Comparison of Fractured Horizontal-Well Performance in Conventional and Unconventional Reservoirs

Abstract

Abstract This paper presents a discussion of fractured horizontal-well performance in conventional (milli-Darcy permeability) and unconventional (micro- to nano-Darcy permeability) reservoirs. It provides interpretations of the objective of fracturing horizontal wells in both types of formations. By using a trilinear-flow model, it is shown that the drainage volume of multiply-fractured-horizontal-wells is limited to the inner reservoir between the fractures even for relatively large matrix permeabilities. Unlike conventional reservoirs, favorable productivities are not warranted in unconventional-tight reservoirs because of high reservoir permeability and high hydraulic fracture conductivity. The most efficient mechanism to improve the productivity of unconventional-tight formations is to increase the density of natural fractures. High natural fracture permeabilities may not necessarily contribute to productivity. Decreasing fracture spacing increases the productivity of the well, but the incremental production for each additional fracture decreases. The trilinear-flow model presented in this work can be used to determine optimum hydraulic fracture properties for a multiply-fractured-horizontal-well. The model can also be used as a predictive tool. The information given in this paper should help the design of multiply-fractured-horizontal-wells and predict their performances. Introduction The objective of hydraulic fracturing in conventional-tight reservoirs (as in tight sands with permeabilities in the milli- to micro-Darcy range) has been to create a high-conductivity flow path to improve flow convergence in the reservoir. Fig. 1 provides the definition of fracture conductivity on a sketch of a hydraulic fracture intercepting a vertical well. Accordingly, a practical interpretation of the objective of fracturing a horizontal well is to create a system whose long-term performance is identical to that of a single effective (total) fracture of length equal to the spacing between the outermost fractures (Raghavan et al., 1997, and Chen and Raghavan, 1997). Fig 2 provides a sketch of this interpretation. With this interpretation, performances of fractured horizontal wells can be correlated in terms of an effective fracture conductivity and effective fracture half-length. The conductivity of this effective fracture depends on the permeability of the reservoir and the number, distance between, and conductivities of the individual hydraulic fractures as demonstrated in Fig. 3.