Multi-step solvent extraction process for heavy oil reservoirs

#This invention relates to the field of hydrocarbon extraction and more particularly to the extraction of heavy oil from underground formations. Particularly, this invention relates to a multi-step heavy oil extraction technique to be used, for example, after primary extraction is no longer effective. Most particularly this invention relates to a solvent based multi-step enhanced extraction process for heavy oil.

#Heavy oil is a loosely defined term, but heavy oil is generally understood to comprehend somewhat degraded and viscous oils that may include some bitumen. Heavy oils typically have poor mobility at reservoir conditions so are hard to produce and have very poor recovery factors. Heavy oil is generally more viscous than light or conventional oil, but not as viscous as bitumen such as may be found in the oil sands. Heavy oil is generally understood to include a range of API gravity of between about 10 and 22 with a viscosity of between about 100 and 10,000 centipoise. For the purposes of this specification the term heavy oil shall mean oil which falls within the foregoing definition.

#Heavy oil exists, in situ, in large quantities, but is difficult to recover. A recent (2003) estimate of the resource by the US Geological Survey, using an estimated recovery factor of 19% puts the theoretically recoverable heavy oil in North America alone at 35.3 billion barrels. This USGS estimate implies that the total domestic North American heavy oil resource is about 200 billion barrels and that more than 80% of this domestic heavy oil is unrecoverable using the best currently available extraction process technology. The USGS report also implies that the worldwide heavy oil resource is 3.3 trillion bbls and that 87% of this resource is unrecoverable or “stranded” with current technology. The commercial opportunity for a better extraction technology is therefore substantial. More specifically, an advance in extraction technology which raises the recovery rate of heavy oil from the current 13% level to only 25%, would contribute an additional 400 billion bbls of recoverable oil worldwide.

#Reference will now be made, by way of example only, to preferred embodiments of the present invention by referring to the following figures, in which:

#FIG. 1 shows a representation of target heavy oil reservoir with a horizontal well positioned near the bottom of the pay zone and a vertical injection well.

#FIG. 2 is a graph of permeability in milli-darcies against total permeability for a typical heavy oil reservoir;

#This present invention is most applicable to heavy oil reservoirs which have undergone a primary extraction and also which demonstrate good confinement. According to the present invention the primary extraction has resulted in an oil extracted region in the reservoir having either gas or water filled voids. A preferred reservoir has had a primary extraction which has recovered between about 5% and 25% of the original oil in place with a most preferred amount being between 8% and 15%. Most preferably a suitable target reservoir will have a significant pay thickness without extensive horizontal barriers so that when the viscosity of the in situ heavy oil is sufficiently reduced, gravity drainage can occur. While a primary extracted reservoir is preferred the present invention is also suitable for virgin reservoirs of the type having naturally occurring drainable voids having a volume of between about 5% and 25% of the original oil in place. An example of such a reservoir is one with a 20-40% water saturation and 60-80% oil saturation, but well confined reservoir in a porous formation.

#FIG. 1 shows a schematic of a target oil reservoir with a vertical well 20 and a horizontal production well 22. The horizontal well 22 is generally placed near the bottom of pay zone 24, and is a production well through which fluids draining through the reservoir by gravity drainage, can be removed. The typical pay zone 24 has layers of different permeability shown as 28, 30, 32, 34, 36, 38, and 40. Most preferably the pay zone 24 is confined by an impermeable overburden layer 25 and an impermeable under burden layer 26, but as will be appreciated by those skilled in the art of reservoir engineering, the present invention also comprehends that man made means for confinement can also be used. Preferably the pay zone 24 has been produced using conventional primary extraction techniques, such as CHOPS (cold heavy oil production with sand), to the full extent possible which has left significant void volumes in what may be called an oil extracted zone. Although the pay zone layers 28 to 40 may be fairly uniform there are typically some permeability variations due to, for example, the original depositional process. There is also typically some natural variation in the oil quality and viscosity with position in the reservoir.

#As a consequence of the primary oil recovery from the reservoir, the highest permeability zones in the pay zone 24, in this case layers 30 and 38 will have been preferentially depleted of heavy oil, while the slightly less permeable zones 28, 32, 34, 36 and 40 will have been mostly bypassed thus having higher proportions of “stranded oil”. If the reservoir was on primary depletion with no pressure support, the depleted regions will likely also have some gas saturation as the naturally occurring in situ dissolved gas comes out of solution and fills the pores as the oil is removed. Significant water or brine is also likely to be present in the voids of the extracted oil regions of the pay zone, especially where waterflooding has been applied. Solvent is being injected as shown by arrow 44 in vertical well 20 and a mixed solvent and oil blend 46 is being removed, for example by a pump 48.