Characterization of Devonian Shale Gas Reservoirs Using Coordinated Single Well Analytical Models

Abstract

SPE Member Summary We present here a simple, fast methodology which can be used to provide information about the two least-measurable parameters of Devonian Shales, fracture spacing and fracture continuity. The results of these calculations can assist in establishment of field development programs. Although this technique may provide for the most detailed characterization possible for the shales, we show the clear need for further research into geophysical and other geological methods for detection of fracture densities. Introduction Reservoir engineering pertaining to gas production from Devonian shales has been limited previously to two approaches, the application of either single well analytic models or large-scale numerical simulations. In this paper we describe a methology which coordinates the use of single well models for field-wide characterization. This simple methology provides relative estimates about the two least-measurable quantities in the shales, natural fracture network spacing and fracture network continuity. The simple methology is significant not only because it gives the capability of estimating fracture density and continuity in a relative way, but also because it is the first methodology whose focus is the determination of the natural fracture network characteristics. we believe that natural fracturing is the reservoir trait which most dominates the flow behavior from the Devonian Shale gas reservoirs. In this paper we discuss the basic mechanisms of gas production from Devonian Shales and summarize the parameters which influence the production process. The basic analytic mathematical model describing flow in the shales is given, and the reservoir characteristics which dominate the solution are summarized. we then present the methodology for field coordination of the analytic model. Finally, we show the results of a sample reservoir study which uses the new technique. The results currently provide the best information as to where infill wells should be drilled in the field. However, this information may not be good enough for reliable application because, as the calculations show, fracture spacing and fracture continuity vary widely and inconsistently throughout the field. In this paper we establish the critical need for further research into geological and geophysical detection of fracture densities in the shales and in other tight-gas formations. THEORY PHYSICAL PROCESSES Unlike production from conventional gas reservoirs, gas in Devonian Shales does not usually flow directly from storage in the rock to the well. In the shales, gas particles can only travel short distances through the rock during reasonable time spans because the rock is so tight. Instead of flowing directly through the rock to the well, the gas flows from storage in the tight rock, usually called the rock matrix, to an adjacent fracture segment, and then through a natural fracture network to the well. Figure 1 shows a two-dimensional plan view of the hypothetical path of a given gas molecule from storage In the tight matrix to a fracture segment and then to the well. Storage in the rock matrix is probably different than that for conventional gas reservoirs. Gas in conventional reservoirs is stored in (relatively) large, open pores. Gas in the shales can be stored in the molecular size micropore space of the shales, can be adsorbed on the surface of the shale, or may be dissolved in the organic content of the shales. Furthermore, flow through the rock matrix is probably due more to molecular diffusion than to Darcy flow. These unusual flow and storage processes in the rock matrix are discussed elsewhere in detail and will not be repeated here. P. 385^