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.
