Three-Phase Saturation Evaluation Using Advanced Pulsed Neutron Measurement

Abstract

Accurately monitoring saturation change mechanisms requires adequate surveillance methods and techniques. We present a methodology to evaluate three-phase saturation using an advanced pulsed neutron measurement. This is a complex reservoir monitoring situation, where gas saturation must be monitored in addition to oil saturation, in a variable water salinity environment. An advanced pulsed neutron logging tool provided robust thermal neutron measurement (hydrogen index) for gas quantification. Formation capture cross section (sigma) was not used for water saturation because of its sensitivity to water salinity, which changes vertically and laterally in the subject field. The apparent volume of oil from the tool's improved-precision carbon/oxygen (C/O) method provided a salinity-independent indicator of oil saturation. Since this C/O apparent oil volume combines the carbon contributions from oil and gas, elemental modeling provided the apparent oil volume response to gas. Lithology information and porosity from initial formation evaluation were also entered in a linear solver to resolve water, oil, and gas volumes. This methodology was applied in wells where all three fluid saturations (water, oil, and gas) were expected to change over time. Surveys were taken at regular intervals over a span of several years. With the improved precision of the advanced pulsed neutron measurement, it was possible to precisely map the saturation changes with time in the field and identify variations in the fluids’ volumes down to a few porosity units. This information was critical in understanding fluid movements inside the reservoir. This is the first implementation of this technique. The precision brought by the advanced pulsed neutron tool provides superior results for monitoring a complex fluid mixture.