The Release of Solution Gas from Waterflood Residual Oil
- Robert I. Hawes | Richard A. Dawe | Richard N. Evans
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 1997
- Document Type
- Journal Paper
- 379 - 388
- 1997. Society of Petroleum Engineers
- 5.4.2 Gas Injection Methods, 4.6 Natural Gas, 1.2.3 Rock properties, 4.1.2 Separation and Treating, 5.3.1 Flow in Porous Media, 4.1.5 Processing Equipment, 5.1 Reservoir Characterisation, 5.1.1 Exploration, Development, Structural Geology, 5.4.1 Waterflooding, 5.2.1 Phase Behavior and PVT Measurements, 1.6.9 Coring, Fishing
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A series of experiments have been undertaken to visually observe the formation and growth of bubbles formed when solution gas is released from waterflood residual oil. The experiments were performed using an etched glass micromodel which represented a two-dimensional section of a sandstone core. This micromodel was mounted vertically within a temperature controlled water bath, and the flow of fluids within the micromodel was observed through a microscope and recorded on video. Experiments were performed under water-wet conditions, using a mixture of low boiling point hydrocarbons, which had a positive spreading coefficient for the oil phase in contact with water and its own vapour. In these experiments the gaseous phase was produced by raising the temperature of the micromodel until it reached the bubblepoint of the oil, at atmospheric pressure.
The following features were observed:
In keeping with the spreading oil, all gas that was generated as bubbles within the oil ganglia from which it was produced. Bubbles did not form in all ganglia, nor did they all form at the same time.
As more gas was released from solution, the bubbles expanded, pushing the gas/oil and oil/water interfaces outwards. Buoyancy forces had little initial influence on the expansion, which occurred in all directions.
The bubbles themselves were unable to migrate, and the gas remained immobile until the volume of individual bubbles had increased to the extent that adjacent bubbles became connected.
The oil phase was transformed from being immobile droplets or ganglia, into films surrounding the gas bubbles, in which the oil was mobile.
A mathematical model for the growth of a single bubble has been developed, which includes the effect of gravity as well as capillary forces in controlling the development of the bubble. Calculations using this model show that the initial growth pattern is dominated by capillary forces, but as the bubble becomes larger buoyancy forces start to play an important part in the way that the bubbles grows.
Reservoir depressurisation at a late stage of waterflood production is seen to be an attractive enhanced recovery project for the Brent reservoir in the UK sector of the North Sea1. Lowering the reservoir pressure provides a way of producing some of the solution gas from the oil remaining in the field at the end of the waterflood, and extending the economic life of the field. In the case of Brent, it is expected that field life will be extended by at least ten years, with an increase in gas production of more than 1 Tscf, and of oil and condensate production of 30 MMstb2. Reservoir depressurisation is also thought to have applications in other reservoirs on the UKCS, and the overall potential for increasing hydrocarbon recovery has been estimated to be 0.2 Bstb equivalent at an oil price of $16/bbl3.
A considerable amount of research has been undertaken in the past to study depressurisation phenomena associated with natural depletion and solution gas drive in virgin reservoir4-25. Much of the existing literature is focused on the role of nucleation and diffusion in the development of the gas phase, and the occurrence of supersaturation in the oil phase. These processes have been modelled theoretically, and good agreement has been achieved between the models and experimental measurements4-7. More recently, visual studies of the release of solution gas in virgin situations have been reported by Li and Yortsos 31, and the processes have been simulated with pore network models32,33. However, the release of solution gas from waterflood residual oil has received comparatively little attention, and this is the subject of the present paper.
In reservoirs that have previously been waterflooded, it is very important to be able to mobilise the gas that is evolved so that it can be recovered at the producing wells. It is well known, however, that when gas is released from solution the gas saturation has to build up to a critical value, Sgc, before it can start to move. This has been observed both in experiments performed with virgin cores7-13, and with cores that were waterflooded before being depressurised 26.
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