not considered in the existing models. As reservoir pressure decreases due to production, fracture aperture can be influenced by both the matrix elasticity and fracture compressibility. This will result in permeability change in the fracture system. To include the impact of the geomechanical properties, a model is derived to understand how pressure change can affect the fracture permeability and behavior of NFRs. It is important to find out this relation since the fractures would close, or at least diminish, after a long period of producing time. The original model of dual-porosity developed by Warren and Root is extended to include the effect of geomechanical properties on the behavior of the reservoir. From this, a new approach for the decline curve analysis of fractured formations is developed that relates the production decline trend not only to the Warren and Root's key parameters (ω, λ), but also to the elasticity parameter (ε) which reflects the mechanical behavior of both the rock skeleton and the fractures. The concept of stress-dependent skin around the wells producing from NFRs is discussed and formulated in this work. The behavior of such NFRs at different flow periods are described, from which we found that the effective-stress change can affect the decline-curves from stress-sensitive fractured formations.