Wells – Processes – In situ combustion
Reexamination Certificate
2000-01-24
2001-12-11
Tsay, Frank (Department: 3672)
Wells
Processes
In situ combustion
C166S059000, C166S261000
Reexamination Certificate
active
06328104
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for simultaneously upgrading and recovering heavy crude oils and natural bitumens from subsurface reservoirs.
2. Description of the Prior Art
Worldwide deposits of natural bitumens (also referred to as “tar sands”) and heavy crude oils are estimated to total more than five times the amount of remaining recoverable reserves of conventional crude [References 1,5]. But these resources (herein collectively called “heavy hydrocarbons”) frequently cannot be recovered economically with current technology, due principally to the high viscosities which they exhibit in the porous subsurface formations where they are deposited. Since the rate at which a fluid flows in a porous medium is inversely proportional to the fluid's viscosity, very viscous hydrocarbons lack the mobility required for economic production rates.
Steam injection has been used for over 30 years to produce heavy oil reservoirs economically by exploiting the strong negative relationship between viscosity and temperature that all liquid hydrocarbons exhibit. This relationship is illustrated in the drawing labeled
FIG. 6
, which includes plots
601
,
603
,
605
, and
607
of viscosity as a function of temperature for heavy hydrocarbons from, respectively, the Street Ranch, Saner Ranch, Athabasca, and Midway Sunset deposits [Reference 6].
In one method of steam-assisted production, steam is injected into a formation through a borehole so that a portion of the heavy oil in the formation is heated, thereby significantly reducing its viscosity and increasing its mobility. Steam injection is then halted and the oil is produced through the same borehole. In a second method, after the oil-bearing formation is preheated sufficiently by steam injection into all boreholes, steam is continuously injected into the formation through a set of injection boreholes to drive oil to a set of production boreholes.
Referring again to
FIG. 6
, the plots show that heating the heavy hydrocarbons from say 100° F., a typical temperature for the subsurface deposits in which the hydrocarbons are found, to 400° F., a temperature that could be achieved in a subsurface deposit by injecting steam from the surface, reduces the viscosity of each of the four hydrocarbons by three to four orders of magnitude. Such viscosity reductions will not, however, necessarily result in economic production. The viscosity of Midway Sunset oil at 400° F. approaches that of a conventional crude, which makes it economic to produce. But even at 400° F., the viscosities of the bitumens from Athabasca, Street Ranch, and Saner Ranch are 50 to 100 times greater than the levels required to ensure economic rates of recovery. Moreover, the high viscosities of many heavy hydrocarbons, when coupled with commonly encountered levels of formation permeability, make the injection of steam or other fluids which might be used for heating a hydrocarbon-bearing formation difficult or nearly impossible.
In addition to high viscosity, heavy hydrocarbons often exhibit other deleterious properties which cause their refining into marketable products to be a significant challenge. These properties are compared in Table 1 for an internationally-traded light crude, Arabian Light, and three heavy hydrocarbons.
TABLE 1
Properties of Heavy Hydrocarbons Compared to a Light Crude
Light Crude
Heavy Hydrocarbons
Properties
Arabian Light
Orinoco
Cold Lake
San Miguel
Gravity, ° API
34.5
8.2
11.4
−2 to 0
Viscosity, cp @
10.5
7,000
10,700
>1,000,000
100° F.
Sulfur, wt %
1.7
3.8
4.3
7.9 to 9.0
Nitrogen, wt %
0.09
0.64
0.45
0.36 to 0.40
Metals, wppm
25
559
260
109
Bottoms (975° F.+),
15
59.5
51
71.5
vol %
Conradson carbon
4
16
13.1
24.5
residue, wt %
The high levels of undesirable components found in the heavy hydrocarbons shown in Table 1, including sulfur, nitrogen, metals, and Conradson carbon residue, coupled with a very high bottoms yield, require costly refining processing to convert the heavy hydrocarbons into product streams suitable for the production of transportation fuels.
Two fundamental alternatives exist for the upgrading of heavy hydrocarbon fractions: carbon rejection and hydrogen addition.
Carbon-rejection schemes break apart (or “crack”) carbon bonds in a heavy hydrocarbon fraction and isolate the resulting asphaltenes from the lighter fractions. As the asphaltenes have significantly higher carbon-to-hydrogen ratios and higher concentrations of contaminants than the original feed, the product stream has a lower carbon-to-hydrogen ratio and significantly less contamination than the feed. Although less expensive than hydrogen-addition processes, carbon rejection has major disadvantages—significant coke production and low yields of liquid products which are of inferior quality.
Hydrogen-addition schemes convert unsaturated hydrocarbons to saturated products and high-molecular-weight hydrocarbons to hydrocarbons with lower molecular weights while removing contaminants without creating low-value coke. Hydrogen addition thereby provides a greater volume of total product than carbon rejection. The liquid product yield from hydrogen-addition processes can be 20 to 25 volume percent greater than the yield from processes employing carbon rejection. But these processes are expensive to apply and employ severe operating conditions. Catalytic hydrogenation, with reactor residence times of one to two hours, operate at temperatures in the 700 to 850° F. range with hydrogen partial pressures of 1,000 to 3,000 psi.
Converting heavy crude oils and natural bitumens to upgraded liquid hydrocarbons while still in a subsurface formation, which is the object of the present invention, would address the two principal shortcomings of these heavy hydrocarbon resources—the high viscosities which heavy hydrocarbons exhibit even at elevated temperatures and the deleterious properties which make it necessary to subject them to costly, extensive upgrading operations after they have been produced. However, the process conditions employed in refinery units to upgrade the quality of liquid hydrocarbons would be extremely difficult to achieve in the subsurface. The injection of catalysts would be exceptionally expensive, the high temperatures used would cause unwanted coking in the absence of precise control of hydrogen partial pressures and reaction residence time, and the hydrogen partial pressures required could cause random, unintentional fracturing of the formation with a potential loss of control over the process.
A process occasionally used in the recovery of heavy crude oil and natural bitumen which to some degree converts in the subsurface heavy hydrocarbons to lighter hydrocarbons is in situ combustion. In this process an oxidizing fluid, usually air, is injected into the hydrocarbon-bearing formation at a sufficient temperature to initiate combustion of the hydrocarbon. The heat generated by the combustion warms other portions of the heavy hydrocarbon and converts a part of it to lighter hydrocarbons via uncatalyzed thermal cracking, which may induce sufficient mobility in the hydrocarbon to allow practical rates of recovery.
While in situ combustion is a relatively inexpensive process, it has major drawbacks. The high temperatures in the presence of oxygen which are encountered when the process is applied cause coke formation and the production of olefins and oxygenated compounds such as phenols and ketones, which in turn cause major problems when the produced liquids are processed in refinery units. Commonly, the processing of products from thermal cracking is restricted to delayed or fluid coking because the hydrocarbon is degraded to a degree that precludes processing by other methods.
The present invention concerns an in situ process which converts heavy hydrocarbons to lighter hydrocarbons that does not involve in situ combustion or the short reaction residence times, high temperatures, high hydrogen partial pressures, and catalysts which are
Mantooth Geoffrey A.
Tsay Frank
World Energy Systems Incorporated
Zobal Arthur F
LandOfFree
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