The Use of Pulsed-Neutron Capture Logs for Reservoir Management in the Midway-Sunset Field

Abstract

Summary Systematic time-lapse pulsed neutron capture (PNC) logging was conducted during a two-year steam foam mechanistic field trial in a previously steamflooded reservoir. The producing reservoir tested was the Monarch Sandstone, which is composed of thinly inter-bedded sandstones, siltstones, and diatomaceous mudstones at the base, coarsening-upward to massive conglomerates and sandstones with a few interbedded mudstones at the top. Observation wells were drilled in the study area, one of which was continuously cored through the reservoir interval (578 feet). Two of these cased wells were emptied to facilitate better temperature logging. Uncalibrated PNC log responses in the air-filled wells were normalized (calibrated) to water-filled conditions with transform functions empirically determined in the same wells. The normalized logs were used to generate a series of steam/gas saturation profiles that indicate clearly the dynamics of the fluid and foam in the reservoir during the experiment. Enhanced precision and bed resolution of the PNC logs were necessary for interpretations in the thinly bedded part of the reservoir. The time-lapse profiles were capable of delineating flow units down to a few feet in thickness and showed excellent consistency with lithologic variations described in the core. In addition, the PNC log detected mudstone layers as thin as six inches. Correlations between neutron capture cross sections (sigma), mineralogy, and rock chemistry, all from analyses of core samples, were examined to investigate additional use of the PNC logs for steamflood reservoir characterization. As expected, sigma increased with increasing clay/mica content. However, a more significant correlation was noted between sigma and diatomite content, which is due to the boron in the diatomite. This finding may result in improved stratigraphic correlations of the more laterally continuous diatomaceous mudstones based on PNC logs, with significant impact on the characterization of reservoirs of this type. The PNC log data from the field trial, when used in conjunction with core, temperature, and pressure data, was critical in developing a better understanding of foam generation and propagation in the reservoir. Furthermore, it minimized the time and cost required to successfully complete the field trial. Introduction In the management of a steamflooded reservoir, it is important to both monitor and predict steam movement in order to optimize steam sweep efficiency and heating energy usage. To these ends, the pulsed neutron capture log (PNC) has been providing useful information in steamfloods in the San Joaquin Valley and elsewhere, along with other logging methods. In most steamflood operations, steam movement and distribution are primarily controlled by gravity and reservoir geology - vertically by barriers, and horizontally by sand geometry. The more complex the geology, the more difficult it is to predict steam movement Cores and openhole logs have been providing geologic information before and during a steamflood.

Reservoir Characterization, Production Characteristics, and Research Needs for Fluvial/Alluvial Reservoirs in the United States

“…These surveys are conducted in injection, production, and observation wells. Recent advances in steamflood monitoring techniques include the use of cased-hole pulsed neutron capture logs to quantitatively predict the in situ steam distribution and steam breakthrough the (Guo et al 1993;Guo 1991;Masse et al 1991), pressure survey in observation wells, cross-hole seismic profiles (Paulson et id. 1992) and use of surface deformation measurement tools to map areal migration of steam (Bruno and Bilak 1994).…”