Cyclic CO2 Injection for Heavy-Oil Recovery in Halfmoon Field: Laboratory Evaluation and Pilot Performance

Abstract

SPE Members Abstract This paper describes an immiscible CO2 project conducted in Halfmoon field, Wyoming. Laboratory results indicated that CO2 could improve recovery of the asphaltic, 17 degrees API gravity crude. The primary project incentive was that a gas source existed in the field. Incremental oil was produced in the field, but the project was not economic at present oil prices. Introduction Halfmoon field is located in Park County, northwestern Wyoming, in the Big Horn basin. The field is developed on 10 acre spacing, and has 27 active producers and 1 water injector. Reservoir properties are summarized in Table 1 and well locations are shown in Figure 1. The Phosphoria producing interval is a limestone/dolomite of Permian age, with open natural fractures on the crest of an anticlinal structure. The Tensleep producing interval is a sandstone of Pennsylvanian age. The structure is also an anticline, somewhat more fractured than the Phosphoria. The reservoir drive mechanism is weak, natural water drive from the deeper Madison formation, except for the northern portion of the Tensleep which experiences stronger water support. The water is believed to entrain production gas, composed of 94 mole percent CO2. Immiscible gas injection was considered because response to the existing water injector suggested the field was a poor waterflood candidate. The major reason for proposing a CO2 project was that a field CO2 source was available. The project was conceived as a field-wide CO2 huff 'n' puff operation because cyclic economics could be evaluated quickly. Several concerns were identified in the beginning stages of project design. CO2 injection rate would be limited by production supply. Natural fractures would cause conformance problems. The distribution of remaining oil was unknown, and mobilization of altered oil might be inefficient due to oil heaviness and the weak reservoir drive. The influence of rock type on process response was unknown. Finally, asphaltene precipitation was possible. A laboratory evaluation was undertaken to alleviate project concerns. In particular, scoping corefloods would address the influence of ROS, rock type, and reservoir drive. The tendency for asphaltenes to precipitate would be determined. Coreflood and modeling results would assist deliberations on whether cyclic CO2 injection should evolve into a flood drive. Field testing would gain operational experience and define the ultimate potential of using CO2 to improve oil recovery. LABORATORY EVALUATION Literature Review Changes in the properties of Halfmoon crude from CO2 contact were estimated from the literature. For the range of current reservoir pressures of about 500 to 900 psia, oil swelling was 5 to 10%, and oil viscosity reduction was 3 to 9 fold. Oil viscosity reduction was expected to be the primary mechanism of enhanced oil recovery. P. 155^