Reservoir Characterization for Naturally Fractured Reservoirs

Abstract

Abstract Reservoir characterization and simulation modeling of naturally fractured reservoirs (NFRs) presents unique challenges that differentiate it from conventional, single porosity continuum reservoirs. Not only do the intrinsic characteristics of the fractures, as well as the matrix, have to be characterized, but the interaction between matrix and fractures must also be modeled accurately. Three field case studies have been evaluated combining the "forward" modeling approach, typically used by geo-scientists, with "inverse" techniques, usually incorporated by reservoir engineers. The forward approach examines various causes of natural fractures and its' associated properties (e.g. fracture spacing, height, stress distribution, etc.) while the inverse approach focuses more on the effect created by the NFR (e.g. decline analysis, material balance, productivity, etc.). This study shows how a more powerful methodology is created, for the evaluation of naturally fractured reservoirs, when combining two techniques that have, historically, been applied in relative isolation. Introduction The development of reservoir modeling and reservoir characterization for Naturally Fractured Reservoirs (NFRs) has lagged behind simpler matrix flow dominated rock systems due to the practical difficulty in quantifying both matrix and fracture parameters. The complexities of, numerical and mathematical calculations have historically constrained the development of NFR modeling1. This paper shows a number of integrated field studies, which have addressed the difficulties in characterizing NFRs, and presents a proven methodology to characterize and model them. Reservoir characterization presents a unique challenge in NFRs because of:the need to characterize the fractures as well as the matrixthe need to characterize the matrix-fracture interaction. Characterization of the fracture includes defining parameters such as inter-fracture spacing, length, orientation, porosity, connectivity, aperture and permeability. As well, it is important to include realistic areal and vertical heterogeneity in both the matrix and the fracture systems. A fractured medium represents a highly heterogeneous system. Fluid transport and pressure dynamics cannot be fully replicated in a model using a homogeneous three-dimensional system. Recent work has emphasized the need to better characterize heterogeneities in matrix properties. The same attention, if not more, needs to be given to the characterization of fracture heterogeneities. Reservoir characterization is highly dependent upon the integration of skills from geologists, geophysicists, petrophysicists and reservoir engineers to an even greater extent in NFRs than in conventional reservoirs. The methodology presented ensures compatibility between the geological and engineering models. Specifically, this paper will show how NFR parameters were determined for the three fields in which this methodology has been applied.