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Abstract

Carbon Dioxide (CO₂) sequestration and enhanced recovery projects require the evaluation of rocks containing mixtures of CO₂, water, and gaseous or liquid hydrocarbons. Pulsed neutron logs of various designs and measurement types have been used since the 1960s to evaluate formations containing gaseous hydrocarbons, but they were not originally designed or characterized specifically for quantitative CO₂evaluation. Computer modeling, test pit data, and field examples are presented in this work to highlight the issues of CO₂ evaluation and to compare these with gaseous hydrocarbons.

Pulsed neutron tools emit 14 MeV neutrons from an accelerator source, but a wide variety of timing sequences, detector types, source-detector spacings, and signal processing techniques are employed by the industry to extract formation description parameters from the recorded counts. For the non-specialist petroleum engineer this variety can confuse and distract from effective use of the measurements. We organize all categories of pulsed neutron logs into simple types based upon the measurement physics to provide an effective guide to field use of these logs.

Examples of commercial and experimental tools in clastic and carbonate environments are presented. The examples show how CO₂ can be quantified and demonstrate critical design requirements for successful pulsed neutron logging campaigns. We outline the lessons learned and make recommendations for the design of logging programs and interpretation of the acquired data in stand-alone or in time-lapse modes.

Introduction

CO₂ sequestration and enhanced recovery projects from around the world include the Statoil Sleipner project in Norway (Baklid et al. 1996), Chevron’s  Barrow Island project in Australia (Flett et al. 2008) and the Rumaitha pilot project in Abu Dhabi (Al Hajeri et al. 2010). In many of these wells Pulsed Neutron Capture (PNC) and Pulsed Neutron Spectrometry (PNS) logs have been run for reservoir monitoring or surveillance. Pulsed neutron tools are very versatile and are used to evaluate the distribution and saturations of the various reservoir fluids.

However, pulsed neutron log interpretation is complicated in the presence of CO₂: as CO₂, which contains no hydrogen, gives rise to a tool response very different from water and hydrocarbons. We study the physics of pulsed neutron tools in the specific situations where the borehole and reservoir either are filled with water or with CO₂. We then extend the understanding we gain to practical CO₂ flood projects.