Abstract

The author has worked on a number of fractured reservoirs in Western Canada, which show a number of common characteristics. Evaluating this type of pool is normally very complex and presents a number of difficulties. Production performance is discussed. They also exhibit distinct types of pressure transient response, which do not correspond to Warren and Root type dual porosity description. Stimulation results have proven difficult to predict.

A reservoir characterization is presented which is consistent with observed production performance, pressure transient responses, production logging results, core analysis and well stimulation. A key component is structural geological style.

This description has been applied to a number of different reservoir situations. It has application to stimulation design, predicting reservoir performance, numerical simulation and pressure transient analysis. An example is highlighted for a gas condensate reservoir.

Introduction

The material in this paper was derived from a number of large detailed reports. Such a detail and volume of material cannot be covered in a single technical paper. The paper therefore only presents a summary of a number of key concepts derived based on the author's experience. The following is discussed:

  • Systematic Approach

  • Core Descriptions

  • Structural Style

  • Fracture Patterns

  • Effect Of Stress

  • Discrete Element Analysis

  • Pressure Transient Observations

  • Mechanisms

  • Example Application: Gas Condensate

This paper is complemented with a companion paper1 "Optimization of the Blueberry Debolt Oil Pool: Significant Production Increases for a Mature Field", which is also presented at this conference.

Systematic Approach

Evaluating a fractured reservoir (after Nelson2) involves four main steps:

  1. Interpreting the origin of the fracture system. This information allows one to predict geometry and the extent of communication.

  2. Determining petro-physical properties of the fractures and matrix. This allows for prediction of the variation in reservoir response. The relative storage (i.e. porosity) must be determined as well as effective permeabilities. Another important property is compressibility.

  3. The flow interaction between the matrix and fracture system is evaluated to determine ultimate reserves from the reservoir.

  4. Classification of the reservoir. Depending on the type of flow interaction the reservoir will fit one of several depletion strategies. Note that most of the variations in strategy apply to waterflooding oil reservoirs.

Conventional Core Analysis

To obtain most of the base information requires some basis of observation, which for most reservoirs starts with core. Information can be derived from the following conventional core analysis data and plots:

  1. Core permeability vs core porosity - all data.

  2. Core permeability vs core porosity - sorted by lithology.

  3. Vertical permeability vs horizontal permeability.

  4. K90 plotted vs Kmax.

Fractured reservoirs do not show the typical straight line relation on core permeability vs core porosity (semilog) plots. Typically lower porosity rock is more prone to fracturing. Fractured reservoirs also tend to have higher anisotropy, which is seen as large variation in K90 vs Kmax.

Core Description
Example From Central Foothills

In the author's experience, based a number of studies in Western Canada, there is a typical core permeabiliy vs core porosity relati

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