The Effect of Gas Injection on Oil Recovery During SAGD Projects

Abstract

Abstract The effect of hydrocarbon gas injection on oil production during Steam Assisted Gravity Drainage (SAGD) projects was investigated using numerical simulation. The results indicate that oil production rates as well as total oil production are significantly reduced when gas is injected with steam from the early period of a SAGD operation. This is because most of the injected gas gathers at the upper part of the leading edge of the steam chamber and prevents the growth of the steam chamber there, which results in a reduction of ultimate oil recovery. However, if the gas injection is initiated during later periods of the process, an improved steam oil ratio is obtained without significant reduction in oil production rates and the total oil production. In this case, the injected non-condensable gas migrates to the upper part of the reservoir and does not prevent the growth of the steam chamber, since the chamber had already grown to the desired size. Gas injection slows down the growth of the steam chamber in the upper part of the reservoir and induces its growth downwards. This mode of growth of the steam chamber results in an improvement in steam oil ratio as is illustrated in this paper. An understanding of this mechanism and its optimal timing are important in the enhancement of the SAGD process. Introduction More than 52 billion cubic metres (330 billion barrels) of bitumen are believed to be in place in the Athabasca oil sands deposit in Canada, which are recoverable using the Steam Assisted Gravity Drainage (SAGD) in situ recovery process(1). The SAGD process has been successfully field tested at the UTF (Underground Test Facility) project, which was initiated in 1988. The project consisted of a small scale pilot test (Phase A) with three pairs of 50 m horizontal wells, and a commercially scaled pilot test (Phase B) with three pairs of 500 m horizontal wells. The performance of these tests has been reported elsewhere(2, 3). Japan Canada Oil Sands (JACOS) has participated in the UTF project since 1992, when Phase A of the project was completed. From its participation, JACOS acquired useful field data and operating experience. The data were extensively analysed through numerical simulation. Based on this analysis, JACOS started a demonstrative SAGD pilot operation in the Hangingstone oil sands reservoir. It involved two pairs of 500 m horizontal wells, and an additional three pairs of 750 m horizontal wells were started late in 1999. The SAGD process is a simple one. It consists of two horizontal wells, an injector placed directly above a producer. At first, both wells are heated by steam circulation. When communication between the two wells is established, steam is continuously injected through the upper injector while oil and steam condensate are produced from the lower producer. The original recovery mechanism of the process was described by Butler(4, 5), who developed the energy and oil flow equations associated with it. The energy flow by thermal conduction and drainage of the heated oil by gravity are the major components of this recovery concept.