Using a Multilayer Reservoir Model To Describe a Hydraulically Fractured, Low-Permeability Shale Reservoir

Abstract

SPE Members Abstract This paper summarizes the development and application of a multi-layer reservoir description for the Gas Research Institute (GRI) Comprehensive Study Well 2 (CSW2), which is completed in the Devonian Shales and located in Calhoun Co., WV. We determined that a two-layer reservoir description best describes the complex reservoir and hydraulic fracture system of the CSW2. This reservoir description, determined using a numerical simulator, matched all the data collected, including pre- and post-fracture flow/buildup tests, muld-layer communication tests, individual-layer injection/ falloff tests and pre- and post-fracture production data. The results of this study indicate that a multi-layer reservoir description provides better estimates of post-fracture performance compared to a more conventional, single-layer reservoir description. This result also explains previous, optimistic estimates of post-fracture well performance which were based on single-layer interpretations of pre-fracture test data. This paper reviews and discusses briefly the many diagnostic tests performed on the CSW2, but focuses on the history match analysis and results determined with a two-layer reservoir model. The testing and analysis approach presented in this paper can be applied generally to other tight formations. Introduction The Devonian Shales of the eastern U.S. are a significant source of domestic U.S. natural gas. The complex storage and production mechanisms of the Shales have been studied extensively. In mid-1987, GRI initiated a multi-year research effort on several highly-instrumented study wells in the Devonian Shales of the Appalachian Basin. These wells, called Comprehensive Study Wells (CSW's), were drilled and studied in cooperation with Appalachian Basin operators. Extensive data were collected and special experiments were conducted on these wells in addition to the operator's normal operations. The objectives of the CSW program were (1) to develop a better understanding of the geologic controls on production, (2) to refine previously developed formation evaluation tools for selecting completion intervals, and (3) to improve reservoir description and stimulation practices in the Devonian Shales. In the CSW program, we first began evaluating the Devonian Shales using simple, single-layer reservoir models until, as this paper demonstrates, the need to develop more complex models become evident. Now, it appears the large intervals completed in the Shales cannot generally be analyzed or classified as an equivalent single-layer system; they are much more complex. In the CSW2, the post-fracture production performance was much less than we predicted using a single-layer reservoir and fracture description based on pre-fracture well Lest analysis results and the fracture treatment design. Our initial post-fracture well test analysis results indicated a short, 28-ft, infinite-conductivity hydraulic fracture, using a single-layer reservoir description. Because of these results, several post-stimulation diagnostic tests were performed to evaluate the location and geometry of the propped hydraulic fracture, to provide a more detailed reservoir description, and to determine the cause of the less-than-predicted post-fracture well performance. The purpose of this paper is to present our CSW2 reservoir description based on results obtained using a two-layer reservoir model to history match all the numerous field data collected. Our analysis results indicate an unusual and complex hydraulic fracture geometry which intersects more than one "pay" layer, but only communicates with the wellbore in one layer. A more detailed discussion of our results can be found in Reference 6. P. 237^