Pulsed-Neutron Monitoring of the First CO2 Enhanced-Oil-Recovery Pilot in the Middle East

Abstract

Summary Logging measurements in the borehole are vital for monitoring carbon dioxide (CO2) floods—for assessing the fluid changes in the reservoir rock as well as in the wellbore. The saturation profile at each well location provides the efficiency of the flood process for fluid displacement within the pore and the vertical sweep across and within the reservoir zones. A snapshot from multiple well locations in the reservoir enables the creation of a picture of the flood flow pattern, and the time-lapse surveys track the progress of the flood with time. Pulsed-neutron logs provide essential measurements for the evaluation of saturation in the injectors, producers, and observers. However, the CO2 environment, with the fluid in the borehole, remains uncharacterized in the industry. Hence, reliable inferences require either that 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 modeled after the physics of the measurement to the maximum extent possible. On the first CO2 enhanced-oil-recovery (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 openhole evaluation to help create a coherent picture of the reservoir. The fluid analysis from the pilot wells confirms the high displacement efficiency of CO2 as an EOR fluid. The saturation profiles from individual wells portray the vertical sweep of the flood, and the snapshot from the multiple wells gives the areal sweep. Combined with the data from production-log sensors and permeability from the magnetic resonance, the flood-breakthrough layers are identified.