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.
The bitumen containing oil sands of Canada have received a much attention due to their immense store of hydrocarbon. However, it would only take a tiny change in the average recovery factor for worldwide heavy oil from 13% to 18% of oil in place to provide an equivalent amount of oil to that which is considered recoverable from the Canadian oil sands. With concerns about peak oil and a limited scope for new reservoir discovery, the ability to recover stranded heavy oil is becoming increasingly important. Furthermore, being able to recover additional oil using energy efficient extraction technology is also very desirable. Solvent has long been recognized to have the theoretical potential to mobilize and recover the stranded heavy oil. Solvent would potentially not require the application of high temperatures and consequent liabilities of high energy consumption and greenhouse gas emissions which plague steam driven bitumen extraction processes for example.
It is currently understood by those skilled in the art, based on best available computer simulation models, that solvent diffuses quickly and deeply into in situ heavy oil. This is evident in the published results from computer simulations (Tadahiro et al, May 2005 JCPT pg 41, FIG. 18) that shows propane solvent penetrating 8 meters (25 feet) beyond the edge of a vapour chamber into a 5200 cp heavy oil. Similarly Das (2005 SPE paper 97924 FIG. 12) comments that it is realistic to expect propane solvent will penetrate 5 meters beyond the edge of the chamber in an Athabasca reservoir.
However, lab studies by the inventor (Nenniger CIPC paper 2008-139, FIGS. 1 and 2) have shown that the solvent extraction mechanism for heavy oil and oil sands is quite different than as predicted by the computer simulations. In particular, rather than easily diffusing deep into an oil bearing zone, the solvent is observed to form a well defined interface with undiluted oil at what might be called a concentration shock front. The concentration shock front arises because the solvent has a very difficult time diffusing or penetrating into the high viscosity oil like heavy oil or bitumen. In a sandpack experiment, the inventor observed asphaltene deposition within a pore length of the raw bitumen, which means that the concentration gradient is extraordinarily steep over a very small length scale.
The physical length scale of the dissolution process of solvent into heavy oil observed is that of individual pores, which are about 100 microns long in 5 Darcy sand. It seems reasonable to assume that two miscible hydrocarbon fluids such as oil and solvent should mix quickly and fairly easily as shown in the simulations of Tadahiro and Das. Consequently, the experimental observation of a concentration shock was surprising and unexpected. More specifically, the observation of a concentration shock front indicates that conventional wisdom regarding rapid dilution of heavy oil and bitumen via solvent diffusion is incorrect.
Many attempts have been made in the prior art to develop solvent based extraction processes. For example, U.S. Pat. No. 5,720,350 teaches a method for recovering oil left behind in a conventional oil reservoir after the original conventional oil has been recovered. This process uses gravity drainage from a formation in which an oil miscible solvent having a density slightly greater than a gas contained in a gas cap is injected above the liquid level in the formation. Following solvent injection the production of oil is commenced from a lower portion of the formation. The idea seems to be that the solvent sweeps the remaining oil to the production well. However, conventional recoveries are generally very good meaning that 30 to 60% or more of the oil in place can be recovered, consequently very large and potentially uneconomic volumes of solvent may be required to recover any significant portion of the remaining oil.
U.S. Pat. No. 5,273,111 teaches a laterally and vertically staggered horizontal well hydrocarbon recovery method, in which a continuous process is used combining gravity drainage and gas drive or sweep (ie pressure drive) to produce the oil from a specific configuration of vertical and horizontal wells. The configuration of the wells is said to be optimized to reduce coning and solvent breakthrough between the wells, but the use of a gas drive or sweep will result in preferential recovery through the higher permeability portions of the reservoir. Thus, even if the coning and solvent breakthough is reduced, it will still be significant, meaning that the drive process will likely bypass much of the stranded oil.
U.S. Pat. No. 5,065,821 teaches a process for gas flooding a virgin reservoir with horizontal and vertical wells which involves injecting a gas through a first vertical well concurrently with performing a cyclical injection, soak and production of gas through a horizontal well, to eventually establish connection to the vertical well, after which time the vertical well becomes the production well and the horizontal well becomes the injection well. Again this process teaches the continuous solvent gas injection (i.e. a pressure drive) through the reservoir once connection is established between the wells. During the initial steps, into a virgin reservoir it will be very difficult to get the solvent to diffuse into and dilute the oil making this process slow and impractical.
Canadian patent application 2494391 to Nexen discloses a further solvent based extraction technique which uses a continuous solvent injection or extraction of the type that may be characterized as a solvent sweep or drive with a pattern of horizontal and vertical wells. Again, however, any attempt to push out the oil with a solvent drive process is anticipated to lead to rapid coning, short circuiting, by-passing and only marginal recovery.
Notwithstanding these and many other prior attempts to perfect a solvent based extraction process for heavy oil, the results remain unsatisfactory. There is a clear need for a different and better understanding of how to effectively use solvent to improve heavy oil recovery, in a way that reduces bypassing of stranded heavy oil. What is desired is a solvent extraction process which comprehends this understanding of how slowly the solvent penetrates into the in situ heavy oil and addresses this problem directly.