Technical Paper: Pulsed Neutron Monitoring of the First CO2 EOR Pilot in the Middle East

Society: SPE
Paper Number: 141490
Presentation Date: 2011
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Using borehole measurements to monitor carbon dioxide (CO2) floods in the reservoir is essential to properly understand the behavior of the injected fluids and the flow patterns. Pulsed Neutron logs provide essential measurements for the evaluation of saturation in the injectors, producers and observers. However, the CO2 environment is as yet uncharacterized in the industry. Hence, reliable inferences require that either the measurement is immune to the borehole environment or that the perturbation is minimal and can be easily corrected. Where corrections are required, suitable benchmarks should be planned in advance to verify the accuracy of the corrections. These corrections should be based on the physics of the measurement to the maximum extent possible.

On the first CO2 EOR pilot project in the Middle East—unique in the world because the CO2 flood was implemented with the reservoir at original oil saturation—several pulsed neutron surveys were recorded in the injector, observer, and producer wells. The surveys included capture and inelastic mode acquisition. Several novel techniques of data acquisition and interpretation were successfully tried. This paper presents the steps in planning and executing the jobs and the results of the surveys. Limitations of existing characterization and those imposed by the measurement environments in the subject wells are discussed and we show, through comparison with benchmarks, that correction for the unusual borehole environment is possible. The paper illustrates how the different modes of pulsed neutron data acquisition complement each other in the individual wells in assessing the borehole environment, providing adequate input data to enable a multiphase reservoir fluid analysis, and yielding independent fluid saturations for effective comparison. The results of the analysis are compared with an openhole evaluation to help create a coherent picture of the reservoir.

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