Abstract

Oil recovery by tertiary CO2 flooding was pilot tested in a watered-out area in the SACROC pilot tested in a watered-out area in the SACROC Unit, Scurry County, Texas, during 1974–1975. Most of the SACROC Unit has been under a full scale enhanced recovery project using CO2 since 1972 in areas not yet watered out. A 2.3 BSCF slug of CO2 was injected into six wells in two adjacent, five-spot patterns in a watered-out portion of the reservoir over a period of nine portion of the reservoir over a period of nine months. Residual oil was displaced by the CO2 approximately 64,000 STBO (3% of OOIP) was recovered.

This paper reports on the analysis of the field data including chemical tracer, pulse test, produced flood analyses and pressure measurements. produced flood analyses and pressure measurements. Performance was history matched with a Performance was history matched with a compositional simulator. The largest uncertainty in the project was in the CO2 capture factor due to poor definition of the areal travel of the injected poor definition of the areal travel of the injected CO2. The volumetric sweep efficiency of the CO2 was calculated to be approximately 0.33. Fluid composition data showed that a significant gas saturation was created by CO2 injection and that interphase mass transfer enriched the produced fluids in intermediate hydrocarbons. The data and simulator results suggest that the flooding mechanism was not strictly miscible displacement. This study found evidence of CO2 dissolving rock and aggravating the heterogeneities and tendencies Of CO2 to channel. The data from the pilot produced a range of CO2 requirements for a large produced a range of CO2 requirements for a large scale project between 15 and 20 MSCF/STBO to give incremental recovery of 4% - 6% of OOIP for a 30% PV slug Of CO2. This recovery efficiency was not PV slug Of CO2. This recovery efficiency was not economic at $14.85 per barrel of oil.

Introduction

Pilot Area Selection and Design Pilot Area Selection and Design The project took place in Tracts 147 and 118 as shown on Figure 1. This area had the following characteristics:

  1. The chosen area had been under water injection for sufficient time to indicate that watered-out zones would be located.

  2. Cross-section work indicated that the area had a uniform pay section.

  3. The patterns formed by the injectors and newly drilled wells were uniform.

  4. Backup water injectors were available to the north and south to help confine the CO2 to the pilot area.

  5. The reservoir pressure was at a level at which miscible or near miscible behavior could be expected.

The following table summarizes pertinent water injection data for the six water injectors converted for CO2 injection into selected zones:

DATE ON CUM WTR INJ INJ RATEINJ THROUGH 1972 DEC 1972 WELL NO. MO/YR BBLS. B/D

118-2 5/70 1,800,000 1600 118-10 5/70 1,000,000 850 147-2 1/71 800,000 600 147-4 1/71 1,000,000 1000 147-7 5/54 11,900,000 1375 147-8 5/54 11,200,000 850

Two producers were drilled between these injectors to result in two adjacent normal five-spot patterns. The new producers were pressure-cored and extensively logged to yield oil pressure-cored and extensively logged to yield oil saturation measurements. Then the producers were tested to locate intervals producing oil-free water, and all eight project wells were completed in those intervals. Any oil production from the zones showing oil-free water could be interpreted unambiguously as tertiary oil.

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