The production capacity of a liquid hydrocarbon-containing subterranean formation may be related to a wide array of factors, including the quantity of hydrocarbons present in the formation, the porosity and permeability of the formation itself, the pressure within the formation, the temperature within the formation, the viscosity of the hydrocarbons contained within the formation, the length of the wellbore that is exposed to the hydrocarbon-bearing strata, the presence of water, gas, and/or other materials within the formation, and a host of additional variables. Due to the variety of potential interactions among these various factors, the presence of hydrocarbons within a subterranean formation does not, in itself, indicate that the hydrocarbons may be economically recovered.
Historically, reservoirs containing conventional oil reserves that may be economically produced using traditional techniques have been the first to be developed. Many of these reservoirs are currently in a state of decline and/or have been depleted, at least with respect to oil that may be recovered with traditional techniques. Even when these reservoirs contain a large quantity of conventional oil, this oil may only make up a fraction of the total hydrocarbons contained within the well. In addition, conventional oil reserves only make up a fraction of the total, worldwide oil reserves. Thus, a wide variety of techniques have been developed to increase the overall recovery of conventional oil from a subterranean formation, as well as to facilitate the recovery of unconventional oil. Illustrative, non-exclusive examples of such methods include water injection, which may increase the pressure within the formation, and steam injection, which may increase both the pressure within the formation and the temperature of the oil contained therein, thus decreasing the oil's viscosity and allowing it to flow more readily. Other techniques include advances in well design and construction, such as the development of horizontal drilling technology, and the use of solvents to dissolve high-viscosity oil.
In the case of steam injection, high-pressure steam may be injected into the subterranean formation. As stated above, this steam may increase the pressure within the formation, increasing the driving force for oil flow out of the formation through a well. In addition, the steam may carry a significant amount of thermal energy, both sensible and latent, into the well. As the steam cools, it may release both sensible and latent heat and increase the temperature of the oil within the formation. As the oil temperature increases, its viscosity may decrease, allowing it to flow more easily from the formation and thereby increasing the overall oil recovery. Steam injection may be accomplished utilizing a variety of known techniques. For example, see: S. M. Farouq Ali, “Heavy Oil—Evermore Mobile,” Journal of Petroleum Science and Engineering, 37(1), pp. 5-9, February 2003. Illustrative, non-exclusive examples of such techniques include steamflooding, steam assisted gravity drainage (SAGD), cyclic steamflooding, steam soak, and/or cyclic steam stimulation (CSS). While steam injection may be quite effective under certain conditions, it also has inherent limitations.
For example, as the pressure of steam is increased, its latent heat of vaporization decreases. At pressures approaching the critical pressure (3200 pounds per square inch absolute pressure (psia) (22 MPa) for pure water), the latent heat of vaporization of steam approaches zero. This decrease and/or elimination of the latent heat of vaporization at high pressures translates to a significant decrease in the ability of a given volume of near-critical and/or supercritical steam stream to transfer thermal energy to a subterranean formation and thus to oil within the formation. In addition, the density of this high-pressure steam may become liquid-like, and the volume change upon cooling may decrease as the pressure approaches the critical pressure, thereby decreasing the pressure increase within the formation for a given mass of steam injected. While this may be at least partially compensated for by a corresponding increase in the steam temperature, doing so may result in operating temperatures that may be in excess of the temperatures desirable for steam injection. Thus, at high pressures, traditional steam injection may become much less beneficial.
As a general rule of thumb, the pressure within many undisturbed reservoirs may be considered to increase by approximately 0.5 psia for each additional foot of reservoir depth (11 kPa for each additional meter of reservoir depth). Thus, the ambient pressure of deep oil reservoirs may approach and/or exceed the critical pressure of pure water and may preclude the efficient use of traditional steam injection methods for the reasons discussed herein. However, since steam injection is a well-established and generally cost-effective method for shallower reservoirs, systems and methods to extend steam injection to deep viscous oil reservoirs are of interest and would be of utility.
Traditional steam injection techniques have been modified in a variety of ways. One such modification is through coinjection with a noncondensable gas species. Illustrative, non-exclusive examples of these modifications are disclosed in U.S. Pat. Nos. 4,324,291 and 4,565,249, the disclosures of which are incorporated by reference. Additional illustrative, non-exclusive examples are disclosed in Canadian Patent No. 1,228,020, the 1984 SPE Paper 11702 by K. C. Hong and J. W. Ault, which is entitled “Effects of Noncondensable Gas Injection on Oil Recovery by Steamflooding,” and the 1998 SPE Paper 30297 by N. P. Freitag and B. J. Kristoff, which is entitled “Comparison of Carbon Dioxide and Methane as Additives at Steamflood Conditions,” the disclosures of which are incorporated by reference. However, these modifications have been made to aid sweep efficiency, to act as a solvent to reduce oil viscosity, and/or to act as an insulating blanket by forming a gas cap, and not to increase the depth at which steam injection may be effectively utilized.
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