Abstract

It is recognised that depositional and tectonic processes can result in anisotropy in horizontal permeability. Previous authors have described inter well interference tests which can be used to determine the magnitude and direction of permeability anisotropy. permeability anisotropy. Earlougher describes the elliptical flow regime resulting from anisotropic permeability. He suggests that it is impossible to recognise anisotropy from a single well test as the pressure response curve is the same as for isotropic reservoirs, This is thought to be valid when the anisotropy occurs as a result of very small scale permeability variations.

The authors have observed that interpretations of well tests conducted on certain formations in the North Sea yield negative skins. The common feature of these formations is that they are prone to anisotropy.

This paper describes the flow regime resulting from large scale anisotropy. The reservoir modelled is assumed to contain a regular series of parallel partial baffles in an isotropic permeability field. Conventional radial flow is followed by an permeability field. Conventional radial flow is followed by an elliptical pseudo radial flow regime, akin to that described by Earlougher. In certain types of anisotropic formation, the skin associated with this flow regime is negative.

A new family of dimensionless type curves is presented which can be used to quantify permeability anisotropy from the pressure response at the disturbing well only. The type pressure response at the disturbing well only. The type curves have been generated numerically but limiting cases have been compared with available analytical solutions to confirm accuracy. The effects of wellbore storage and producing time on pressure response are also considered. Pressure responses are observed to be similar to those from other lateral hetrogenieties. This lack of unique model reinforces the need to interface with other disciplines in well test interpretation.

Field and synthetic examples are used to illustrate use of these type curves.

Introduction

It has long been recognised that many formations exhibit varying degrees of horizontal permeability anisotropy, horizontal in this sense meaning bed parallel. The geological basis for this anisotropy can be either depositional or tectonic. Scale and magnitude can vary widely. For example, depositional environments which can result in anisotropy include aeolian and fluvial environments. In the former, centimetre scale cross-bedding perpendicular to the paleo-wind direction can reduce permeability in that direction. In the latter case, shaley channel boundaries which lie parallel to the paleo-current, can reduce permeability perpendicular to paleo-current, can reduce permeability perpendicular to channel axes. In this latter case the anisotropy is often only apparent at a scale of tens or even hundreds of feet (metres).

The occurrence of fractures in formations as a result of tectonic activity may also influence directional permeabilities. Open fractures will enhance effective permeability parallel to the fractures, cemented fractures will degrade permeability perpendicular to the fractures. The magnitude of permeability perpendicular to the fractures. The magnitude of permeability anisotropy is a function of the -x and -y directional permeability ratio. This can range from unity over several permeability ratio. This can range from unity over several orders of magnitude.

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