BACKGROUND OF THE INVENTION
Steam assisted gravity drainage (SAGD) is now the leading in situ thermal enhanced oil recovery (EOR) process to recover bitumen from Alberta's oil sands. The oilsands are one of the world's largest hydrocarbon deposits. SAGD has two parallel horizontal wells up to about 1000 m long, in a vertical plane, separated by about 5 m. The upper steam injector is controlled by injection steam rate to attain a target pressure set by the operator (i.e. “pressure control”). The lower bitumen and water producer is controlled by pumping rate (or other methods) to maintain a fluid temperature lower than saturated steam (sub-cool or steam-trap control) to ensure no live steam breaks through to the well.
The above control methods work well where the steam chamber is contained, even if the target pressure is higher than the native reservoir pressure. But, the oil sands have a significant portion of the resource that is impaired by water zones (top water, bottom water, interspersed lean zones). These can cause the reservoir to be “leaky” with significant water influx or egress. Under these conditions, SAGD pressure control for steam injection does not work well. Pressure gradients need only be modest to transport large volumes of water and disrupt SAGD. It is hard to choose an appropriate pressure target or to accurately measure an appropriate pressure to minimize the harmful effects of a leaky reservoir. This invention describes an alternate volume control method for SAGD steam injection in leaky reservoirs. The technique involves using WRR (the water recycle ratio) as the key measurement and control parameter. WRR is volume ratio (measured as water) of water produced to steam injected.
The Athabasca bitumen resource in Alberta, Canada is one of the world's largest deposits of hydrocarbons. As describe above, a significant portion of the resource can be impaired by a water zone—causing the reservoir to be “leaky.” Also, The Athabasce bitumen resource in Alberta, Canada is unique for the following reasons:
(1) The resource, in Alberta, contains about 2.75 trillion bbls. of bitumen (Butler, R. M., “Thermal Recovery of Oil & Bitumen”, Prentice Hall, 1991), including carbonate deposits. This is one of the world's largest liquid hydrocarbon resources. The recoverable resource, excluding carbonate deposits, is currently estimated as 170 billion bbls −20% mining (34 billion bbls.) and 80% in-situ EOR (136 billion bbls) (CAPP, “The Facts on Oilsands”, November 2010). The in situ EOR estimate is based on SAGD, or a similar process.
(2) Conventional oil reservoirs have a top seal (cap rock) that prevents oil from leaking and contains the resource. Bitumen was formed by bacterial degradation of lighter source oil to a stage where the degraded bitumen is immobile under reservoir conditions. Bitumen reservoirs can be self-sealed (no cap rock seal). If an in situ EOR process hits the “ceiling”, the process may not be contained and it can easily be contaminated by water or gas from above the bitumen.
(3) Bitumen density is close to the density of water or brine. Some bitumens are denser than water; some are less dense than water. During the bacterial-degradation and formation of bitumen, the hydrocarbon density can pass through a density transition and water can, at first, be less dense than the reservoir “oil”. Bitumen reservoir water zones are found above the bitumen (top water), below the bitumen (bottom water), or interspersed in the bitumen net pay zone (water lean zones (WLZ)).
(4) Most bitumen was formed in a fluvial or estuary environment. Focusing on reservoir impairments, this has two consequences. First, there will be numerous reservoir inhomogeneities. Second, the scale of the inhomogeneities is likely to be less than the scale of a SAGD recovery pattern (FIG. 1) or less than about 1000 m in size. The expectation is that an “average” SAGD EOR process will encounter several inhomogeneities within each recovery pattern.
SAGD is a delicate process. Temperatures and pressures are limited by saturated steam properties. Gravity drainage is driven by a pressure differential as low as 25 psia. Low temperatures (in a saturated steam process) and low pressure gradients make the SAGD process susceptible to impairments from reservoir inhomogeneities, as above.
This invention describes an alternate volume control method for SAGD steam injection in leaky reservoirs. The technique involves using WRR (the water recycle ratio) as the key measurement and control parameter. WRR is volume ratio (measured as water) of water produced to steam injected.
SUMMARY OF THE INVENTION
The following acronyms will be used herein.
American Oil & Gas Reporter
Canadian Association of Petroleum Producers
Computer Modeling Group (Calgary)
Cyclic Steam Stimulation
Enhanced Oil Recovery
Energy to Oil Ratio (MMBTU/bbl)
Electric Submersible Pump
Gravity Drainage (chamber)
Journal of Canadian Petroleum Technology
Steam Assisted Gravity Drainage
Steam to Oil Ratio