Fractured Reservoirs: Turning Knowledge into Practice
Authors: Nelson R.A. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Fractured reservoirs have always been difficult to describe and toevaluate. Great advances have been made in understanding how to incorporatediverse data sets from several disciplines into coherent reservoir studies. Numerical simulation models offer an effective vehicle for such synergism. Theneed for better laboratory testing methods for fractured rock samples. conceptual models of broader applicability, and numerical modeling systemscapable of more complex reservoir descriptions and conditions remains.

Introduction

The exploration, evaluation, and development of fractured reservoirs havebeen important topics among both major and independent oil companies over thepast 10 to 15 years. During that period, fractures have gone from a relativeoddity in reservoir engineering, to a major aspect of many of the new fieldsdiscovered each year. Indeed, our knowledge of subsurface fracture systems hasprogressed to the point that both their qualitative and quantitative effectscan be evaluated in a variety of technical ways. Such approaches can vary fromthe use of simple two-dimensional (2D) flow anisotropy in depletion and sweepefficiency calculations to complex dual-porosity/dual-permeabilitycompositional-phase-behavior reservoir performance predictions.

This paper highlights some of the important advancements made in dealingwith fractured reservoirs in the past 10 years, how we might use this knowledgein our work today, and what we will need to learn in the next 10 years to doour job better. In accomplishing this, emphasis is placed on integration ofdata from several disciplines and iteration through reservoir modeling. Therefore, those techniques that either have or will facilitate generalizediterative models for fractured reservoirs are stressed.

Models

The petroleum industry has historically favored previous experience as aguide to technical prediction. So-called "look-alike" fields, or producedfields that appear to have characteristics similar to the one in question, arefrequently depended on heavily in much of our development work. Theselook-alikes are generally the vehicle for data integration in that once theproper reservoir is chosen by predominantly descriptive methods, the response, stimulation techniques, and development plans can be inferred by analogy. Infractured reservoirs, however, this approach has not proved as successful as inmore conventional reservoirs. It appears that the complex interaction offractures, matrix, and fluids is sufficiently variable to render each fracturedreservoir unique. In addition, most older produced fields that could be used asanalogies or look-alikes lack the necessary quantitative characterization ordescription of the fracture system present. The result has been a tendency torely less on look-alike models in fractured reservoir work and more onconceptual and numerical models.

Conceptual models generally are used as a guide for interpreting individualdata sets; for example, short-term flow rate or pressure-transient data. Conceptual models for fractured reservoirs have run the gamut from very uniformengineering approaches (Fig. 1) to very complicated geologic ones (Fig. 2). Real fracture distributions can be quite simple in some cases, such as withregional fractures or orthogonal joints (Fig. 2a). In other situations, such aswith tectonic fractures on a fold, however, the fracture patterns can be quitecomplex (Fig. 2b).

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