I. Field of the Invention
This invention is concerned with a method for recovering viscous petroleum including bitumen from viscous petroleum-containing formations including tar sand deposits, and more particularly is concerned with an improved carrier gas vaporized solvent flooding method especially useful in viscous petroleum-containing formations including tar sand deposits.
II. Description of the Prior Art
There are many subterranean petroleum-containing formations in various parts of the world from which petroleum cannot be recovered by conventional means because the petroleum is too viscous to flow or be pumped. The most extreme example of viscous petroleum-containing formations are the so-called tar sand or bituminous sand deposits. The largest and most famous such formation is the Athabasca Tar Sand Deposit in the northeastern part of the Province of Alberta, Canada, which contains over 700 billion barrels of petroleum. Other extensive deposits are known to exist in the western United States and in Venezuela, and smaller deposits exist in Europe and Asia.
Tar sands are defined as sand saturated with a highly viscous crude petroleum material not recoverable in its natural state through a well by ordinary production methods. The petroleum constituent of tar sand deposits are highly bituminous in character and very viscous. The sand present in tar sand deposits is generally fine quartz sand coated with a layer of water, with the bituminous petroleum material occupying most of the void space around the water wetted sand grains. The balance of the void space is filled with connate water, and some deposits contain small volumes of gas such as air or methane. The sand grains are packed to a void volume of about 35 percent, which corresponds to 83 percent by weight sand. The balance of the material is bitumen and water, and the sum of bitumen and water is fairly consistently 17 percent by weight, with the bitumen portion thereof varying from about 2 percent to about 16 percent. One of the characteristics of tar sand deposits which differs considerably from conventional petroleum-containing formations is the absence of a consolidated material matrix within the formation. The sand grains are generally in contact although mostly uncemented and the bitumen occupies most of the void space. The API gravity of the bitumen ranges from about 6 to about 8, and the specific gravity at 60.degree. F. is from about 1.006 to about 1.027 and the viscosity is in the millions of centipoise range at formation temperature.
The methods for recovering bituminous petroleum from tar sand deposits are classified as strip mining and in situ separation processes. Most of the recovery to date has been by means of strip mining, although this method is economically feasible at the present time only when the ratio of overburden thickness to tar sand deposit thickness is around 1 or less. Vast quantities of petroleum are known to exist in the form of tar sand deposits which are not within a range which is economically suitable for strip mining, and so there is a serious need for some form of in situ process wherein the bitumen or bituminous petroleum is separated from the sand by some means and recovered therefrom through a well or other production means drilled into the tar sand deposit.
In situ separation processes described in the literature include thermal techniques, such as fire flooding (or in situ combustion) and steam flooding, and emulsification drive processes. To be successful, an in situ separation process must accomplish two functions: the viscosity of the crude oil must be reduced and some form of oil displacement or driving mechanism must be supplied to the formation. Emulsification processes frequently also employ steam, plus a basic material such as sodium hydroxide which induces formation of an oil-in-water emulsion having a viscosity substantially lower than the viscosity of the formation petroleum. Thermal processes are restricted to formations having sufficient overburden thickness to tolerate injection of high pressure fluids. Many tar sand deposits exist in which the overburden thickness is too thin for thermal flooding and too thick for strip mining.
One other possible process for recovering bitumen from tar sand deposits by in situ separation which does not require the presence of sufficient overburden thickness to contain high pressures, is solvent flooding. Solvent flooding involves injection of a solvent into the tar sand deposit, which solvent dilutes and reduces the viscosity of the bituminous petroleum to render it mobile and recoverable by means of a well as is normally employed in conventional oil recovery operations. Although many solvents including aromatic hydrocarbons such as benzene, toluene and xylene, as well as carbon tetrachloride and carbon disulfide, readily dissolve bituminous petroleum, these materials are expensive and since huge quantities are required, solvent flooding has not been considered to be economically feasible. Paraffinic hydrocarbons such as ethane, propane, butane, pentane, etc. are more readily available and less expensive than those solvents described above, but it has always been uniformly assumed by persons skilled in the art that paraffinic hydrocarbon solvents could not be used in bituminous petroleum because of the danger of precipitating asphaltenes, which would cause formation plugging. Indeed, the asphaltic constituents of crude oil are frequently defined as pentane insoluble materials. Asphalt removal from oil by contacting the crude with propane is a well known refinery process. Furthermore, the cost of solvent flooding has always been considered prohibitive because of the vast quantities required to saturate the formation.
It can be seen from the foregoing that there is a substantial need for a method for recovering viscous petroleum such as bitumen or bituminous petroleum from tar sand formations by use of moderate quantities of readily available, inexpensive solvents in a relatively low pressure procedure that can be used in intermediate depth deposits as well as in deep deposits.