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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper reviews design and performance data on sixteen (16) CO 2 huff 'n puff projects conducted in different wells in the Forest Reserve oilfield of Trinidad and Tobago over the last twenty (20) years. Specific inferences on conditions under which these projects succeeded in increasing oil production were generalized taking into account published results of similar projects elsewhere.

Summary Four immiscible carbon dioxide (CO2) pilot floods were implemented in the Petroleum Co. of Trinidad and Tobago's (Petrotrin's) reservoirs at its Forest Reserve and Oropouche fields, Trinidad, over the period 1973 to 1990. The projects were conducted in a gravity-stable mode after primary, secondary, and tertiary production (after natural-gas and water injection). CO2 was injected into thick sands of variable continuity containing medium-gravity crude (17 to29°API). Production increases were observed in all projects. It is postulated that injected CO2 swelled the oil, reduced viscosity and helped form oil banks that could move more easily under gravity. Oil-production rates and recovery improved as a consequence. In some of the projects, these beneficial effects continued for several years, even after discontinuation of CO2 injection(supply interruptions), with recovery aided by water influx. Interruptions in CO2 supply did not appear to harm incremental oil recovery materially. Channeling was observed at high injection rates and was promoted in reservoirs with low transmissibility. Oil recovery improved as more offtake (production) wells were added downstream of the injection wells. This phenomenon reinforced the importance of optimizing volumetric sweep and of capture during CO2 flooding by judiciously selecting injection and offtake locations. Incremental recovery ranges between2 and 8% of the original oil in place (OOIP), with predicted ultimate recoveries of 4 to 9% of OOIP. Cumulative CO2 use improved with efficient production practices and ranges from 3 to 11 Mcf/bbl to date. Introduction The Forest Reserve and Oropouche fields are located in the southwest peninsula of the island of Trinidad, as shown in Fig. 1. In 1973, CO2 injection was initiated into a former natural-gas-injection project in Forest Reserve when there was a shortage of natural gas. Three immiscible pilot floods and one cyclic-injection project were later implemented between 1974 and 1986. Another immiscible pilot flood was implemented in 1990 in the Oropouche field. These projects were implemented in a "poor boy" mode using existing wells and equipment. CO2 is piped 25 miles from an ammonia plant, compressed, and injected into target reservoirs. This paper documents Petrotrin's 30 years of experience1 with CO2 immiscible injection into these projects and presents a comparative analysis of the performance of the four enhanced-oil-recovery (EOR) projects with some immiscible CO2-flood projects from the literature. Results and lessons learned will be used to guide the extension of CO2 injection to other similar reservoirs in the company's operations and to improve the management of existing projects.

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