Abstract

Until recently, the only parameter that usually could be accurately determined from log analyses of wells that had been flooded for secondary recovery was porosity. Now, using new techniques, it is often possible to determine whether the primary fluid produced from a zone will be connate water, flood water, oil, or gas. Actual gas saturations can be calculated, and, in open hole, wireline formation-pressure tests can be made.

This paper will discuss several log-interpretation techniques which can be used to help define the zone-by-zone production of a producing well in a field being flooded. Their further implementation by wireline-formation-pressure tests in open hole will also be mentioned. All of these techniques have been used successfully in the Permian Basin. Their uses and limitations will be briefly outlined.

Introduction

A major problem in log evaluation of fields being flooded has been that, due to the encroachment of flood water, the waters in different zones frequently have different salinities. It has been impossible, therefore, to compute water saturations accurately with just a resistivity log or just a pulsed-neutron-capture (PNC) log, since such pulsed-neutron-capture (PNC) log, since such calculations require that the analyst know the water salinity in each zone. However, using the two tools together it is usually possible to differentiate oil-producing zones from flood-water-producing zones. Thus, flood-breakthrough zones may be identified and squeezed or plugged off.

In addition to producing unwanted flood water, the operator often finds that a secondary gas cap has developed in his reservoir, causing gas production to increase drastically. Using cased-hole logs, the gas-oil contact can be defined. Gas saturation can be determined through the use of PNC and porosity logs for either open-hole or cased-hole completions. Under some conditions, oil and water saturations may also be determined.

In the Permian Basin oil wells have commonly been completed open hole. In these wells, a Repeat Formation Tester (RFT*) can be used to take as many formation-pressure tests as desired on one trip in the hole.

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