Non-thermal Heavy Oil Recovery a report by Pablo Barajas and I Yucel Akkutlu School of Petroleum and Geological Engineering, Mewbourne College of Earth and Energy, University of Oklahoma
Enhanced oil recovery (EOR) techniques are practical engineering solutions to a set of challenging scientific problems related to the aided recovery of oils. The techniques fundamentally involve injection of a fluid into a subsurface reservoir and, consequently, mobilisation of the fluid-contacted oil in place, followed by its displacement and, finally, production. The latter stages are driven by potential, chemical and/or temperature gradients, which are artificially generated in the reservoir by the injection process, under the influences of gravity, capillarity, and in situ chemical reactions. The success – or failure – of these techniques in the field is strongly tied to our ability to predict the complex reservoir phenomena of oil recovery and to develop operational strategies for efficient production.
Our understanding and predictive ability of the steam-based thermal recovery techniques, such as cyclic steam stimulation and steam ‘flooding’, has traditionally been at relatively advanced levels. Steam injection techniques are applied to shallower depths (<2000 ft) and heavy (<26oAPI) oil reservoirs with reasonable success. They have, therefore, been the dominant thermal heavy oil recovery techniques in the industry, in particular in Canada, the US, and Venezuela. However, recent production trends (Figure 1) show a gradual decline in thermal recovery in the US during the last two decades. Large heat requirements make the thermal methods uneconomic in mature fields and less efficient in deep tight reservoirs with thin pay zones. In these cases non-thermal – primarily miscible gas injection – methods are emerging as the techniques more convenient for the recovery of heavy oils.
Below, we illustrate and emphasise this point of view by discussing the current status of non-thermal methods based on trends in number of field projects and total heavy oil production in the US. For this purpose, we use the biennial Oil and Gas Journal EOR survey data from 1988–2008. Following, we briefly delve into technical and operational challenges of the non-thermal heavy oil recovery methods. Although they take place at nearly isothermal conditions, these methods involve intrinsically complex reservoir physics, theoretical description and numerical modelling of which is outside of the current bounds of the conventional reservoir flow simulators. The focus will be on the hydrocarbon miscible gas injection and on hydrocarbon ‘solvent vapour extraction’ (VAPEX), a new generation of promising non-thermal heavy oil recovery technique – which integrates the horizontal well technology and the concept of oil gravity drainage with the conventional hydrocarbon-miscible gas injection process.
Improved Waterflooding – Was it a Myth? Do we Even Want It?
Based on the field projects and ongoing research, the non-thermal recovery methods are classified in Table 1 into two main categories:
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improved waterflooding and gas injection. Improved waterflooding techniques – also known as chemical flooding – were widely used in the 1980s as a waterflooding follow-up. Biopolymers were the most popular additives used to increase the oil displacement and sweep efficiencies by controlling the rheological properties of the injected water. Other, more expensive chemicals used included caustic/alkaline and surfactants. Various operational problems were encountered with the improved waterflooding techniques such as shear-, thermal- and biodegradation of the injected chemicals, low or reduced injectivity, and loss of the chemicals in reservoir rock by physical adsorption and entrainment. These problems are serious and they increase the operational cost of injection significantly; consequently, the number of projects has been reducing. A series of encouraging technological breakthroughs and innovations are currently needed for companies to take the improved waterflooding beyond government-supported – tax incentives or cost-share – pilot tests in the field.
Ushering in a New Era for Enhanced Oil Recovery – Gas Injection
Nowadays, gas injection methods are consolidated as the most widely used non-thermal EOR techniques in the field over improved waterflooding methods. The increased non-thermal enhanced oil production trend observed in Figure 2 is due to gas injection. Inefficient improved waterflooding projects were terminated in the 1990s and new gas injection projects have been implemented since 1998. To put this in perspective, the general EOR trends in the US indicate that the era of thermal and chemical EOR is being progressively replaced by the hydrocarbon and non-hydrocarbon
(flue gas, N2, CO2, air) gas injection techniques. These techniques – with the exception of air – are often practiced under the influence of local and federal environmental regulations, e.g., as part of the efforts to reduce gas flaring.
Air injection involves a group of oil recovery processes, which develop under on the reservoir conditions when air comes in contact with the oil. In situ combustion is only one of these recovery processes and – due to appearance of a high-temperature oxidation wave in the reservoir – traditionally considered in the context of thermal recovery, in particular, from shallow low-pressure heavy oil reservoirs. Until recently, air injection was synonymous with in situ combustion. More subtle distinctions among the other recovery processes of air injection are lately beginning to emerge with the application of air into deep high-pressure light oil reservoirs.
N2-miscible displacement of air with low-temperature oxidation (oxygen of the injected air being consumed by the liquid hydrocarbons
in-place at or near the reservoir temperature) can be an effective improved recovery method. Williston Basin air injection projects in South and North Dakota are examples of them. A reactive immiscible
© TOUCH BRIEFINGS 2011
Heavy Oil
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