Extending Production Petrophysics Applications in Monitoring Complex Recovery Mechanisms

摘要

Due to variable reservoir properties, displacement mechanisms and well access issues, cased hole reservoir surveillance for fluid saturation is often challenging. Traditional methods such as capture cross section (sigma) and carbon/oxygen logging are limited in these environments and new techniques are often needed to successfully monitor fluid changes. This paper discusses the application of two new pulsed neutron based techniques that utilize capture and inelastic gamma ray data acquired with a three detector pulsed neutron instrument. Measurements are combined with theoretically modeled tool responses for different fluids (water, oil and natural gas or carbon dioxide) to determine saturations. Inelastic and capture ratio measurements from the first and third detectors are used because they exhibit higher formation sensitivity than conventional two-detector instruments. These ratios respond differently to changes in oil/gas densities, compositions and formation water salinities, and these differential responses can be used simultaneously to solve for a three-phase saturation solution. This paper presents results from two mature fields in Alaska. The first example is a three-phase saturation analysis in a light oil reservoir with pressures close to bubble point where traditional sigma and carbon/oxygen logging has been unsuccessful. The new technique located bypassed oil which was produced by adding perforations and confirmed by running production logs. In a second example, the problem was the surveillance of gas cap injection water with seasonal source water salinity variations injecting into a low porosity conglomeratic zone. Trapped gas saturations derived using the new technique were consistent with lab derived core flood measurements. This new generation neutron data has been used to help reduce the range of uncertainty in water salinity and improve the estimation of water saturation within the gas cap even in the low porosity conglomerates. The new techniques provide an effective solution in environments where conventional pulsed neutron measurements have high uncertainty. Future applications of this measurement in similar areas include the monitoring of reservoirs with complex lithologies where the calibration points are difficult to define and the distinction between gas condensate and dry gas have been problematic for traditional techniques.