Statistical Calculation of Elementary Matrix Blocks in Fractured Reservoirs
Authors: Ghez Fabien , Janot Pierre 
Published: December 2007

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The search of optimum development conditions of a fractured reservoir rests upon the proper description of the fracture system. Consequently, sizes and volumes of matrix blocks have to be defined at each point of the structure.

Geometry of the considered medium (juxtaposition and shape of blocks) is generally too intricate to lend itself to calculations. In a previous communication, this difficulty had to previous communication, this difficulty had to be overcome by a consideration of mean values (dips, azimuths, fracture spacing), which led to a magnitude of volumes being considered.

A block-volume distribution law cannot be rendered by a mean volume, though. It is precisely this distribution law that determines precisely this distribution law that determines selection of one or several successive recovery processes. An original statistical calculation processes. An original statistical calculation method of the distribution law of matrix blocks, which can be applied at any point of the pool, is thus presented. The pool part concerned with blocks of a given volume can then be derived.

General knowledge of natural fracturing phenomenon is the basis of the model. phenomenon is the basis of the model. Subsurface observations as to reservoir fracturing provide the model data (fracture orientation and provide the model data (fracture orientation and spacing histograms).

Application to Eschau pool (Alsace, France) is reported here to illustrate the method.

Introduction

In a fractured reservoir, recovery ratio and the means to obtain it are closely connected to volume and sizes of matrix blocks.

Petroleum field geologists have observed that the fracturing process does not take place in an arbitrary manner, but develops generally along a pattern of nearly parallel fractures, of which cross cut results in a juxtaposition of regular blocks. Volume, sizes and orientation of these blocks vary from one point of the structure to another, depending on tectonic stresses that took place.

Conclusions drawn from outcrop observations, as well as fracturing study methods at each structure point based on cores, led to qualitative determination of block-size magnitude through reasoning upon mean values.

It rapidly became evident that previous estimates of mean volume were insufficient to characterize efficiently the natural fracturing of a reservoir.

However, in order to permit a more accurate evaluation of recovery ratio (by comparison with laboratory studies) knowledge of block volume and size distribution is very useful.