Non-thermal Heavy Oil Recovery Figure 4: Status of Gas Injection for Heavy Oil Recovery
1 2 3 4 5 6 7 8
0 1998 2000 Hydrocarbon miscible
60,000 50,000
40,000 30,000 20,000 10,000 0
1998 2000 Hydrocarbon miscible 2002 2004 CO2 Immiscible 2006 2008 Nitrogen Immiscible
as the injectant fluid. In its simplest form, it requires continuous injection of solvent to a subsurface oil reservoir using a horizontal injector and maintenance of a constantly evolving solvent chamber. Following an initial transition period of injection, during when the chamber grows mainly vertically, the solvent chamber is expected to grow laterally – in parallel – and above a close by horizontal production well so that, once mobilised inside or near the chamber wall, oil could readily be drained by gravity and produced. Ideally, solvent injection, chamber growth and oil drainage should proceed at a steady rate in order to have a stable and successful recovery.
The wall of the chamber is a nonlinear moving boundary, i.e. a thin layer or rather an interface, separating the injected hydrocarbons and in-place oil. The solvent interface has similarity to the mass transfer-dominated mixing zone of the other mentioned gas injection methods above. Locally, however, the interactions of vapour solvents with the heavy oil are complex and more important for recovery. It is believed that a continuous oil renewal mechanism aided by gravity and capillarity could yield much higher rates of diffusive mass transfer inside the solvent interface. Further, the efficiency – and complexity – of the interface is enhanced by the so- called ‘de-asphalting’ effect of the solvent which potentially leads to in situ heavy oil upgrading during the VAPEX process.
Pilot projects are currently active in Saskatchewan and Alberta, Canada, testing economic viability of the VAPEX process on heavy oil recovery. Propane and natural gas are being injected into shallow (~1,500 ft) and heavy oil reservoirs with gravity as low as 12oAPI
44 2002 2004 CO2 Immiscible 2006 2008 Nitrogen Immiscible
reservoirs. An initially slow response time is reported, nonetheless, after a few years of operation and once the solvent breakthrough occurs, oil production increases about ten-fold. It is anticipated that the ultimate recovery will be 40% of the initial oil in place, which is a significant enhancement in production compared to the typical 2–3% recovery with primary production.
Should I use Steam-assisted Gravity Drainage or Solvent Vapour Extraction?
VAPEX is the non-thermal equivalent – rather an offspring – of the steam-assisted gravity drainage (SAGD), specifically designed to avoid the heat-requirement and related heat-management problems of the latter. It is, however, a delicate process and has its own unique operational problems.
Mobilization of the oil is mass transfer-controlled, namely the oil drainage towards the horizontal producer takes place at times comparable to the rates of solvent diffusion and dissolution in oil; therefore, it is widely recognized as a very slow recovery process. In addition, the solvent vapor chamber growth is more difficult to predict and control, compared to the steam chamber. Furthermore, the solvent chamber is often under detrimental influence of depositional flow barriers, such as thin shale layers.
Non-thermal Future of Heavy Oil Recovery It is our opinion that the recent EOR trends are likely to continue into the future, which is going to further decrease the contribution of thermal methods in total heavy oil recovery and increase the role of gas injection methods.
Miscible hydrocarbon gas injection, in particular the new variants of VAPEX, are going to prevail in near future. The brood is likely to involve multiple horizontal wells with carefully designed
It is our opinion that the recent EOR trends are likely to continue into the future, which is going to further decrease the contribution of thermal methods in total heavy oil recovery and increase the role of gas injection methods.
multi-stage fracturing to create an artificial network of fractures – fast highways – for the slow mass transfer of the vapour extraction process and for bypassing the flow barriers. Further it is likely to call for multi-component fluids as the gas injectant, constituting several chemical species proportions of which are designed at the injection pressure and temperature conditions simply to better control and optimise the chamber location and growth during the injection process. Finally, even stricter environmental regulations are likely to appear and promote hybridisation of the gas
injection techniques, e.g. CO2- and/or N2-hydrocarbon miscible gas injection. n
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
US heavy oil EOR (gas) production [b/d]
US heavy oil EOR (gas) projects
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