An Investigation of the Effect of Oil Composition on Heavy Oil Solution-Gas Drive

SPE Journal ◽  
2006 ◽  
Vol 11 (01) ◽  
pp. 58-70 ◽  
Author(s):  
Guo-Qing Tang ◽  
Yi Tak Leung ◽  
Louis M. Castanier ◽  
Akshay Sahni ◽  
Frederic Gadelle ◽  
...  
Keyword(s):  
Crude Oil ◽  
Gas Phase ◽  
Heavy Oil ◽  
Gas Bubbles ◽  
Gas Saturation ◽  
Mineral Oils ◽  
Solution Gas Drive ◽  
Depletion Rate ◽  
Gas Drive ◽  
Solution Gas

Summary This study probes experimentally the mechanisms of heavy-oil solution gas drive through a series of depletion experiments employing two heavy crude oils and two viscous mineral oils. Mineral oils were chosen with viscosity similar to crude oil at reservoir temperature. A specially designed aluminum coreholder allows visualization of gas phase evolution during depletion using X-ray computed tomography (CT). In addition, a visualization cell was installed at the outlet of the sandpack to monitor the flowing-gas-bubble behavior vs. pressure. Bubble behavior observed at the outlet corroborates CT measurements of in-situ gas saturation vs. pressure. Both depletion rate and oil composition affect the size of mobile bubbles. At a high depletion rate (0.035 PV/hr), a foam-like flow of relatively small pore-sized bubbles dominates the gas and oil production of both crude oils. Conversely, at a low depletion rate (0.0030 PV/hr), foam-like flow is not observed in the less viscous crude oil; however, foam-like flow behavior is still found for the more viscous crude oil. No foam-like flow is observed for the mineral oils. In-situ imaging shows that the gas saturation distribution along the sandpack is not uniform. As the pattern of produced gas switches from dispersed bubbles to free gas flow, the distribution of gas saturation becomes even more heterogeneous. This indicates that a combination of pore restrictions and gravity forces significantly affects free gas flow. Additionally, results show that solution-gas drive is effective even at reservoir temperatures as great as 80°C. Oil recovery ranges from 12 to 30% OOIP; the higher the depletion rate, the greater the recovery rate. Introduction Solution gas drive has shown unexpectedly high recovery efficiency in some heavy-oil reservoirs. The mechanisms, however, that have been proposed are speculative, sometimes contradictory, and do not explain fully the origin of high primary oil recovery and slow decline in reservoir pressure. Smith (1988) first identified this effect. He hypothesized that gas bubbles smaller than pore constrictions are liberated from the oil, but are not able to form a continuous gas phase and flow freely. Instead, the gas bubbles exist in a dispersed state in the oil and only flow with the oil phase. Smith stated that oil viscosity is reduced significantly, resulting in high recovery performance. Later, many researchers focused on so-called foamy-oil behavior. Claridge and Prats (1995) hypothesized that heavy-oil components (such as asphaltenes) concentrate at the interfaces between oil and gas bubbles, thereby preventing bubbles from coalescing into a continuous gas phase. Bubbles are assumed to be smaller than pore dimensions. Claridge and Prats stated that the concentration of heavy-oil components at the interfaces results in a reduction of the viscosity of the remaining oil. Bora et al. (2000) discussed the flow behavior of solution gas drive in heavy oils. Based on their studies, they found that dispersed gas bubbles do not coalesce rapidly in heavy oil, especially at high depletion rate. They stated that the main feature of the gas/oil dispersion is a reduced viscosity compared to the original oil. Models to explain the experimental results were also established (Sheng et al. 1994, 1996, 1999, 1995).

Effect of Depletion Rate on Gas Mobility and Solution Gas Drive in Heavy Oil

SPE Journal ◽  
2002 ◽  
Vol 7 (02) ◽  
pp. 213-220 ◽  
Author(s):  
R. Kumar ◽  
M. Pooladi-Darvish ◽  
T. Okazawa
Keyword(s):  
Heavy Oil ◽  
Solution Gas Drive ◽  
Depletion Rate ◽  
Gas Mobility ◽  
Gas Drive ◽  
Solution Gas

Flow Visualization Studies of Solution Gas Drive Process in Heavy Oil Reservoirs Using a Glass Micromodel

2000 ◽  
Vol 3 (03) ◽  
pp. 224-229 ◽  
Author(s):  
R. Bora ◽  
B.B. Maini ◽  
A. Chakma
Keyword(s):  
Flow Visualization ◽  
Heavy Oil ◽  
Gas Bubbles ◽  
Oil Reservoirs ◽  
Pore Scale ◽  
Glass Micromodel ◽  
Heavy Oil Reservoirs ◽  
Solution Gas Drive ◽  
Gas Drive ◽  
Solution Gas

Summary A series of flow visualization experiments was carried out to examine the pore scale behavior of the solution gas drive process in heavy oil reservoirs. The main objective was to testify several speculative theories that had been put forward to explain the anomalous production behavior of heavy oil reservoirs producing under the solution gas drive process. Contrary to previous postulations, the asphaltene constituents did not appear to play a significant role in the nucleation and stabilization of the gas bubbles that evolved during the solution gas drive process. Experimental evidence also suggests that the production of heavy oil is not accompanied by a large population of microbubbles. These observations suggest that the production enhancement in the solution gas process in heavy oil reservoirs may be related to other mechanisms such as viscous coupling effects, sand production, wormhole effects, etc. Introduction Primary production of heavy oil reservoirs operating under the solution gas drive mechanism exhibits an unexpectedly higher primary recovery with a slower pressure decline rate, lower than expected gas oil ratios, and higher oil production rates. These reservoirs which are prolific during the primary production phase have shown very poor response to secondary recovery techniques, such as thermal recovery. Ongoing observations in the fields 1–4 and preliminary observations in laboratories 5–7 strongly suggest that the cold production process of heavy oil reservoirs by the solution gas drive process involves a multitude of effects. A detailed analysis of such unusual production behavior was first provided by Smith.1 He suggested that the solution gas drive in heavy oil reservoirs involves simultaneous flow of oil and gas in the form of microbubbles. Following this, the flow behavior of such gas-oil dispersions has been the subject of several investigators and considerable speculation.2–9 However, the solution gas mechanism in heavy oil reservoirs remains controversial and poorly understood. Background In the solution gas drive process, the main source of energy driving the oil towards the wellbore is the evolution and expansion of the gas bubbles initially dissolved in the oil. The role of the gas bubbles in the oil displacement process has been studied for a long time.10--16 The first visual studies of the behavior of the solution gas process at the microscopic level was performed by Chatenever et al.14 using thin glass bead packings and thin sections of natural sandstone and limestone. With the advent of glass micromodels, flow visualization studies were conducted to examine the microscopic behavior of the solution gas drive process.17–22 All these studies provided a direct observation of pore level events. However, a comprehensive understanding of the pore scale physics in the solution gas drive process has not yet been attained. Moreover, recent observations in the field led to revised thinking of the mechanisms involved in the solution gas drive process in heavy oil reservoirs. The flow of heavy oil under the solution gas drive process appears to be more complex than what is expected from conventional solution gas drive theories. None of the previous studies focused on the behavior of the solution gas process in heavy oil reservoirs. To acquire an improved understanding of the solution gas drive mechanisms, it is necessary to consider the pore scale physics. Most of the questions concerning nucleation, growth, coalescence, and flow of the gas bubbles dispersed in oil can be answered only by direct examination of individual pore scale events. Although it is not possible to visually examine the processes occurring at the pore level in actual reservoir rocks, a very close approximation can perhaps be achieved in a micromodel. Micromodels provide a very convenient means of directly observing the formation, growth, flow, and trapping of gas bubbles. The main objective of this work was to carry out a series of flow visualization experiments, using a high pressure etched glass micromodel, to make a detailed investigation of the effects of asphaltene particles, pressure depletion rates, and sand wettability on the pore level flow mechanisms in the solution gas drive process. To the best of our knowledge, there has been no such systematic investigation of pore scale physics of the solution gas drive process in heavy oil reservoirs. The applications and technical contributions of such a study include the following:an improved understanding of the solution gas drive mechanism in heavy oil reservoirs,planning optimum development strategies for heavy oil reservoirs, andunderstanding of the condition of the reservoir at the end of the primary production phase which is helpful for developing an effective follow-up secondary recovery technique. Micromodel Apparatus The experimental setup is shown schematically in Fig. 1. The heart of the test rig is the high pressure etched glass micromodel. Conceptually, it is simple in design. Two glass plates were held together by overburden pressure inside a windowed pressure vessel. One of the glass plates had a detailed flow pattern chemically etched onto it, the other plate was unetched and had parallel sides. The flow pattern used in this work is displayed in Fig. 2. Here, the black dots represent sand grains while the white area represents the flow channels. The center to center distance between adjoining "sand grains" was 500 µm and the diameter of each dot was 334 µm. The average depth of etched flow channels was about 50 µm. The pore volume within the boundaries of the etched pattern was approximately 75 µL. The etched flow patterns were illuminated with high intensity halogen light bulbs underneath the bottom window of the pressure vessel. The overburden pressure in the pressure vessel was maintained at 600 psi (4.14 MPa) throughout the entire study.

Heavy-Oil Solution Gas Drive in Consolidated and Unconsolidated Rock

SPE Journal ◽  
2006 ◽  
Vol 11 (02) ◽  
pp. 259-268 ◽  
Author(s):  
Guo-Qing Tang ◽  
Akshay Sahni ◽  
Frederic Gadelle ◽  
Mridul Kumar ◽  
Anthony R. Kovscek
Keyword(s):  
Heavy Oil ◽  
Bubble Nucleation ◽  
Gas Bubble ◽  
Oil Composition ◽  
Oil Viscosity ◽  
Overburden Pressure ◽  
Solution Gas Drive ◽  
Depletion Rate ◽  
Gas Drive ◽  
Solution Gas

Summary Solution gas drive is effective to recover heavy oil from some reservoirs. Characterization of the relevant recovery mechanisms, however, remains an open question. In this work, we present an experimental study of the solution gas drive behavior of a 9°API crude oil with an initial solution gas/oil ratio (GOR) of 105 scf/STB and live-oil viscosity of 258 cp at 178°F. Constant rate depletions are conducted in a composite core (consolidated) and a sandpack (unconsolidated). The sandpack does not employ a confining pressure, whereas the consolidated core does. The evolution of in-situ gas saturation vs. pressure is monitored in the sandpack using X-ray computed tomography. The two different porous media allow us to develop a mechanistic perspective whereby the effects of depletion rate and overburden pressure on heavy-oil solution gas drive are investigated. The results are striking. They show that the overburden pressure offsets partially the pore-pressure decline. This compaction, in turn, modifies the size and shape of mobile gas bubbles, and as a result the oil and gas relative permeabilies are greater within the confined, consolidated core. Additionally, the supersaturation in the sandpack is markedly larger, but recovery is greatest from the composite core at identical rates as a result of compaction. Introduction Solution gas drive in some heavy-oil reservoirs yields unexpectedly large oil recovery. Remarkably, the reservoir pressure declines more slowly than expected and the produced GOR increases slowly below the equilibrium bubblepoint pressure. Since 1988, when Smith identified the phenomenon (commonly referred to as foamy oil), experimental and theoretical studies have aimed to elucidate gas-flow and oil-production mechanisms. Results indicate that the factors governing the efficiency of heavy-oil solution gas drive are oil viscosity (Tang and Firoozabadi 2003, 2005), depletion rate (Tang et al. 2006; Kumar et al. 2000; Sahni et al. 2004), solution GOR (Tang and Firoozabadi 2003), oil composition (Tang et al. 2006; Bauger et al. 2001), and gas-bubble morphology (Li and Yortsos 1995; Tang et al. 2006). Obviously, these factors are not mutually exclusive. Among them, depletion rate as well as the size and shape of bubbles play a key role in recovery. Additionally, the oil composition is important because it plays a determining role in the flowing gas-bubble size that ultimately determines gas-phase mobility (Tang et al. 2006). Gas bubbles grow as a result of supersaturation (the difference between equilibrium and dynamic pressure) as well as pressure depletion. Gas-bubble nucleation is usually described as progressive or instantaneous (Li and Yortsos 1995; Firoozabadi and Kashchiev 1996), depending on the oil composition and porous medium (Tang et al. 2006; Kumar et al. 2000). Experiments with (El Yousfi et al. 1997; George et al. 2005) and simulation of (Arora and Kovscek 2003) gas nucleation in porous media indicate that the gas phase forms progressively. The period of active bubble nucleation is, however, relatively short compared to the time needed to deplete the sysem. Therefore, the process might be approximated as instantaneous nucleation if the longer time behavior is of interest (El Yousfi et al. 1997).

A Study of Heavy Oil Solution Gas Drive for Hamaca Field: Depletion Studies and Interpretations

2004 ◽  
Author(s):  
Cengiz Satik ◽  
Carlon Robertson ◽  
Bayram Kalpakci ◽  
Deepak Gupta
Keyword(s):  
Heavy Oil ◽  
Solution Gas Drive ◽  
Gas Drive ◽  
Solution Gas
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