BACKGROUND OF THE INVENTION
The present invention is generally related to development of energy resources, and more particularly to producing hydrocarbons from oil shale.
One kind of oil shale is sedimentary rock that contains little liquid hydrocarbon, but includes a significant amount of a solid organic phase known as kerogen. Kerogen has not been exposed to the temperatures and pressures required to completely convert it into oil and gas. Kerogen cannot be pumped directly out of the ground because it is solid. Further, kerogen is insoluble in common organic solvents. However, kerogen can be mined and processed to generate a liquid hydrocarbon similar to conventional oil. For example, mined oil shale can be crushed and then heated in a process known as retorting in order to convert the kerogen into various useful products.
Retorting mined oil shale has certain drawbacks including being costly and environmentally problematic. Mining generally requires movement of large quantities of material to a treatment facility. Further, a byproduct of the process is large quantities of spent shale, the disposal of which is environmentally problematic. Surface mining can be less costly than underground mining, but tends to alter the site of the mine in an environmentally problematic manner and still produces spent shale. An alternative technique known as “in situ retorting” is currently being studied because it helps to mitigate these problems. The in situ retorting technique involves heating the oil shale in the subterranean reservoir in order to produce a fluid product which can be produced to the surface, thereby leaving the spent shale in place. However, heating a reservoir to a sufficiently high temperature, e.g., greater than 325° C., in order to convert the kerogen requires considerable energy input relative to the amount of liquid product returned.
SUMMARY OF THE INVENTION
A method in accordance with one aspect of the invention includes the steps of: calculating input conditions under which production of vaporizable oil and production of bitumen are optimized; and recovering vaporizable oil and bitumen using the calculated input conditions.
Apparatus in accordance with one aspect of the invention includes an analyzer which calculates input conditions under which production of vaporizable oil and production of bitumen are optimized; and production equipment which recovers vaporizable oil and bitumen using the calculated input conditions.
In contrast with prior art techniques based simply on recovery of vaporizable oil, oil shale conversion is enhanced by calculating conditions to optimize recovery of both oil and bitumen. This can be accomplished by either separately calculating a program of heating times, temperatures, pressures, and additives for which production of vaporizable oil and production of bitumen are optimized, or calculating a program of heating times, temperatures, pressures, and additives for which production of vaporizable oil and production of bitumen are optimized by applying a weighted maximizing function to combined vaporizable oil and bitumen data. An advantage of this technique is that greater efficiency is achieved because the amount of heating associated with the retorting process can be reduced and the yield of valuable products can be increased.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 illustrates apparatus for recovery of hydrocarbons from oil shale.
FIGS. 2 and 3 illustrate products generated as a function of heating time.
FIG. 4 illustrates an example of heating time calculation.
FIG. 5 illustrates a method in accordance with an aspect of the invention.
DETAILED DESCRIPTION
Substances associated with recovery of hydrocarbons from oil shale include kerogen, bitumen, oil, hydrocarbon gas, nonhydrocarbon gas, and coke. In the description that follows the terms are used with the following meanings. Kerogen is a native state component of oil shale which is a solid, organic in origin, and insoluble in common organic solvents. Bitumen is a hydrocarbon which does not migrate out of the heated reservoir and is soluble in one or more organic solvents. Oil is a hydrocarbon product which does migrate out of the heated reservoir (this includes wax, which is solid at room temperature but melts below 100° C.). Hydrocarbon gas is a mixture of noncondensable (at standard temperature and pressure) products such as methane, ethane and propane. Nonhydrocarbon gas includes carbon dioxide. Coke is solid residuum, insoluble in common organic solvents, mainly composed of carbon.
The general characteristics of oil shale conversion have been described by R. L. Braun and A. K. Burnham, Chemical Reaction Model for Oil and Gas Generation from Type I and Type II Kerogen, Lawrence Livermore National Laboratory Report UCRL-ID-114143, June 1993, which is hereby incorporated by reference. Braun and Burnham describe ten transformations associated with the pyrolysis of Type I kerogen. In accordance with an aspect of this invention, oil shale conversion includes the following two steps: (1) kerogen is converted to bitumen, oil, gases and coke; and (2) bitumen is converted to oil, gases and coke. These steps are described in greater detail below.
Referring to FIG. 1, in the first step in situ retorting is performed by introducing heat into the subterranean reservoir (100) of oil shale via a borehole (102) in order to convert kerogen to bitumen, oil, gases and coke For example, a heating element (104) may be disposed in the borehole proximate to a retort interval in the formation. The borehole may include equipment which facilitates production, such as a casing, electrical cabling, pipes, and a downhole pump (110). The electrical cabling enables downhole devices such as the heating elements, sensors and pumps to be powered and controlled (the heating element may be powered by gaseous products). The pipes and pumps enable fluids to be introduced to and removed from the borehole. The heat raises the temperature of the reservoir near to the heating element and converts kerogen in the oil shale into various products, some of which are in a liquid or gaseous state. The conversion tends to occur near to the formation-borehole interface so it is not necessary for the products to traverse the formation. In particular, the conversion tends to create a retorting chamber which increases in volume in relation to the duration of the retorting process. Supplemental gases can be introduced to change the composition of the resulting products and the amounts and types of products produced. The fluid products are removed from the chamber to the surface via the pipes. In particular, oil and gases are removed to a surface processing facility (106).
In the second step bitumen is recovered from the partially spent oil shale after the oil and gases produced by the retorting process have been recovered. Active or passive cooling of the reservoir at this stage may be desirable. In the illustrated embodiment a solvent from a storage tank (108) is introduced to the reservoir via the borehole 102 to facilitate bitumen recovery. Generally, bitumen is too viscous to be recovered via the pipes. However, mixing of the solvent with the bitumen results in a product characterized by a viscosity that permits recovery via the pipes. The solvent based extraction process could be somewhat similar to the technique known as VAPEX which is used for heavy oil production (see e.g. Oliveira et al., SPE 122040 (2009), which is hereby incorporated by reference). The bitumen is also processed at the surface processing facility to produce oil and other residual materials.
Referring now to FIGS. 1 through 3, the two steps of oil shale conversion are enhanced by modifying conditions of the process. As already mentioned, products of the retorting process include oil, bitumen, hydrocarbon gas, nonhydrocarbon gas, and coke. Of these products, oil currently has the highest commercial value. Bitumen is also commercially valuable, but not as valuable as oil. Hydrocarbon gas can be used as a source of heat for the retorting process, or can be sold through usual channels. Coke and nonhydrocarbon gas are primarily waste products. Whereas typical prior art techniques focused on recovering a maximum amount of vaporizable oil, an aspect of the present invention is calculating conditions to optimize recovery of both oil and bitumen, as discussed by F. P. Miknis, P. J. Conn, and T. F. Turner, Isothermal Decomposition of Colorado Oil Shale, DOE/FE/60177-2288, May 1985 which is incorporated by reference. An advantage of this technique is that greater efficiency is achieved because the amount of heating associated with the retorting process can be reduced and the total yield increased.
Referring now to FIGS. 2 and 3, the illustrated sums of oil and bitumen generated from a sample as a function of heating time show that heating can be stopped sooner and more product obtained than when only the vaporizable oil fraction is considered. For a particular interval or intervals of interest, evaluation of samples enables calculation of inputs such as a program of heating times, temperatures, pressures, and additives to achieve a particular result. For example, experiments can be conducted to calculate the amount of the various products that will be produced as a function of controllable inputs. In the illustrated embodiment an exemplary oil shale was rapidly heated to 380° C. and held at that temperature for varying lengths of time. Vaporized products were allowed to exit the reactor at a pressure of 1 atm and were separated into condensable (oil and water) and noncondensable (gas) fractions. At pre-determined stopping times the partially spent oil shale samples were cooled to room temperature. Total amounts of oil, water, and gas collected during pyrolysis were then measured. Remaining spent oil shale was removed from the reactor vessel and measured using 2 MHz proton magnetic resonance. Bitumen was extracted from the partially spent shale using Soxhlet extraction with a solvent consisting of a 90:10 mixture of dichloromethane:methanol, and the amount of extractable bitumen was measured. The bitumen-free shale was then remeasured by nuclear magnetic resonance (NMR). As indicated in FIG. 2, the scaled NMR signal amplitudes (+) are in fair agreement with the estimates of bitumen content from solvent extraction (X). Following solvent extraction, the NMR signal returns to a low level consistent with its value for native state oil shale. The experimental data qualitatively agree with the kinetic model of Braun and Burnham. Vaporizable products increase steadily with reaction time, tending toward an asymptote for reaction times on the order of a day. Bitumen content at first increases, attaining a maximum at about six hours in these experiments, and then declines. After approximately 24 hours, little or no bitumen is observed in the spent shale, it having been converted to oil, gas, and coke. It will be appreciated that the equipment for analyzing the samples is well known to those skilled in the art. Such equipment will be referred to collectively as an “analyzer.”
Referring now to FIG. 4, an algorithm such as a maximizing function can be used to calculate duration of heating based on the evaluated samples. A primary cost (400) associated with the in situ conversion of oil shale is heating the formation to a temperature that is adequate for retorting. Consequently, the intensity and duration of heating should be reduced as much as practical in order to improve production efficiency. However, not all reservoirs exhibit the same response to the same heating schedule. Optimal processing protocols depend on the individual characteristics of the oil shale interval being treated and on the product mix desired by the operator. Typically, the amount of oil and oil+bitumen produced increase with heating time initially as kerogen is broken down, reach a maximum as the kerogen becomes exhausted, and potentially then decrease if secondary reactions are significant. The costs associated with production increase linearly with time if constant heat generation is assumed to be the dominant cost, although more complicated functions will occur if capital, solvent extraction, or other costs are significant. Therefore, it is possible to calculate a time duration for which the formation should remain on heat in order to balance energy input cost (400) against the value (402) of returned products. The illustrated plot shows product value and production costs over time under the above assumptions. An operator may therefore calculate a treatment schedule for which the heating process ends at the time (404) which corresponds to the maximum delta between value (402) of product and production cost (400). In other words, the calculation may be performed in two steps with the oil data and then the bitumen data from FIG. 3 as the value (402), or in one step with the oil+bitumen data from FIG. 3 as the value (402).
FIG. 5 illustrates steps of a method in accordance with an embodiment of the invention. Some or all of the steps may include operation of computer software stored on a computer readable medium. Step (500) is to obtain samples of oil shale from the reservoir interval or intervals of interest. The samples may be obtained, for example, from outcrops, drill core, or drill cuttings. Step (502) is to perform a series of pyrolysis experiments to determine the effects of temperature, pressure, heating rate, heating time (duration) and additives. Step (504) is quantify the vaporizable oil and gas. Step (506) is to quantify the bitumen. Bitumen quantification may be achieved using NMR. Step (508) is to determine the conditions under which the production of oil and production of bitumen are optimized. As mentioned previously, the calculations can be performed for oil and bitumen either separately or together. Step (510) is to use the calculated treatment program in the oil shale reservoir, collecting vaporizable oil and gas. Optional Step (511) is to passively or actively adjust the reservoir temperature to allow for optimal solvent extraction. Step (512) is to extract remaining bitumen from the reservoir with solvent. Once pyrolysis of the oil shale reservoir is characterized, the method can be simplified to steps (510), (511) and (512).
In view of the description above it will be appreciated that aspects of the invention are not limited to use in association with in situ retorting. For example, aspects of the invention could be utilized to enhance recovery techniques based on mining, such as where mined oil shale is crushed and treated at the surface, or where oil shale is reburied in a treatment tunnel.
While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while the preferred embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.
Patent Application
20110174694
