Practical Solutions for Pressure-Transient Responses of Fractured Horizontal Wells in Unconventional Reservoirs

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 125043, ’Practical Solutions for Pressure-Transient Responses of Fractured Horizontal Wells in Unconventional Reservoirs,’ by M. Brown, SPE, and E. Ozkan, SPE, Colorado School of Mines; R. Raghavan, SPE, Phillips Petroleum Company (Retired); and H. Kazemi, SPE, Colorado School of Mines, prepared for the 2009 SPE Annual Technical Conference and Exhibition, New Orleans, 4-7 October. An analytical trilinear-flow solution was developed to simulate the pressure-transient and production behavior of fractured horizontal wells in unconventional reservoirs. This solution provides an alternative to rigorous solutions, which can be cumbersome to evaluate. These solutions provide insight about and the conditions leading to these flow regimes. Identifying these flow regimes is important to characterize unconventional reservoirs from pressure-transient tests. Introduction Although it is possible to develop detailed analytical models to represent transient fluid flow toward a multiple-fractured horizontal well in tight unconventional reservoirs such as shale, the difficulties of these models are the increased computational requirements, the implicit functional relationships of key parameters, and the inconvenience of their use in iterative applications. Despite the complex interplay of flow among matrix, natural fractures, and hydraulic fractures, the key characteristics of flow convergence toward a multiple-fractured horizontal well may be preserved in a relatively simple, trilinear-flow model. The basis of the trilinear-flow model is that the productive lives of fractured horizontal wells in tight formations are dominated by linear-flow regimes. Physical Model and Assumptions For hydraulically fractured horizontal wells in unconventional reservoirs with microdarcy matrix permeabilities (such as shale), the contribution of the reservoir beyond the stimulated volume usually is negligible. Hydraulic fracturing also changes the stresses in the fracture-drainage area, which may create or rejuvenate natural fractures near the horizontal well. Under these conditions, flow convergence is mainly in the linear direction perpendicular to the surfaces of the hydraulic fractures, and, despite the complex interplay among matrix, natural fractures, and hydraulic fractures, a relatively simple, trilinear-flow model may represent the key characteristics of flow toward a multiple-fractured horizontal well. The trilinear-flow model couples linear flows in three contiguous flow regions, as shown in Fig. 1: the outer reservoir beyond the tips of the hydraulic fractures, the inner reservoir between hydraulic fractures, and the hydraulic fracture. Each region can have distinct properties. The inner reservoir may be homogeneous or naturally fractured, and the hydraulic fractures may have finite conductivity.