Abstract

This paper reports on the feasibility of using carbon dioxide and flue gas in huff-n-puff cyclic gas injection to enhance light oil recovery from a waterflooded reservoir.

Phase behaviour measurements were carried out on recombined reservoir oil/CO2 mixtures in a cyclic pressure mode to simulate field application of the process. Significant amounts of CO2 dissolved in the oil causing oil swelling and viscosity reduction. During pressure depletion (puff cycle) the oil retained CO2 preferentially to methane; thus, the beneficial swelling and viscosity effects were maintained over an extended portion of this cycle. The Peng-Robinson equation of state was successfully tuned to match the laboratory phase behaviour data in both pressure-increasing and -decreasing modes. Corefloods were performed in the huff-n-puff mode to investigate the effect of waterflood residual oil saturation and injection gas composition (CO2 and enriched flue gas) on oil recovery. Incremental oil recovery was observed to be sensitive to waterflood residual oil saturation and to the process application scheme. Gas utilization factors of these tests varied from as high as 17.2 Mscf/STB to as low as 2.8 Mscf/STB.

These results suggest that the process is feasible, but that field application factors have to be optimized for maximum economic benefit.

Introduction

Elswick East Midale field, located in southeast Saskatchewan, was discovered in 1965. As shown in Figure 1, oil production from the field did not occur until 1987. Waterflooding began in late 1991; oil production increased significantly as a result, but decreased sharply after 1994. Additional development of horizontal wells within the field in later 1998 raised the monthly oil production again. However, until September 2003, Elswick East Midale field had produced only about 19.1% of the initial oil in place (IOIP).(1) The water cut has currently exceeded 85%, therefore enhanced oil recovery (EOR) methods are needed for increasing the recovery factor.

Various technologies have been applied in tertiary oil recovery processes, such as gas miscible/immiscible injection and chemical flooding. Among these EOR methods, the huff-npuff process has been reported to be economic at an oil price of less than US$20/STB and CO2 costs of US$40/ton.(2) For example, it was shown in a flue-gas huff-n-puff project(3) that oil production rates stabilized, and the project proved to be costeffective with small investment requirements and low operating costs. In another case,(4) the CO2 huff-n-puff process was not successful in increasing incremental oil recovery. However, there during the injection, soak, and flow phases, which were beneficial to the project.

The principal obstacle to developing EOR technologies is economics, which is affected by the special features of petroleum reservoirs. It is essential to develop cost-effective EOR technologies and to accelerate technology transfer to the field so that more oil resources can be recovered. There is increasing interest in CO2/flue-gas huff-n-puff injection into single wells because the process is relatively easy to apply and comparatively inexpensive. Although the huff-n-puff process is showing substantial oil recoveries in some oil reservoirs, experimental work has to be conducted to characterize the interaction of the injected gas with the crude oil under the candidate reservoir conditions before a field pilot starts.

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