Counter-current Aspect of the SAGD Process

Abstract

Abstract The uniqueness of the steam assisted gravity drainage (SAGD) recovery process lies in the salient role of moving condensing boundaries and counter-current flows. Process effectiveness depends on the balance between rising steam and draining oil-condensate emulsions. Reservoir permeability, well completion and effective drainage-pumping of oil-condensate emulsions can affect such balance. A new, non-steady state, laboratory steam-front dynamic tracking technique was used in measuring steam-liquid counter-current and co-current flows for different permeabilities and initial gas saturations. Steam chamber ceiling propagation rates (process initialization and growth) were determined for different injection and reservoir conditions. The paper highlights critical factors that control steam-oil emulsion counter-current flow and rate of propagation of the steam chamber. In addition, the CMG STARS numerical model was used to simulate SAGD counter-current flows and determine sensitivities to different parameters. Introduction The concept of the SAGD process was developed by Butler et.al.(1) In this concept, two horizontal wells are placed near the bottom of the reservoir and separated by a distance. Figure 1a illustrates the application of the SAGD process in very viscous oil (bitumen) reservoir. The top horizontal well (an injector) is located at a distance of about 5 m above the bottom well (the producer). The process consists of two distinctive phases. An initialization phase, where in most cases, conduction heating in the space between the two wells is chosen for initialization of gravity. FIGURE 1a: Illustration of the initialization and growth phases of the SAGD process in paired horizontal wells (Available in full paper) During this phase, steam is circulated in the tubing and out of the annulus. Initial breakthrough between the wells will generally occur in a localized region. Proper initialization procedures are required to bring the entire length of a well pair into active drainage. In general, the initialization phase is slow and oil rates during this phase are low. Nasr et. al.(2) reported results on two strategies for accelerating the initialization phase of the SAGD process: first, by using inter-well channels, and second, the injection of a hydrocarbon additive (naphtha) with steam. Following the initialization phase, after the oil in the inter-well region becomes mobile enough; the second phase (the growth phase) of the process can be started. During this phase, steam is injected into the top horizontal well and production fluids are obtained from the bottom well. Pressure drop along the horizontal well bore causes a slope in the steam chamber along the well as illustrated in Figure 1a. Improvement of the process during this phase requires improving the growth rate of the steam chamber and as a result improving oil drainage rates. Figure 1b shows a cross-section perpendicular to the well bores shown in Figure 1a and illustrating fluids flow in the steam chamber. Two types of flow exist. One at the ceiling of the steam chamber (ceiling drainage) and the other along the slopes of the steam chamber (slope drainage). Along the slopes of the chamber, mobilized oil accumulates in a progressively thicker layer. However, at the ceiling, oil moves away from the front immediately after mobilization.