Methods for Determining Residual Oil with Pulsed Neutron Capture Logs

Abstract

The special techniques described here appear to be promising for determining precise quantitative residual oil saturations - under certain reservoir conditions. A field test of one method was encouraging. The PNC logs showed saturations that agreed with other measurements, and the degree of uncertainty in the estimates approached theoretical expectations. Purpose and Scope The basic physical principles of pulsed neutron capture (PNC) logging and descriptions of logging instrumentation and operation have been documented in earlier publications.3,4 In addition, several methods of PNC data interpretation for petroleum engineering applications have appeared in the literature.6 These latter reports demonstrate that PNC logging has become a valuable technique for formation evaluation. The most significant use of PNC logging has been to discriminate among gas-bearing, oil-bearing, and salt-water-bearing formations in cased holes. For example, gas-oil, gas-water, and oil-water contacts have been found and followed. Unsuspected hydrocarbon migration between zones also has been detected. In addition, water saturations have been calculated from PNC log data when porosity, rock type, formation hydrocarbon type, and formation water salinity were known or could be estimated. Interpretations of PNC log data are often precise enough for decisions regarding recompletion or for a qualitative assessment of reservoir mechanism and performance. But how useful are these data when we must decide whether or not to conduct a tertiary recovery project in the flooded-out portion of a reservoir? In comparison with waterflooding, tertiary recovery is more complicated; thus its performance is harder to predict. Also, the process costs are higher, so there is much less margin for error. Therefore, many of the parameters must be measured with great accuracy for designing a tertiary recovery project. One of the most important parameters is the residual oil saturation - the target for profit. A decision to undertake a tertiary recovery project could depend entirely on the estimate of residual oil saturation and the margin of error associated with the estimate. Minimum expected errors in oil saturation and saturation distribution (among other parameters) are essential for a tertiary recovery project. The purpose of this paper is to describe in some detail the possibility of using PNC data to estimate more precisely the residual oil saturation remaining at the end of a waterflood (either natural or induced).