A new flood monitoring technique is presented that models both the capture cross section (sigma) and ratio/porosity responses of a pulsed neutron logging tool using known reservoir, hydrocarbon and flood agent properties. Open hole logs and pulsed neutron logs acquired before, during and after flooding are combined to determine individual hydrocarbon, saltwater and flood agent saturations. This new model uses total neutron migration length and direct ratio computations1  to compute the flood agent volume fraction. Improved accuracy of the computed fluid saturations and the ability to handle specific flood agents are two enhancements to flood monitoring provided by this new approach. Accuracy is especially important because often the results from a limited number of monitor wells are used to project the sweep efficiency and economics of the flood process for an entire reservoir.

This new method is applied to a CO2 flood and the resulting fluid saturations for oil, saltwater and CO2 are analyzed and discussed. These results are compared with those based on the older excavation effect method.2,3,4 

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