CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No. 60/606,755 filed Sep. 1, 2004, and U.S. Provisional Application No. 60/606,756 filed Sep. 1, 2004.
1. Field of the Invention
This invention provides a novel concept for the mobility enhancement of heavy crude oil, thus making possible more efficient and effective recovery of oils including oils that are presently accessible using existing techniques. Such oils include not only the balance of the oil left after primary and secondary recovery operations, but heavy oils and bitumen such as found in Athabaskan oil sands. More particularly, the present invention provides a downhole crude oil processing method that improves reservoir sweep efficiency and enhances quality of the crude oil delivered to the surface. In addition, coke formed may be recovered in the form of natural gas. Cost per barrel is consequently reduced. The method of the present invention significantly increases available domestic oil reserves, and consequently decreases dependence on oil imports by making oil available from the abundant deposits of otherwise in-accessible heavy oils.
2. Description of the Related Art
The industrial world depends heavily on petroleum for energy. However, it has been increasingly clear that production cannot keep up with the rapidly growing need, particularly in view of the growing demand from countries such as China and India. Moreover, the bulk of new production must come from the politically unstable Middle East. Fortunately, it is feasible to produce crude oil from unconventional deposits such as heavy oils and bitumen which represent a resource much greater than conventional petroleum. Such deposits may be recovered by mining and upgrading the recovered oil. However, by far the bulk of such heavy oil reserves occur at depths greater than that from which it can be recovered by surfaces mining. Thus steam flooding extraction methods such as SAGD (steam assisted gravity drainage) have proven useful.
Steam flooding from surface steam generators is an effective and broadly applicable thermal recovery approach to enhanced oil recovery. The primary effects are heating the oil to an elevated temperature thereby reducing the oil viscosity sufficiently low enough to allow flow and displacing the oil toward a production wellhead. The oil removed tends to be the more mobile fraction of the reservoir, and the combustion emissions of the steam generator can be limiting (as in California). Such steam flooding faces limiting technical and economic obstacles relating to conductive heat losses through the wellbore and incomplete reservoir sweep efficiency, especially in heterogeneous reservoirs. With a large fraction of heavy oils there is a further problem: even if the heavy oil is heated sufficiently to promote flow to a wellhead, heat loss in flowing to the surface from typical reservoir depths will result in some loss of fluidity of the heavy oil before reaching the surface. In addition, the oil produced cannot be transported by pipeline without dilution with a lighter oil. Further, although surface mining can recover close to 100 percent of the oil in place, steam flooding typically can recover as little as fifty percent of the oil in place. The oil recovered must be upgraded, typically by coking. However, because thermal coking produces a low-grade oil, there is a trend towards hydrogenation and hydrocracking thus producing a synthetic crude oil similar to a high quality conventional light crude. This requires a supply of hydrogen. Capital costs are high and the availability of natural gas to produce the needed hydrogen can be an issue.
Thus there has been renewed interest in fire flooding techniques which can in principle recover close to eighty percent of the oil in place and upgrade the oil by in-situ thermal coking. This would produce an oil similar to that from coking of surface mined and SAGD produced heavy oils. Unfortunately, fire flooding is difficult to control and the in-situ coking can plug the deposit. Thus there have been only a few pilot operations.
- BRIEF SUMMARY OF THE INVENTION
Technology that would promote economic extraction of such deposits and produce a higher quality oil is thus much needed. With worldwide consumption of petroleum increasing year-by-year, production of oil from heavy crude oil deposits in accordance with the present invention can play an important role in limiting dependence on importation of petroleum to meet consumption demand.
It has now been found that heavy oil can be upgraded downhole to produce a lower viscosity oil which does not require dilution for transport to a refinery or an upgrader. By contact with steam at a temperature high enough to promote steam cracking of the heavy oil in-situ, upgraded oil is produced. Preferably, coke formed can react with the steam to form hydrogen and/or methane.
In one embodiment of the present invention, steam from a conventional surface boiler is mixed with sufficient oxygen wherein the oxygen is combusted with carbon to form CO2. The fluid temperature would be high enough to heat the oil to a temperature sufficient to induce cracking of the oil. In this embodiment, all the heat of combustion released is delivered to the reservoir to enhance steam cracking. The steam and oxygen present react with carbon thus maintaining reservoir porosity. The optional injection of catalyst into the steam combustion product stream further promotes cracking of the heavy oil and allows control of pH.
Temperatures above the critical temperature of steam are required. Temperatures of 800 to 1000 degrees Fahrenheit are advantageous allowing production of upgraded oil and methane. Even higher temperatures allow production of hydrogen and further upgrading of the in place oil. The required oxygen may be supplied either as air, oxygen enriched air, or pure oxygen. Use of pure oxygen minimizes dilution of produced methane or hydrogen. The amount of oxygen should be such that upon injection into a heavy oil reservoir, in-situ combustion releases enough heat to raise the temperature sufficiently for steam cracking of the oil. Such temperatures are well known in the art.
It is preferred that reservoir oil be heated at least to a temperature of 700 degrees Fahrenheit. Thus steam temperatures above this level are desired. This enables in-situ refining of heavy crude oils with minimal formation of carbon deposits. Not only is oil viscosity reduced by heating the oil, as in conventional steam flooding, but also intrinsic viscosity is reduced by thermal cracking the oil in the presence of steam thereby altering its chemical composition. Unlike conventional in-situ combustion, carbon produced in the cracking process reacts with steam and is converted to a mixture comprising methane and carbon dioxide (or hydrogen and carbon oxides at higher temperatures), thermodynamically favored reactions, expressed as follows:
C+2H2O=2H2+CO2 & C+2H2=CH4
Free hydrogen is produced from carbon even at temperatures as low as 700 degrees Fahrenheit. Thus, hydrogen is available for in situ desulfurization of reservoir oil. Both cracking and desulfurization lower oil viscosity thereby upgrading the oil. Sweep efficiency is improved via enhancement of mobility and control of reservoir permeability as a result of the reduction of intrinsic oil viscosity. Thus effective recovery of the oil in place can exceed ninety percent.
In another embodiment of the present invention, steam at a temperature high enough for steam cracking may be produced by downhole combustion of a fuel supplied from the surface together with water. The water is converted to steam of a desired temperature downhole thereby eliminating the need for a surface steam boiler. In this embodiment all combustion products are delivered downhole and boiler heat losses are avoided. Any known combustor system may be used.
Preferably, a catalytic combustion system is provided downhole. Catalytic combustors have two interrelated features that allow downhole combustion of hydrocarbon fuels for generation of a higher temperature steam: combustion stability and soot free operation. Operation within normal flame stability limits is not required allowing use of low BTU fuels. This approach retains all the benefits of downhole steam generation while adding the benefits of in-situ oil upgrading and thereby significantly reducing costs and improving sweep efficiency.
In another embodiment of the present invention, stoichiometric amounts of hydrocarbon fuel and a gas containing oxygen are combusted downhole in a catalytic combustor to produce heat and an admixture comprising carbon dioxide and steam. Water is injected into the hot combustion products to produce a cooled mixture at a temperature of about 1160 degrees Fahrenheit, a temperature more than adequate for rapid cracking of oils. The cooled mixture is then passed into contact with the heavy oil deposit. The mixture temperature may be varied to maintain the oil deposit at a desired temperature depending on the degree of cracking desired.
BRIEF DESCRIPTION OF THE DRAWINGS
In another embodiment of the present invention, steam from a conventional surface boiler is admixed with compressed oxygen in sufficient quantity and reacted with carbon to form CO2. The mixture temperature is increased by at least one hundred degrees Kelvin, more preferably by at least one hundred fifty degrees Kelvin. The amount of oxygen provided may be varied to maintain the oil deposit at a desired temperature depending on the degree of cracking desired. Optionally, compressed air or oxygen-enriched air may be used to supply the required oxygen.
FIG. 1 depicts a diagrammatic representation of one embodiment of a steam production and oil extraction system according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 depicts a diagrammatic representation of another embodiment of a steam production and oil extraction system according to the present invention.
The present invention yields a superior and more flexible enhanced oil recovery process comprising methods for providing steam downhole at a temperature high enough for in-situ steam cracking of oil deposits and for production of methane from coke. If available, supercritical temperature steam can be supplied from the surface. Catalysts known in the art including boric acid and potassium hydroxide may be added to the steam admixture, to enhance cracking efficiency and/or the reaction of carbon with steam.
The hot fluid comprising steam is injected into the oil-bearing formation to provide the heat and the hydrogen necessary for production of methane from carbon. Steam cracking of the oil reduces oil viscosity and disrupts oil-sand bonding. The hot fluid may be injected into an oil-bearing reservoir via a vertical well or, more advantageously, through a horizontal well. In huff-and-puff operation, injection of the hot fluid is stopped after a chosen portion of the reservoir has reached a selected temperature. Oil is then withdrawn through the heated zone into the injection well which becomes the production well. Preferably, the heated portion of the reservoir may be allowed to soak at temperature for a selected period of time, allowing for greater reaction of the oil in contact with steam. This permits lower reaction temperatures. The result is a process system that offers numerous benefits with a number of controllable variables. Because oil fields differ and the task of recovery varies in each case, these variables can be adjusted to adapt the process to fit the particular reservoir conditions.
As in conventional steam flooding, heat and pressure may be used drive oil from the source and towards a producing well. Gravity drainage may be employed. Because steam cracking reduces the oil molecular weight, the oil remains fluid even at low ambient temperatures. Thus, flow through the production well is no longer limited by heat loss during transport to the surface.
Optionally, a catalyst may be added to the steam to enhance cracking of the oil in the reservoir while weakening molecular polarity thereby promoting displacement of the oil from the sand. Water/steam miscible catalysts include materials such as boria, potassium and sodium carbonate as well as compounds of known catalytic metals including nickel, cobalt and chromium. The catalysts injected can be selected to adjust the pH of the steam. This can help control permeability of oil-bearing clays, the swelling of which is a function of pH.
The CO2 and methane produced in the steam cracking reactions offer pressure maintenance for enhanced product flow. The downhole CO2 displaces the oil through preferential adsorption of the CO2 in the sand/clay particles, although at higher temperatures this effect can be limited. Where the presence of nitrogen would unduly dilute associated natural gas, pure oxygen can be used instead of air to supply the oxygen. For very deep wells, the costs of compressing air can outweigh the costs of pure oxygen production, leading to preferential use of pure oxygen. Using gaseous oxygen instead of air reduces the amount of oxidant that must be compressed. The capability to control fluid pH enables control of clay permeability in clay-bearing strata, also important in improving sweep efficiency.
FIG. 1 depicts an application 10 of the present invention within a sloping viscous oil deposit. Injection well 12 extends vertically downward from surface 14 to oil reservoir formation 20. Injection well 12 is in fluid communication with horizontal well section 16 which proceeds along a lower elevation region of formation 20. Formation 20 is underlain by impermeable region 22 and capped by impermeable layer 24. The hot fluid flow 18 enters formation 20 migrating upward from horizontal well section 16 into formation 20. The reacted oil will migrate downslope and collect within formation 20. In a huff and puff operation, reacted oil may be recovered through injection well 12 and horizontal well section 16 after termination of fluid injection.
FIG. 2 depicts an application 110 of the present invention within a heavy oil deposit. Injection wells 112 and 113 extend vertically from surface 114 to the bottom of deposit 120. Injection well 112 and then extends horizontally along horizontal well section 116, along the bottom of deposit 120 which slopes uphill. Injection well 113 similarly extends horizontally along horizontal well section 117 along the bottom of deposit 120. Deposit 120 is underlain by impermeable region 122 and capped by impermeable layer 124. Hot fluid flow 118 enters deposit 120 migrating upward from horizontal well sections 116 and 117 into deposit 120. Production well 126 extends vertically from surface 114 to a downhill region of deposit 120 where it can collect and extract cracked oil. A horizontal section, horizontal well section 128, of production well 126, extends nominally perpendicular to horizontal well sections 116 and 117 in a downhill location of deposit 120. Hot steam-bearing fluid may be provided from the surface or generated downhole using a catalytic combustor.
This invention improves sweep efficiency through steam cracking and provides improved downhole temperature control. In contrast, conventional surface generation of steam provides oil temperatures that are too low for effective steam cracking thereby producing unrefined oil and limiting the achievable recovery of oil in place. Since in-situ steam cracking improves crude quality and oil recovery efficiency, the present invention significantly reduces the oil price required for profitability after extraction thus further augmenting the advantage compared to conventional steam flooding. No coking of produced oil is needed since sufficient coking can occur in-situ with the coke reacted with steam to produce methane and carbon dioxide. The present invention effectively increases the recoverable reserves of heavy oils by allowing high recovery of oil in place and also offers benefits in enhanced recovery of lighter oils and oil from shale. Methane produced can be utilized as fuel for the steam production.
Although the invention has been described in considerable detail, it will be apparent that the invention is capable of numerous modifications and variations, apparent to those skilled in the art, without departing from the spirit and scope of the invention. Such modifications and variations should be considered within the scope of the present invention.