Flow Regimes During Cyclic Steam Stimulation At Cold Lake

Abstract

Abstract From the analyses of production data from thousands of cyclic steam stimulation (CSS) cycles at Cold Lake, a conceptual model of the flow regimes has been developed. It indicates that the traditional reservoir flow conceptualization of bitumen and water slipping past each other according to the relative permeability curves is, to a large extent, inapplicable at Cold Lake, and very likely inappropriate to visualizing steam stimulation in general. Upon completion of steam injection, three distinct and sequential flow regimes characterize the production phase: type I, free water with little if any bitumen production: type II, slugs of free water alternating with slugs of water in bitumen emulsion, resulting in scattered production rates; and type III, a single phase flow condition comprising of water in bitumen emulsion with very smooth, slowly varying production rates. In early cycles the majority of the bitumen production occurs during type III flow, with the type II regime being small. Over time, as the cycle number increases, the water content of the emulsion in type III flow reaches a maximum of about 50%; the flow then increasingly remains in type II condition, corresponding to a decline in the well's OSR and increase III the wafer/oil ratio (WOR) as the well begins to approach its economic limit. The swelling of the bitumen by the entrained water permits the emulsion to occupy essentially the whole pore space during type III flow, incorporating an otherwise continuous free water that could compete for production at the wellbore. This may be the physical reason for the need to introduce hysteresis to depress the water relative permeability during numerical stimulations of the production phase of CSS. Introduction Using cyclic steam stimulation (CSS), Esso Resources Canada Limited produces in excess of 14,000 m3/day of 10 API gravity bitumen from the Clearwater formation at Cold Lake, Alberta(l). A significant technological effort supports this large commercial thermal recovery project. Progress has been made in delineating the relative importance of key recovery mechanisms of CSS, such as formation recompaction, solution gas drive and gravity drainage(2). This paper reports the reservoir flow regimes during the production phase of CSS deduced from the analyses of production data from thousands of cycles. This analysis leads to the view-point that the traditional reservoir flow conceptualization of bitumen and water slipping past each other according to the relative permeability curves is, to a large extent, inapplicable at Cold Lake, and very likely inappropriate to visualizing steam stimulation in general. Perusal of the production data shows that the water and bitumen production rates often track each other with considerable coherence; this is most striking when the WOR is one. This is illustrated in Figure 1 for well A in its fourth cycle production. Note that in the later half of the cycle, for a period of 150 days, the WOR persists as one even during wide variations in fluid rates. During this period the reservoir is cooling, with the viscosity of the bitumen increasing much faster than that of the water; by the end of the period, it is estimated that the ratio of bitumen to water viscosity has increased at least by a factor of two.