Abstract

Imperial Oil Limited is using the Cyclic Steam Stimulation (CSS) process to recover bitumen from oil sands at Cold Lake, Alberta, The clean oil sands at Cold Lake generally produce well, but some of the oil sands with imbedded clasts have poor production. This paper presents the results from laboratory studies which show that some clasts are richer in carbonates. These carbonates react with aluminosilicates in the clasts under steaming conditions to generate formation damaging products like swelling clays and carbon dioxide, Swelling clays can damage the formation by plugging pore throats, whereas carbon dioxide can lead to near-wellbore scaling. There is an inverse correlation between the carbonate content of the clasts and bitumen production in the field. The amounts of carbon dioxide generated in lab tests also correlate inversely with the field bitumen production. The paper discusses the application of portable XRF and near-infrared tools for rapid core description, and of photoelectric absorption logs for identifying reservoirs wtth reactive minerals. It proposes diagnostic tests in the field to identify the extent and type of damage in a producing well. Finally possible methods for damage remediation and prevention are discussed.

Introduction
Cyclic Steam Stimulation Process (CSS)

Cold Lake In northeastern Alberta has a vast resource base of oil sands. The bitumen in the oil sands has a viscosity of about 100,000 cp at reservoir temperature. Imperial Oil Limited is using the Cyclic Steam Stimulation (CSS) process to recover the bitumen. In one cycle, steam is injected into a well and a hot bitumen and water mixture is produced from the same well. Each well goes through several cycles of Injection and production until steam Injection becomes uneconomic. The wells are drilled directionally from one surface location and there are twenty wells in one pad. CSS piloting at Cold Lake started in 1964. Piloting operations were expanded leading to the start-up of commercial production, known as CLPP(Cold Lake Production Project) in 1985. Cold Lake operations have the capacity to produce 14,000 m3/d of bitumen.

"Complex" Reservoirs

Most or Cold Lake's production prior to CLPP has been from good reservoirs with clean oil sands. These reservoirs in the Clearwater formation have thick, clean oil sands with high bitumen saturation. Variable reservoir quality and increased heterogeneities were encountered in CLPP. Although the current Cold Lake operations are, in general, in good quality reservoir, the future development areas will have to deal with reservoirs with lower bitumen saturation, top gas, top water and bottom waler. In addition, there are reservoirs with varying amounts of shale interbeds and clasts, which are relatively consolidated and are imbedded in the oil sands (Figure 1). The latter reservoirs are called "complex" reservoirs. The present study deals with the problems associated with the exploitation of the huge reserve in "complex" reservoirs.

Production from some of the "complex" reservoirs has been less than satisfactory. For example, in one pad steam injection in the first cycle was normal, but the production rate was 1/4 to 1/3 of a normal first cycle at Cold Lake.

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