The eighty mile [130 km] long Cedar Creek Anticline structure, trending from eastern Montana to southwestern North Dakota, contains at least a dozen carbonate reservoirs at an average depth of 9000 feet [2745 m]. All of the major oil accumulations have been under waterflood, starting from the mid-1950's to the mid-1960's. These waterfloods are now reaching a mature stage.
Laboratory PVT, core flood studies, and field measurements of waterflood residual oil saturation indicated that there was potential for carbon dioxide floods in the Cedar Creek Anticline water-flood units. A three well carbon dioxide injectivity test was initiated in 1983 in the South Pine Field to define carbon dioxide injectivity and extent of tertiary oil mobilization for the important Red River U4 interval.
The test consisted of an injection well, a logging observation well, and a pressure/sampling observation well. The three bottom hole locations were within 90 feet [27.4 m] of each other. The injection sequence was to inject twelve percent brine, a carbon dioxide slug, then resume the twelve percent brine injection. Pressures were monitored continuously in the injection well, the pressure/sampling observation well, and a passive offset well 650 feet [198 m] from the test location. Detailed surveillance of fluid movement in the test area was obtained by frequent monitor logging using compensated neutron, induction and gamma ray logs. A post-flood pressure core was cut to measure the oil saturation remaining after the carbon dioxide slug injection.
Analysis of pressure measurements indicate the observed carbon dioxide injectivity was approximately fourteen times that of the preflood brine. Log and core measured saturations show substantial desaturation of waterflood residual oil in the well swept areas of the test interval; mobilized tertiary oil was also produced at the pressure/production observation well, located 70 feet [21.3 m] from the injector. The detailed analysis of fluid movement during the test has been complicated by the heterogeneous nature of the test interval at the location of the pilot. Flow horizons within the U4 with permeability contrasts on the order of a factor of fifty have been identified through analysis of brine tracer and carbon dioxide encroachment data. Pressure transient data, thermal front arrival times, brine tracer data, carbon dioxide and follow-up brine encroachment data, and tertiary oil desaturation log and core measurements were analyzed to obtain an estimate of the values of parameters needed for estimation of full scale flood response.