Pressure-Transient Testing of Low-Permeability Multiple-Fracture Horizontal Wells
Authors: Denney Dennis 
Published: February 2015

This article, written by Dennis Denney, contains highlights of paper SPE 163983, ’An Approach to Practical Pressure-Transient Testing of Multiple-Fracture-Completed Horizontal Wells in Low-Permeability Reservoirs,’ by Richard Volz, Elvia Pinto, and Omar Soto, SPE, BP America, and J.R. Jones, SPE, NSI Fracturing, prepared for the 2013 SPE Middle East Unconventional Gas Conference and Exhibition, Muscat, Oman, 28-30 January. The paper has not been peer reviewed.

A common well-completion configuration for shale-gas wells is a horizontal well with multiple transverse hydraulic fractures. This configuration is becoming common for tight gas reservoirs. Pressure-transient testing of this completion configuration has not been considered practical or useful because extracting completion parameters (e.g., fracture conductivity and fracture half-length) from the recorded response requires estimating the effective formation permeability. Estimating permeability directly from a buildup or drawdown test can be performed only if data from the radial-flow period, which reflects this parameter, are recorded. Unfortunately, for this completion configuration in a low-permeability reservoir, this flow period occurs only after extremely long shut-in or flowing times.

Introduction

The usual shale-gas completion is a cased-and-cemented horizontal well, perforated with multiple perforation clusters. Each perforation cluster is treated with an independent fracture stimulation with a large volume of fluid and relatively low proppant concentration. The goal is to create an induced-hydraulic-fracture system, spaced along and covering the length of the horizontal well to provide a large, effective surface flow area in the reservoir. Interpreted microseismic images of these multistage-stimulation treatments indicate that, in some reservoirs, the fractures created at each perforation cluster have dominant transverse components. These dominant transverse components may or may not be interpreted to be overlain with or coupled to a more-complex system of secondary fractures. Even if the secondary system of more-complex fracturing occurs, the relative importance of this secondary fracture system and its relative contribution to the deliverability of the overall induced fracture system remain subjects of debate. It is equally difficult to establish or deny the importance of the overprint of a natural-fracture system to the flow response and performance of multiple-fracture horizontal wells (MFHWs) in unconventional reservoirs.

Therefore, the natural starting point for developing a useful pressure- transient- analysis method for horizontal-shale- well data would be on responses from horizontal wells affected by induced transverse fractures alone. Building viable pressure-transient-data interpretation methods for this simple base-case problem was the goal of this work. This interpretation must be accomplished before considering how to solve the more general cases that include induced- and natural-fracture complexity. The main issues in analyzing the pressure-transient response from an MFHW are the same as those with a vertically fractured well.