An Approach to Evaluating Fractured Reservoirs
Authors: Nelson R.A. 
Published: December 2007

Fractured reservoirs are much more difficult and expensive to evaluate than more conventional reservoirs. In evaluating fractured reservoirs, a particular sequence of study is shown to be effective. This evaluation sequence leads the evaluator to delineate the fracture system properties and their role in overall reservoir performance in a timely and efficient manner.

Introduction

The petroleum industry must evaluate and produce fractured reservoirs with increasing frequency. This increase is caused partly by the necessity to explore in progressively poorer quality reservoirs with time and partly by an increased awareness of the presence and the effect of fractures in subsurface reservoirs. This is especially true as more economic interest is generated in effective secondary and tertiary recovery, where permeability anisotropy imparted by fracture systems plays a large role in field development and ultimate recovery.

Evaluation and development of such reservoirs are very difficult. Many parameters must be investigated in either a qualitative or a quantitative manner. This paper presents a particular sequence of study that proved efficient in organizing and advancing a fractured reservoir evaluation. This sequence includes the following steps.Interpret fracture system origin by using geologic and rock mechanics data to increase the predictability of fracture distribution and morphology.Determine reservoir properties of the fracture network and how they change with depth and reservoir pressure depletion.Determine the degree of interaction between matrix porosity and the fracture system.Determine the type of fractured reservoir, on the basis of what properties the fracture system provides to the overall reservoir, to evaluate recoverable reserves and potential production problems.

This evaluation sequence, which delineates the fracture system properties and their role in overall reservoir performance, is addressed in the next section.

Discussion

The following addresses each of the four major headings in this particular evaluation sequence, including the general types of interpretations and analyses required. The purpose of this discussion is primarily a development of the study sequence and not a detailed discussion of the individual analyses and evaluations performed.

Fracture System Origin

After natural fractures first are detected in a subsurface reservoir from cores, logging tools, or anomalously high flow rates, it is extremely important to postulate the origin of the fracture system. This is done by applying data on fracture dip, morphology, strike (if available), relative abundance, and the angular relationships between fracture sets to various empirical models of fracture generation. Such data can be obtained from full-diameter core (oriented or unoriented), borehole televiewer output, or several other less appropriate logging tools. The fracture models available range from various types of tectonic models to others that are primarily diagenetic. These are discussed in detail by Stearns and Friedman1 and Nelson.2 Only by a proper fit of the available fracture data to one of these genetic fracture models can any effective extrapolation or interpolation of limited data be made.

Fracture System Origin

After natural fractures first are detected in a subsurface reservoir from cores, logging tools, or anomalously high flow rates, it is extremely important to postulate the origin of the fracture system. This is done by applying data on fracture dip, morphology, strike (if available), relative abundance, and the angular relationships between fracture sets to various empirical models of fracture generation. Such data can be obtained from full-diameter core (oriented or unoriented), borehole televiewer output, or several other less appropriate logging tools. The fracture models available range from various types of tectonic models to others that are primarily diagenetic. These are discussed in detail by Stearns and Friedman1 and Nelson.2 Only by a proper fit of the available fracture data to one of these genetic fracture models can any effective extrapolation or interpolation of limited data be made.