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Abstract

Vertical permeability is an important input to coning and water influx studies and also choice of perforation policy. For British Gas Exploration and Production knowledge of this parameter is important for a number of small gas fields underlain by very large aquifers where water influx could significantly reduce reserves.

Conventional ways of measuring Kh/Kv (anisotropy) at reservoir scales have either proved unsuccessful or can only be undertaken once differential depletion effects become apparent.

A new openhole testing technique is described. It uses a dual inflatable packer run with a wireline formation tester as part of the logging program. The measurement scale is greater than that for core measurements and offers both versatility and precision to measure anisotropy within and between selected reservoir units at in-situ conditions.

The technique and analysis are briefly described, and results compared with other sources of permeability data. The paper concludes that this type of interference test can provide valuable and timely data that reduces the risk involved in a development decision and the cost of using conventional testing equipment.

Introduction

Production experience at the South Morecambe field has shown that vertical permeabilities are significantly lower than horizontal permeabilities in this area, ref. 1. Anisotropy varies from 5 to 500, but is typically 50. Figure 1 shows how this uncertainty affects reserve estimates.

The majority of the satellite fields are likely to be developed by one or two wells each, with either subsea completions tied back to existing platforms, or small 'not normally manned' platforms. As a result it will be difficult to determine vertical permeability after the start of production.

The MDT* Modular Formation Dynamics Tester tool was run in an exploration well to provide more information on anisotropy. The well was a technical success, but only encountered uncommercial quantities of oil. Nevertheless the MDT tool provided useful information on anisotropy within and between selected reservoir units at in-situ conditions.

Vertical and horizontal permeability were determined with the MDT tool using an inflatable dual packer. The packer was inflated using borehole fluids, and the formation straddled was then flowed to a sample chamber to create a sink pressure pulse. The drawdown and ensuing pressure build-up were monitored at an observation probe spaced 5 ft above the packer using a conventional formation tester probe.

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