SPE 139250 Analysis of Mechanisms of Flow in Fractured Tight-Gas and Shale-Gas Reservoirs

In this paper we analyze by means of numerical simulation the mechanisms and processes of flow in two types of fractured tight gas reservoirs: shale and tight-sand systems. The numerical model includes Darcy’s law as the basic equation of multiphase flow and accurately describes the thermophysical properties of the reservoir fluids, but also incorporates other options that cover the spectrum of known physics that may be involved: non-Darcy flow, as described by a multi-phase extension of the Forschheimer equation that accounts for laminar, inertial and turbulent effects; stress-sensitive flow properties of the matrix and of the fractures, i.e., porosity, permeability, relative permeability and capillary pressure; gas slippage (Klinkenberg) effects; and, non-isothermal effects, accounting for the consequences of energy balance and temperature changes in the presence of phenomena such as Joule-Thompson cooling in the course of gas production. The flow and storage behavior of the fractured media (shale or tight sand) is represented by various options of the Multiple Interactive Continua (MINC) conceptual model, in addition to an Effective Continuum Method (ECM) option, and includes a gas sorption term that follows the Langmuir isotherm. Comparison to field data, analysis of the simulation results and parameter determination through history matching indicates that (a) the ECM model is incapable of describing the fractured system behavior, and (b) shale and tight-sand reservoirs exhibit different behavior that can be captured (albeit imperfectly) using some of the more complex options of the multi-continua fractured-system models. The sorption term is necessary to describe the behavior of shale gas reservoirs, and significant deviations from the field data are observed if it is omitted. Conversely, production data from tight-sand reservoirs can be adequately represented without accounting for gas sorption. All the other processes and mechanisms allow refinement of the match between predictions and observations, but appear to have secondorder effects in the description of flow through fractured tight gas reservoirs. Introduction Background. Tight sand and shale gas reservoirs have recently emerged as a potentially huge resource, and production from such reservoirs has seen an explosive growth over the last few years. Ultratight reservoirs present numerous challenges to modeling and understanding. These reservoirs typically require fracture stimulation, which create complex flow profiles. Additionally, according to Hill and Nelson (2000), between 20 and 85 percent of total storage in shales may be in the form of adsorbed gas. Production from desorption follows a nonlinear response to pressure and results in unintuitive and difficult-tomodel pressure profile behavior. Closed or open natural fracture networks in ultratight reservoirs introduce further complexity through interaction with the induced fractures. The explosion in production has not been accompanied with the same level of understanding of the basic principles that govern flow in these ultratight reservoirs. The existence of fractures (natural and induced), the matrix-fracture interference, and the challenges posed by the lack of fundamental knowledge on the release of gases from sorption in nearly-impermeable formations with pores on the same order of magnitude as the mean free path of the gases make this a formidable problem that strains current simulation capabilities. In this study we attempt to investigate whether, with the knowledge gained thus fur (albeit limited), and with an understanding of the main mechanisms and processes involved in mass and heat transfer in these ultratight formation, we may be possible to predict their behavior using simplified geological, geometric and property models. Analysis of flow and gas production from tight sand and shale reservoirs. Type curves have been a continually developing method to characterize the behavior of production from tight gas wells (Thompson (1981), Maley (1985), Neal