Determining the Elementary Matrix Block In a Fissured Reservoir Eschau Field, France
Authors: Janot Pierre 
Published: December 2007

The fracturing of a reservoir is not a random phenomenon; it appears that the rock splits up into regular blocks bounded by networks of parallel fissures. A method is described for combining information parallel fissures. A method is described for combining information deduced from cores and outcrops to obtain a statistical distribution of fracture characteristics. From this we deduce the orientation, the dimensions, and the volume of the average block.

Introduction

In a fractured reservoir, recovery ratio and methods of calculating it are closely connected to the volume and size of the matrix blocks. The reason is that the volume and size of the block influence the way hydrocarbons escape from the rock matrix as the reservoir is depleted. This observation is valid for any depletion process, whether it be imbibition, solution gas drive, or gravity drainage. Furthermore, the orientation of blocks (which is derived from the orientation of bounding fractures) will greatly influence the flow of fluid toward the wellbore, and consequently will directly affect the location of production wells and the choice of recovery process, production wells and the choice of recovery process, both of which, in part, determine the recovery ratio.

Various numerical simulation models have been designed to calculate future production, based on assumptions involving drainage mechanisms; basic to the functioning of the model are data on the characteristics of the matrix block. It would be highly desirable, then, to be able to gather data on the volume, size, and orientation of the matrix block for each well in a field.

Such a goal can be achieved through production geology, because rock fracturing is not a random phenomenon, but takes place mainly under the effect phenomenon, but takes place mainly under the effect of tectonic stresses that accumulated with the rock mass during the geological history of the formation.

In a study of the Laurel Gas Storage (Hocking County, Ohio), the U. S. Bureau of Mines noted that fracture orientation measured at an outcrop was similar to fracture orientation determined from oriented cores (taken at a depth of approximately 2,460 ft), and that hydraulic fracturing induced in an unfractured reservoir sample leads to a fracture orientation conditioned by the stresses that currently exist. The orientation of the induced fractures is superimposed upon that of the existing fractures.

V.N.I.G.R.I. Institute of Leningrad (USSR) published similar observations based on data gathered published similar observations based on data gathered in the field from outcrops as well as on data derived from cores.

From measurements of outcrops, the geologist can arrive at basic laws relating to the orientation and magnitude of fracture networks at major structural features, as both depend essentially upon tectonic stresses.

The theories that now exist are general; and the laws behind them are only partly known; the theories may in some cases be used to classify into families all of the fractures encountered in cores, even if the outcrop corresponding to the subsurface structure being studied is actually missing. Subsurface observations made within a reservoir can thus be explained, and characteristics of the elementary matrix block can be determined.

The aim of this study is to show, with a concrete example, the steps to take in determining the elementary, matrix block. The example is an application of a general method to a particular case.

JPT

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