Applications of Nuclear Techniques to Reservoir Monitoring
Authors: Westaway P. , Whittman M. , Rochette P. 
Published: December 2007

Monitoring the variations with time of water, oil, and gas saturations in producing and observation wells is invaluable in determining the depletion profiles and recovery factors of reservoirs. Conventional nuclear logging techniques used in reservoir monitoring are described as well as an improvement in spectroscopic analysis. Field data obtained in the Middle East are presented.

Introduction

Nuclear logging techniques are based on the responses of the formation near the wellbore to high-energy neutron bombardment. These techniques permit evaluation of porosity and the nature and saturations of the fluids present in the pores. Important reservoir information therefore is accessible from open- or cased-hole producers or strategically placed observation wells.Regular reservoir monitoring, starting with a base or reference log recorded shortly after well completion (leaving sufficient time for filtrate invasion to dissipate), provides a "time-lapse" picture of the reservoir's progress through all stages of recovery.The precision with which conventional neutron methods are able to distinguish hydrocarbon from water deteriorates when formation water is very fresh. Here, accurate shale estimation becomes critical.Our examples have been chosen to summarize the responses and applications of conventional neutron logging tools. Also presented are results obtained with the IGT device, a new spectroscopy tool tested in the Middle East. This tool obtains saturation, porosity, lithology, salinity, and other information, even in conditions unfavorable to conventional neutron techniques.

Neutron Interactions

As neutrons emitted from a logging source travel out into the formation, they interact with atomic nuclei in their path. There are four broad categories of interaction: At high kinetic energies some neutrons undergo activation and inelastic scattering. Others suffer elastic scattering and are successively slowed towards thermal energies, where capture eventually occurs. Various nuclear logging tools measure the results of these neutron interactions, investigating either the resulting neutron population or the gamma rays which are emitted (except from elastic scatter). The principles of neutron logging, and the tool responses to various reservoir phenomena, are described in detail in Refs. 2 through 5.

Logging Tools

Using continuous neutron emission at several MeV (pJ) from a "chemical" source, the thermal neutron population density at a fixed distance from the source will be a measure of the slowing-down power of the formation, primarily through elastic scatter. This is dependent largely on the concentration of hydrogen present. This hydrogen index is related to porosity, the nature of the fluid in the pores, and to a lesser extent, lithology. As well as lending itself to open- and cased-hole porosity measurement, this method will respond to a change in the pore fluid provided there is a sufficient contrast in fluid hydrogen index. In this way gas can be distinguished from oil or water by its lower hydrogen content, which causes an apparent decrease in porosity reading.The following summarizes the neutron logging tools referred to in this paper, together with references for more detailed descriptions. The GNT uses a single detector to measure the thermal neutron population by counting capture gamma rays. It is now used mainly for correlation in cased holes, and for gas detection.

JPT

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