Experimental Modeling of the SAGD Process - Enhancing SAGD Performance with Periodic Stimulation of the Horizontal Producer
- K. Sasaki (Akita U.) | S. Akibayashi (Akita U.) | N. Yazawa (TRC/JNOC) | Q.T. Doan (U. of Alberta) | S.M. Farouq Ali (U. of Alberta)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 2001
- Document Type
- Journal Paper
- 89 - 97
- 2001. Society of Petroleum Engineers
- 4.3.4 Scale, 4.1.5 Processing Equipment, 5.4.2 Gas Injection Methods, 5.2.1 Phase Behavior and PVT Measurements, 1.6 Drilling Operations, 5.4.6 Thermal Methods, 5.8.5 Oil Sand, Oil Shale, Bitumen, 1.7.5 Well Control, 5.3.9 Steam Assisted Gravity Drainage, 5.5 Reservoir Simulation, 5.3.2 Multiphase Flow, 4.1.2 Separation and Treating
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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.
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.
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