Abstract

Evaluation of three-phase reservoirs in gas injection fields has been a major challenge in the Asia-Pacific region. Low-to-mid porosity and complex wellbore completion environments create a difficult, pulsed neutron logging environment for analyses of remaining oil saturation, fluid contacts, and identification of flood breakthrough.

A new, through-tubing, pulsed neutron measurement system has recently been introduced to the industry in the Asia-Pacific region. Refinements have achieved the highest available spectral resolution and accuracy of any slim tool, extending multi-phase reservoir evaluation to lower porosity formations and to more complex wellbore environments. Features of the tool include single-pass simultaneous carbon/oxygen (C/O) and capture cross section (S) logs, quantified water flow analysis, porosity logs, and elemental analysis, all acquired at higher speeds than previously available. Modern tool characterization and adherence to well-established best practices have enabled reliable decision making based on application of the higher quality pulsed neutron measurements to these difficult situations, which were previously considered infeasible.

This paper discusses potential for economic gains that can be realized from enhanced reservoir characterization with the new pulsed neutron system. Additional hydrocarbon reserves can be delivered from application of the modern technology within these more challenging environments. Log examples included in this paper will show that the new tool is capable of determining gas, oil, and water volumes in formations with less than 10 percent porosity. It includes details of an interpretation method tailored to a specific reservoir yet applicable to many multiphase situations. With advanced pulsed neutron devices as part of a well designed investigative package, more robust support of reservoir management can be realized.

Introduction

In the face of recent high oil prices and frequent supply shortages, operators are consistently looking to their existing fields as potential sources of increased production. By locating bypassed hydrocarbon producing intervals and by better managing and monitoring existing production, operators can economically increase returns on these fields.

The Asia-Pacific region is of particular interest in that operators are using water and gas injection technology in an attempt to increase production. Knowledge of how the flood is progressing through the reservoir is extremely critical to the bottom line economic aspect of these projects. A regular logging program of reservoir monitoring logs is essential to obtaining the knowledge needed to manage and maximize product from the reservoir and to yield an improved economic return.

While much has been written about the use of pulsed neutron capture logs (PNC) in reservoir monitoring projects,1,2 it is also known that these tools work best in formations of known, high water salinity. In most water floods, fresh water or seawater is used as the flood agent. When these waters are flooded into formations that had contained water of known salinity, the salinity of the formation water is changed and the use of PNC logs, as a reservoir monitor log, is no longer feasible. (Note, however, that PNC logs are applicable for determining gas saturations in formations containing only fresh water.)

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