J.W. Johnston, Atlantic Richfield Co.
The Willard Unit is located in the Wasson (San Andres) Field in Yoakum County, Texas. The reservoir is a layered dolomite with an average porosity of 8.5% and average permeability of 1.5 md. Secondary recovery by waterflooding has been in progress since 1965. Although secondary operations have been quite successful in the Willard Unit, a substantial amount of oil will be unrecoverable by waterflooding. A CO2 miscible displacement project was conducted in the unit to investigate the applicability of this process for full-scale improved oil recovery. The project consisted of two separate-held tests to study the various operational and reservoir aspects of the CO2 miscible process. The first of these consisted of eight adjacent CO2 injection wells on regular waterflood spacing. Since this was the first effort to conduct a CO2 miscible flood in this unit, this test was called Phase I. Water and CO2 were injected alternately in Phase I from November, 1972, to February, 197.5. This area was planned to provide insight into the extent of reservoir sweep problems that might occur in a regular-size pattern CO2 flood. It would also provide an opportunity to investigate control measures if these problems arose. Additionally, information would be obtained on injection performance and operational procedures that could be used in planning a unit-wideflood. The second test was located and operated separately from Phase I and was called the Pilot. It consisted of four wells: an injector, logging observation well, pressure observation and sampling well, and pressure core well, all on close spacing. The Pilot was designed to allow a more detailed investigation of the reservoir flow behavior of CO2 and water and to determine the reduction in waterflood residual oil levels due to CO2 injection. Phase I injection performance was good. The reservoir pressure was maintained above the minimum required for miscible displacement. Cumulative CO2 injection was 3.8 BCF of CO2, or 4.4% of the hydrocarbon pore volume. Some of the injected CO2 was produced as a result of excessive CO2 injection pressures, but this volume has totaled only 3% of the cumulative injection. There was no evidence of severe gravity segregation or area1 sweep problems. A complete analysis of the Pilot area has not been finalized. This project verified the concept of stratified flow in the reservoir and no significant gravity overriding of the CO2 was observed, The pressure core project was very successful and the Pilot results suggest that additional oil displacement occurred as a result of the CO2 injection.