Several of ADCO's oil fields in Abu Dhabi are giant carbonate reservoirs, some with producing lives in excess of 100 yrs. Generally water-injection is installed to support natural water flood & maintain reservoir pressure, but in some reservoirs gas-injection is being evaluated as an alternative or complimentary recovery mechanism. Potential gains from improved recovery can be large. Several small scale pilots are underway to optimize the next phase of field development. The monitoring of the gas-flood progression and efficiency is a key part of the evaluation of these schemes. High-resolution measurements of formation gas saturation through casing using Pulsed Neutron Capture logs are routinely utilized to establish gas break through times and sweep efficiency.
The recently introduced RPM-C logging tool utilizes three detectors to improve the accuracy of the results by allowing gas detection ratios to be taken over a longer spacing than conventional pulsed neutron instruments. The new tool investigates a larger volume and is potentially more sensitive to gas-filled porosity. The measurements are independent of formation water salinity. Characterization of the gas response of the tool for a variety of reservoir conditions allows for more accurate quantification of gas saturation. In addition to defining the sweep in a qualitative manner, we have investigated the possibility of using these data to estimate the residual oil saturation in gas flooded zones. Key input data include detailed knowledge of variations in porosity and mineralogy as well as fluid properties. We need to know the hydrogen index of the fluids in the reservoir to evaluate these neutron logs accurately. Trial results from wells in Abu Dhabi will be presented to define best practices and demonstrate the value of this approach.
