Monitoring Flood Profiles With Induction Logs
Authors: Richardson J.E. 
Published: December 2007

Monitoring flood profiles with dual induction logs in fiberglass-lined wells was successful in two chemical floods. Response profiles were obtained from the preflood, chemical slug, and polymer drive phases. A series of log runs was conducted for each phase to monitor the change in response profile with time. This information determined the cumulative feet of breakthrough with time, vertical sweep efficiency, and redistribution of the injected fluids.

Introduction
In 1972, Shell Oil Co. began a chemical tertiary flood in the Tar Springs sandstone (average porosity 18%) of Benton Field, Franklin County, IL. Open-hole logs run before the chemical flood are shown in Fig. 1. The reservoir had been under waterflood and was producing at about 98% water cut. Because of the high costs and uncertainties involved, good surveillance techniques were required for the chemical flood. Therefore, in addition to fluid-sampling wells, logging observation wells were included.
Monitoring the movement of formation fluids with logs can provide important information for a supplemental recovery process, including redistribution of the injection fluids, vertical sweep efficiency, and cumulative feet of breakthrough with time. In addition, the results from monitoring are accounted for when considering the timing and location of evaluation core holes.
The choice of a monitoring log is controlled to some extent by the properties of the formation and the respective fluids. Of particular importance are the types and compositions of the fluids. Sometimes, these restrictions may be eased by modifying the injected fluids to fit a particular logging tool. Because cased wells are particular logging tool. Because cased wells are necessary to prevent crossflow, the logging tool also must be able to measure formation properties behind casing. A most important qualification is that the logging tool give good repeatability for months or even years.
The dual induction log was chosen for monitoring the chemical flood. This log met the conditions of the flood best and also could investigate deeper than most logging tools. The two curves (medium and deep induction) from the log had the additional advantage of detecting any communication behind casing. To adapt the wells for use with induction logs, we used high-resistivity fiberglass casing, instead of steel casing. Where possible, the resistivity of a fluid was selected and maintained during injection to obtain maximum information from the logs.
Our major concern was the repeatability of the dual induction log. For reliable monitoring information, log data must be recorded with greater precision than generally is required for normal use. In our experience, the tool never had been tested to this extreme. We have found that the tool can be operated with good repeatability. However, this requires precise calibrations, good quality control, and careful processing of log data. For the first chemical flood, we monitored the preflood, the chemical slug, and the polymer drive. We also monitored a second chemical flood and now are using the technique on a polymer flood and a CO2 pilot flood.

Technique
Each logging observation well was completed with a high-resistivity fiberglass liner (Fig. 2). Shell's experience with cemented fiberglass tubulars is described by Bowers et al. The borehole was filled with fresh water to eliminate the borehole signal. Induction logs run before and after placing the liner showed no effect from the liner.
JPT
P. 19