Abstract

In well testing, an important corroboration of the interpretation can be made from integrated analysis of core and geological information. The kh product, skin, and flow regimes from the pressure data should all be consistent with the petrophysical data and the geological interpretation.

It has been observed that certain types of reservoirs show negative skin despite the absence of any natural or induced fractures. This skin has been interpreted as due to the presence of thin high (with respect to the matrix) permeability channels. These channels have the well testing signature normally associated with fractures and are termed "Pseudo-fracture channels" (PFC). The skin (negative) due to the limited extent of these features around the well bore, results from purely geological phenomena and is termed "geoskin". Modelling studies have mapped out the relationship between thickness, radius, number and location of PFCs, the permeability contrast with the matrix, and the quantity of negative geoskin. With some understanding of the external limits of the reservoir the lateral extent of these phenomena can be estimated.

We show the application of the model in a braided fluvial reservoir. The well test in two wells are solely the response to different architectural organisation of the same matrix and channel permeabilities. The model shows that the two wells lie at the extremes of a continuum in which the scale of channels is the only varying parameter. This knowledge is useful in the understanding of reservoir performance and allows the well test data to be used more effectively in the reservoir model.

This study also shows that the interpretation of the skin can be an important measure of the near well bore properties and should be expected in braided fluvial systems (i.e., high net: gross) where high permeability, small scale (relative to the tested volume) channels can be present. Other high net:gross reservoirs were there are high permeability contrasts may also show such phenomena.

Introduction

Braided fluvial reservoirs are important hydrocarbon reservoirs worldwide that are characteristically highly heterogeneous as a result of the sedimentary processes. In these systems, permeability is largely controlled by the primary textural parameters of the sediment. The variability in permeability is a function of sedimentary architecture and this determines the characteristics of the well performance as shown by, the well test response. P. 87

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