After-Closure Analysis for Naturally Fractured Reservoirs: A Field Validation Study
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 97015, "After-Closure Analysis for Naturally Fractured Reservoirs: A Field Validation Study," by S. Chipperfield, SPE, Santos Ltd., prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9–12 October.
Economic optimization of fracture stimulation designs requires accurate reservoir description. The full-length paper presents a new technique for evaluating a minifracture injection in a manner similar to a conventional pressure-transient test for identification of productive natural fractures. Appendix A in the full-length paper presents a brief theoretical background. Three examples from Australia and one from Algeria review field application of this technology.

Introduction
An understanding of critical reservoir parameters aids in determining whether a zone should be stimulated and the most appropriate stimulation design. Until recently, this evaluation was made by use of pressure-buildup tests. However, these tests are performed less frequently now because of the costs in terms of additional equipment and the delay in bringing the product on line. The advent of after-closure analysis (ACA) has allowed reservoir characterization to be conducted in a more cost-effective manner through merging reservoir engineering and minifracture analysis. The full-length paper presents an extension of ACA to identify and characterize naturally fractured reservoirs. This analysis approach identifies natural fractures that are open under in-situ conditions and assists the engineer in differentiating these natural fractures from those fractures that may be open only under high injection pressures. Because naturally fractured reservoirs may require alternative design strategies, this ACA approach not only provides a means of reservoir characterization but also drives stimulation-design choices.

Theory Highlights
Naturally Fractured vs. Matrix-Style Reservoirs. Flow regimes observable during the after-closure period for a matrix reservoir include pseudolinear and pseudoradial. These flow regimes are identified on a Nolte diagnostic plot (NDP) from a half slope and unit slope, respectively. Reservoir quality can be determined from the pseudoradial-flow period. Fracture half-length Xf can be estimated from the intercept of pseudolinear flow and pseudoradial flow (the "knee" time). Matrix reservoir response is typified by the derivative, and the pressure-difference curves maintain a degree of separation and overlay each other only in late time.
The dual-porosity scenario is typified by a dual-porosity dip feature where the derivative and the pressure difference cross. The time and size of the dip during the decline period depend on the natural-fracture properties—namely the storativity ratio ω and the matrix-permeability/fracture-permeability ratio λ. The terms λ, ω, and Xf can be determined from key time markers during the decline period—namely the start of the dip, the dip base, and the end of the dip. Table 1 in the full-length paper outlines the broad range of possible outcomes for the dual-porosity naturally fractured case.