In-Situ Determination of Permeability Anisotropy and its Vertical Distribution - A Case Study
Authors: Peffer J. , O&apos;Callaghan A. , Pop J. 
Published: December 2007

Vertical permeability anisotropy the ratio Kv/Kh, is a critical unknown in the formulation of an accurate reservoir description. It is of particular importance during field predevelopment planning when errors in the assumed anisotropy could significantly impact the ultimate economic success of a project through poor decisions regarding well spacing, the projected success of water and/or gas injection schemes, and compression and/or water injection requirements, etc.

This paper presents results from a series of four vertical interference tests performed in a fluvial sandstone reservoir located in Algeria. The tests were conducted in openhole using a formation tester that consisted of a straddle packer module and a vertically displaced pressure monitoring probe. The objective of these tests was to determine in-situ permeability anisotropy for the various rock types identified on logs and cores obtained from other wells in the area. The interference test results were compared to laboratory measurements on whole cores and core plugs. These data were subsequently used to refine the permeability anisotropy input to pre-development reservoir simulation models used for full field development planning.

Introduction

The Triassic Argileux Greseux lnferieur (or TAGI) sandstone is fluvial in origin and is believed to have been deposited in a broad, low relief valley system, incised into an eroded Paleozoic surface. The TAGI sandstone units are approximately 5m to 15m thick and are interbedded in shales which are areally extensive. Petrophysical properties in the TAGI are good with average horizontal permeability derived from well tests being in the range 200 md to 700 md. Although the TAGI has long been productive in Algeria (El Borma, Berkoui), newly discovered fields in the central Ghadames Basin region lack both the well control and production history to determine permeability anisotropy and its effect on production. Determining this parameter is particularly important in Algeria as development plans include re-injection of all associated gas. This is a moderate to high GOR oil, 220 m3/m3 [1200 SCF/STB], and therefore the volumes of gas re-injected will be significant.

The major objectives of the openhole testing program were: - To confirm by in-situ measurements that the high Kv/Kh ratios (Kv/Kh &gt;0.1) obtained from core analysis do in fact characterize the TAGI sandstone.–To determine vertical permeability anisotropy for each of the major rock types identified in this fluvial system.–To quantify the impact on vertical anisotropy of the discrete claystone lenses seen in other wells in the area and to estimate their lateral extent.

Selecting the intervals to be tested on the basis of rock type would allow extrapolation of the results to other wells drilled in the area and should permit an improved geologic description. These data would also be used as inputs to the reservoir simulation model and should help to better ascertain the effects of gas and/or water injection on production performance.

Interference Test Design

The tests were performed using the MDT Modular Dynamics Formation Tester. The modularity of this tool allows several configurations to be utilized for the acquisition of data for the determination of horizontal and vertical permeabilities. The packer-probe combination used in Algeria is shown in Fig. 1. The vertical scale of these data - the tool has a 2 m packer-probe spacing - corresponds well with the vertical dimension of cell blocks used in numerical modeling of the reservoir.

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