Gas Injection With Radioactive Tracer To Determine Reservoir Continuity-East Coalinga Field, California
Authors: Tinker Gordon E. 
Published: December 2007

Radioactive tracers injected with natural gas revealed that desaturated intervals were often continuous over wide areas of the field; those intervals could greatly influence an injection project by acting as thief zones for injected water. The tracer study suggested generally better reservoir sand continuity than could be inferred from a related outcrop study.

Introduction
A waterflood pilot project, to start in 1974, has been planned for the Temblor Zone II reservoir of the East planned for the Temblor Zone II reservoir of the East Coalinga field, Calif. This proposed five-spot pattern drive project has required data on reservoir continuity and fluid distribution that have not been readily available from a study of a nearby outcrop nor from modem indirect geological methods., Engineering and geological studies indicated that this gravity drainage reservoir consists of complex channel sand deposits having uncertain interwell continuity complicated by oil desaturated intervals (intervals having a gas saturation and a low gravity-drainage residual oil saturation). Many of these sands contain sufficient movable oil associated with gas intervals to be included in a supplemental oil recovery project. A geological outcrop study was carried out approximately 2 miles northwest of the project study area to develop a conceptual picture of the Temblor Zone II reservoir. To learn more about this reservoir and to follow up the outcrop study, a series of reservoir characterization tests was undertaken. The tests consisted of injecting gas with radioactive tracers into the desaturated intervals. After a study of the out-crop, it was concluded that the subsurface geological features associated with channel boundaries would limit reservoir continuity to the channel width of 300 to 500 ft in all major sands, with better continuity along the trend of the channels. This restricted continuity would severely limit an injection project, but the results of the gas tracer tests indicate that the effective reservoir continuity is much greater than the continuity inferred from the outcrop.

Method of Testing
Gas tracer surveys were carried out successfully in Temblor Zone II sands, which had desaturated intervals with considerable continuity, Fig. 1 is an oil saturation profile based upon core analysis data for the gas injection well. The oil saturations within the sands having no desaturation have been reduced by the flushing action of the mud filtrate during coring. Some desaturated intervals are shown to overlie movable oil intervals and some desaturated intervals cover entire sand members. The sand in this reservoir is unconsolidated, so in obtaining all rock data, modem procedures for handling unconsolidated core were procedures for handling unconsolidated core were used and rock properties were measured under confining pressure. The well was completed with casing set through the Temblor Zone II formation so that individual sands could be isolated by tubing and packers in the overbore for each of the injection test. packers in the overbore for each of the injection test. The well completion consisted of perforating the desaturated intervals in four sands. To insure that the injected gas had been injected into the right sand, normal radioactive gas injection profile surveys were run at the conclusion of each phase of the testing before the packers were moved.
The injection tests were conducted in separate phases using methane containing a radioactive tracer phases using methane containing a radioactive tracer at the constant rate of 500 Mcf/D.
JPT
P. 1251