Pressure Transients Characterize Fractured Midale Unit
Authors: Beliveau Dennis 
Published: December 2007

Summary
The Midale Unit is a naturally fractured carbonate reservoir in the Williston basin of southeastern Saskatchewan. Recently, the Midale working-interest owners initiated a CO2-flood pilot project with several closely spaced wells. A comprehensive interwell pressure-transient test program was conducted for reservoir-characterization purposes before pilot program was conducted for reservoir-characterization purposes before pilot operations began. The combination of conventional, pulse, and interference tests resulted in a detailed local description of this naturally fractured reservoir.
Selected pressure-transient data from the reservoir-characterization program are presented. The data dramatically illustrate the program are presented. The data dramatically illustrate the pressure-transient characteristics of an anisotropic, naturally fractured pressure-transient characteristics of an anisotropic, naturally fractured reservoir. Analytical techniques are reviewed briefly, and an interpretation technique for multiwell pressure-transient tests in wells with negative skins is included. A discussion of the pressure-transient behavior that is observed in wells with typical field spacing is also included.

Introduction
A significant proportion of the world's oil reserves exists in naturally fractured rocks. As a result, the pressure-transient behavior of naturally fractured reservoirs has been studied widely. These reservoirs, referred to as dual-porosity or dual-permeability systems, can be modeled with matrix blocks of various shapes separated by continuous fracture networks. Typically, the system permeability is largely associated with the fractures, while the tighter matrix contains most of the reservoir-storage volume and acts as a fluid source for the fracture network. Initial production from dual-porosity reservoirs is from the more permeable fracture network, which results permeable fracture network, which results in pressure decline in the fractures. Depletion of the fracture system causes pressure gradients to develop between the matrix blocks and the fractures, which results in fluid flow from the blocks into the fractures. This transitional flow period controls the reservoir-pressure behavior until the matrix fracture pressure regime equilibrates, after which the reservoir acts as a uniform system with composite properties.
Pressure-transient testing is an important technique used to describe naturally fractured reservoirs. Each flow period (fractures, transitional, and composite) has distinctive pressure-transient characteristics. Pressure-transient behavior in real Pressure-transient behavior in real dual-porosity reservoirs, however, can be subtle and ambiguous, especially in normal situations with typical well spacing. For example, pressure transients controlled by depletion of the fracture network are rarely seen in the field because of the fast onset of the transitional flow period. Transitional- and composite-system pressure transients can be attributed to such other causes as layers, skin, faults, or boundaries. Wellbore storage can obscure key parts of the pressure-transient curve. For these and other pressure-transient curve. For these and other reasons, few comprehensive pressure-transient data bases exist for naturally fractured reservoirs. The scarcity of good data impedes refinement and validation of precise pressure-transient models.
The Midale field (Fig. 1) produces oil from relatively tight, naturally fractured carbonates. The field was discovered in 1953 and produced competitively under primary depletion until unitization for waterflooding in 1962. Secondary performance is dominated fieldwide by the oriented-fracture system, which causes a permeability anisotropy of about 25:1. Recent implementation of a tertiary miscible CO2-flood pilot project in the Midale Unit, with 10 wells on 4.4 acres [1.8 ha], has allowed for very detailed reservoir characterization. Conditions in the Midale CO2-flood pilot are amenable to both single- and multiwell pressure-transient programs. An extensive, precise pressure programs. An extensive, precise pressure transient data base was acquired during pilot-testing operations. The reservoir pilot-testing operations. The reservoir model that was obtained from pressure-transient analyses is consistent with geological, petrophysical, and operational data, and it petrophysical, and operational data, and it provides significant refinement over earlier provides significant refinement over earlier reservoir models.