Summary Experiments on initial stages of the steam-assisted gravity drainage (SAGD) process were carried out, using two-dimensional (2D) scaled reservoir models, to investigate production process and performance. Expansion of the initial steam chamber, its shape and area, and its temperature distributions were visualized with video and thermal-video pictures. The relationship between isotherms and steam-chamber interface was investigated to study the drainage mechanism. Temperature at the expanding steam-chamber interface was observed to remain nearly constant at close to 80°C. The effect of vertical spacing between the two horizontal wells on oil recovery was also investigated. For the Conventional SAGD case, oil production rate increased with increasing vertical spacing between the wells; however, the lead time for the gravity drainage to initiate oil production became longer. The results suggest that vertical spacing between the wells can be used as a governing factor to evaluate production rate and lead time in the initial stage of the SAGD process. Based on these experimental results, the SAGD process was modified; the lower production well was intermittently stimulated by steam injection, in conjunction with continuous steam injection in the upper horizontal injector. With the modified process (named SAGD-ISSLW), the time to generate near-breakthrough conditions between two wells was shortened, and oil production was enhanced at the rising chamber stage compared with that of the Conventional SAGD process. Introduction The SAGD process was developed by Butler and his coworkers.1,2 In Canada, the SAGD process has proven successful for recovery of bitumen, as demonstrated in the reports on the UTF projects (Phases A and B).3,4 Chung5 and Chung and Butler6,7 reported experimental results for the SAGD process with scaled and visual reservoir models. Furthermore, Chow and Butler8 reported numerical simulation results matching Chung's experimental results5 using Computer Modelling Group Ltd.'s STARS™ simulator. Recently, Mukherjee et al.9 successfully forecasted the performance for Phase B of the UTF project. Butler10 gave a review of the SAGD process. An operational problem of the SAGD process for oil sands reservoirs is the lead time required to generate a steam chamber in near-breakthrough conditions between the two horizontal wells before the production stage. In this study, we first examined characteristics of the Conventional SAGD process, especially the expansion rate of the steam chamber by gravity drainage and the effects of well spacing. It was found that by using smaller vertical spacing between the two horizontal wells, the lead time was reduced, while production rate after breakthrough became lower. As shown in this paper, results from our investigation demonstrated that a more economical SAGD operation could be achieved by a simple modification involving selective intermittent stimulation of the lower horizontal producer by steam injection. For this process, called the SAGD-ISSLW process, the lower horizontal well was modified to enable intermittent stimulation by steam injection along the well design reported by Liderth.11 As such, this well served two functions: selective intermittent steam injection, and continuous fluid production. Steam from this lower well was injected intermittently to prevent steam breakthrough. The experiments using this process were compared to those using the Conventional SAGD process. The results showed that the SAGD-ISSLW process was successful in reducing the lead time to generate the steam chamber in the initial stage. The quick generation of the steam chamber plus the intermittent steam injection provided the advantage of allowing larger vertical spacing to be set between the two horizontal wells. Intermittent steam injection also led to another advantage of enhancing the instability of the steam-chamber interface near the ceiling, and thus it could be used to control the expanding steam chamber more effectively. Experimental Apparatus and Procedures Many experiments were performed in scaled 2D reservoir models with porous packing materials to investigate steam-chamber behavior and oil-production mechanisms, with respect to heat and mass-transfer phenomena. To compare process performance, steam-injection and fluid-production rates were measured. The experimental apparatus was designed and dimensions were determined according to the scaling criteria given in Refs. 5 and 6. Major experimental conditions and the purposes for the four phases of experiments are listed in Table 1. One difference between our experiments and those of Chung and Butler6 is the process used to preheat the reservoir by circulating steam through two wells before injecting it into the reservoir. We did not use preheating in our experiments, as we believed that it would interact with the well structure and materials, and as a result, heat not only the reservoir but also both side plates of the 2D models. Fig. 1 shows a schematic of the experimental apparatus, including the reservoir model. The apparatus consisted of a water pump, steam generator, steam accumulator, 2D scaled reservoir model, production-control mechanism, visualization system, and the data-acquisition system. All components, except the data-acquisition and video-camera systems (DAS), were mounted on a flat steel table designed and built in-house. Scaled Reservoir Model. The 2D scaled reservoir models (Fig. 2) were designed to represent a vertical segment of an oil sands reservoir. The models were made from smooth and transparent acrylic-resin plates 20 mm in thickness. The transparent side plates allowed visualization of the displacement of the oil in the steam chamber. Glass beads (diameter: 0.18 to 0.25 mm, average 0.21 mm) and heavy oil were packed between the two side plates. Motor oil (COSMO #1000, molecular weight=490 g/gmol, ?=998 kg/m3) served as the heavy oil in the experiments. Viscosity of the COSMO #1000 oil and Athabasca bitumen (extracted by Suncor Inc.) was measured as a function of temperature with a rheometer (Shimadzu, RM-1), as shown in Fig. 3. Viscosity of this oil was 93 000 mPa's (or 93 Pa's) at an initial temperature of 20 to 25°C, and 120 mPa's at a steam temperature of 106°C. Thus, the viscosity of the heavy oil used in the present experiments is roughly one-fifth that of the bitumen.
A series of Accelerating Rate Calorimeter (ARC) and Thermo Gravimetric Pressurized Differential Scanning Calorimeter (TG/PDSC) tests was conducted on oil-rock systems from three light-oil reservoirs (Oils A, B and C) to screen and evaluate the potential of the air injection process. ARC tests helped determine, a priori, whether the test oils would autoignite under reservoir conditions of pressure and temperature. Also, the limits of the low temperature range were established and Arrhenius oxidation kinetics parameters were estimated. The goals of the TG/PDSC tests were to identify temperature ranges over which the oil reacted with oxygen in the injected air, and to determine the fraction of the sample responsible for the reactivity. ARC and TG/PDSC tests demonstrate that Oils A and C offer favourable exothermic behaviour in the low temperature range with lower activation energies and low orders of reactions?the conditions typically favouring autoignition. The presence of rock material lowered the ignition temperature, confirming its impact on O2 uptake. Oil A had a lower energy generation (ignition) temperature, and a stronger and smoother transition to the higher temperature region. Both oils responded favourably during isothermal aging with air as manifested by a drop in the initial self-heating temperature (a 15 °C drop for Oil A and a 10 °C drop for Oil C). The third oil, Oil B, showed unusual characteristics, with almost no impact of the core material on the starting temperature of the exotherm; rather, the core material appeared to have acted as a heat sink. Overall, Oil B needed a much higher activation energy to ignite, and its order of reactions was very high. Furthermore, it showed no response to isothermal aging, and hence, it is less likely to autoignite in the reservoir. esults revealed that ARC and TG/PDSC tests could be an effective tool to rank and study the oxidation characteristics in the low temperature range of the candidate oils. Also, it was observed that the oil composition and rock mineralogy are important factors affecting the type and rate of the oxidation reactions occurring in the low temperature range. Introduction The air injection process is now a proven and viable process in improving oil recovery from several light-oil reservoirs. As a result, it has generated much interest in recent years(1–5). Cheap and abundant, air is also touted as a possible alternative to highcost hydrocarbon and CO2 gases in certain circumstances or locations where water is scarce(6–10). Moore et al(11) suggest that in an air injection process in lightoil reservoirs, both oxygen addition and bond scission reactions take place. Oxygen addition reactions are believed to occur at temperatures between 100 °C and 150 °C. These reactions are characterized by heavier oxygenated hydrocarbon products. Bond scission reactions for light oils typically occur at temperatures in the 150 to 300 °C and the 350 to 700 °C range. These reactions produce carbon oxides, water (steam) and heat, which contribute significantly towards mobilizing oil.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractExperiments and numerical simulations on initial stages of the steam-assisted gravity drainage (SAGD) process were carried out. Experimental studies used two-dimensional scaled reservoir models to investigate fluids flow characteristics in the steam chamber and production fluids. The rise and growth of the initial steam chamber were observed and visualization of micro-phenomena at inclined interface on the side of the steam chamber with a high-resolution optical-fiber scope was carried out. Very fine water droplets of 0.01 mm order in size were observed at the interface between steam and heavy oil phases. These droplets entered into heavy oil phase and created emulsion together with the heated oil, which flowed down and was produced from the production well. It was successfully demonstrated that these micro phenomena has an influence on chamber expansion rate in horizontal direction and oil production rate.The numerical simulation of the SAGD process was also performed. The thermal simulator, CMG's STARS TM , was used to simulate the experimental steam chamber growth at the initial stage. The simulation used the two-components (water and heavy oil) black oil, three-phase (water, heavy oil and steam) and three-dimensional numerical model for the physical model. The results from the history-matched numerical simulation were found to be in good agreement with those of the experiments for oil production and steam chamber shape by using the Intermediate3-Stone1 wettability model represents fluids behavior cumulating the microscopic phenomena at the chamber interface. Furthermore, a new process named Surfactant-SAGD injecting a surfactant before starting steam injection to enhance the communication between two wells and mobility of the production fluids was tested.
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