literature Cited (1) Ball, J. S., Rall, H. T., Waddington, Guy, and Smith, H. M., presented as part of the Symposium on Composition of Petroleum, before the Division of Petroleum Chemistry, 119th
c
Meeting, AMERICAN CHEMICAL SOCIETY, Cleveland, Ohio. (2) Eccleston, B. H., Morrison, Marilyn, and Smith, H. M., presented as part of the Symposium on Nonhydrocarbon Constituents of Petroleum before the Division of Petroleum Chemistry, 121st Meeting, AMERICAN CHEMICAL SOCIETY, Milwaukee, Wis. (3) Forziati, A. F., Willingham, C. B., Mair, B. J., and Rossini, F. Proc. Am. Petroleum Inst., 24, sect. 111, 34-48 (1943); J . Research Nail. Bur. Standards, 32,ll-37 (1944). (4) Fred, Mark, and Putcher, Richard, Anal. Chem., 21, 900 (1949). ( 6 ) Haak, F. A., and VanNess, K., J. Inst. Petroleum, 37, 329 (1951).
(6) Hughes, E. C., and Hardman, H. F., “Progress in Petroleum Technology, Advances in Chem. Ser., No. 5 , 262 (1951). (7) Lochte, H. L., IND.ENG.CHEM.,44, 2597 (1952). (8) Mair, B. J., Rossini, F. D., “Science of Petroleum,” Vol. V, pp. 126-52, New York, Oxford University Press, 1950. (9) Richter, F. P., Caesar, P. D., Meisel, S. L., and Offenhauer, R. D., IND.ENG.CHEM.,44, 2601 (1952). (10) Rossini, F. D., Mair, B. J., presented as part of the Symposium on Composition of Petroleum before the Division of Petroleum Chemistry, 119th Meeting, AMERICANCHEMICAL SOCIETY,
Cleveland, Ohio. (11) Skinner, P. A., IND.ENG.CHEM.,44, 1159 (1952). (12) Williams, R. B., Hastings, S. H., and Anderson, J. A,, Jr., Anal. Chem.. 24. 1911 (1952). and Chandler, W. B., Jr., IND.ENG.CHEM.,44, RECEIVED for review October 15, 1952.
ACCEPTED February 20, 195 3
Oil Recovery400 Per Cent? MORRIS MUSKAT Gulf
ON
Corp., Piftsburgh 30, Pa.
Various types of oil-producing mechanisms and their physical limitations with respect to recovery are reviewed. The basic physicochemical reason behind failure to obtain 100% oil recovery lies in the phenomena of fluid immiscibility and interfacial forces. Several approaches are suggested for theoretically overcoming these limitations in order to achieve substantially complete recovery. These
include vaporization of the oil phase by high pressure gas injection and .subsequent cycling, the displacing of the oil by immiscible liquids such as liquefied petroleum gases, and the use of additives in waterflooding which will eliminate or drastically reduce the interfacial tension between the water and oil. The economic importance of research on these problems is pointed out.
HE purpose of the query in the title of this paper is to crystallize the nature of the problem to be discussed rather than to announce a definite answer to it. For even if one could forsee all future technological advances to be made in the oilproducing industry, these alone, when considered aside from the economic aspects of the subject, would still not suffice to fix the answer. Furthermore, any pretense of being able t o predict the long-term future of research or technology would in itself belie the assumption that either the author or subject merits the dignity of a place in a scientific symposium. The question, if in the future we shall recover 100% of the oil presumably available for recovery, evidently implies t h a t such is not being achieved now. Chemists primarily interested in phases of the oil industry other than production, may perhaps be surprised that we do not now recover and bring to the surface all the oil our geologists and geophysicists discover through their highly technical and painstaking efforts. One may well ask why the producing phase of the industry is so far behind the refining chemists, who long ago learned to make 100% conversions of crude oil into useful products. This very proper question is, of course, the one that must first be answered and fully explored in order to understand what the problems are in achieving 100% recovery and the possibilities of overcoming them by future technological progress. The basic technical reason for failure currently to effect 100% recovery from producing oil fields rests on two very simple physicochemical phenomena-fluid immiscibility and interfacial forces. Before exploring how these phenomena are involved in limiting the recovery of oil by methods now in use, let us review briefly the present status of oil recovery efficiency.
In the very great majority of oil-producing reservoirs, the oil itself has no substantial inherent power of self-expulsion from the reservoir rock into the well bore and thence t o the surface, except in an indirect manner t o be discussed later. The process of oil expulsion takes place by virtue of replacement and displacement of the oil by another phase. This observation provides t h e basis for two of the three fundamental producing mechanisms or displacement processes-namely, the solution gas drive and the water drive. I n the former, the oil is replaced and displaced by gas evolving from solution in the oil, as the pressure declines, and by the expansion of the free gas phase developed by prior gas evolution. I n the water drive, the oil is displaced and driven to the producing well bores by a water phase entering the oilbearing rock from contiguous extensions of the latter. In considering further the detailed processes whereby a solution gas drive leads to oil expulsion, the factors of fluid immiscibility and surface and interfacial forces come into play. Consider an’element of porous rock containing gas-saturated oil and located perhaps several hundred feet from the first well drilled into and opened t o production from this producing section. At the instant before the oil is first withdrawn from the producing well the oil is a t its bubble point pressure-that is, a t a fluid pressure equal to the saturation pressure of the gas in solution. As soon as production begins, a pressure reaction-a decline-develops in the immediate vicinity of the well bore, and is propagated outward, with increasing attenuation, t o the distant parts of the rock continuum. Neglecting for the present possible supersaturation effects, the pressure decline front, when impinging on the volume element of interest, leads to local gas evolution and the development of a distinct gas phase immiscible with the oil. (The term “front,” as used here, describes a boundary of separation between the area of substantial pressure reaction and t h a t not yet appreciably affected b y the well withdrawals, rather than a surface of pressure discontinuity.) This creation of the gas phase volume and the similar incremental volumes of gas evolved a t other points passed by the pressure front are automatically
T
‘Bi
Present Methods of Oil Discovery
The essential facts of the present status of oil recovery efficiency, expressed in a very qualitative and elementary fashion, are as follows: July 1953
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reflected in corresponding volumes of oil or liquid phase being removed from the porous rock and produced through the well bore. What happens to this gas phase as production continues? From a static standpoint the gas phase already developed continues t o expand as the total pressure decline increases. I n addition, more gas is evolved from solution. Dynamically, however, this potential growth in local gas phase volume does not imply a corresponding one-to-one displacement of oil throughout the whole history of pressure decline. The initial phase of gas evolution takes place by the formation of gas bubblea in individual pores of the rock. As soon as they grow t o a size comparable t o the dimensions of the passageways between the pores
volumetric velocity of the free gas phase may become as high as twenty times that of the oil. As a result, the reservoir gas is rapidly dissipated, and the oil production ceases or falls to uneconomically low values. Generally this termination of the oil recovery occurs by the time the free gas phase volume occupies only about 30% of the total pore space of the rock and when some 70 to SO% of the initial oil content still remains in the producing formation. The nature of solution gas drive production has been outlined in considerable detail t o show that the very low recovery by the solution gas drive mechanism does not in itself imply a poor efficiency in oil field operation. Rather, it reflects the natural consequence of the combination of physical factors represented by the geometrical structure of oil-bearing rocks and the interaction between the immiscible gas and liquid phases. Further analysis of these processes also explains why gas returned to a producing reservoir, t o replace that lost during production, will also be limited in its capacity to displace additional oil, although in many reservoirs the gain in oil recovery will be of considerably greater value than the cost of such means to supplement the natural reservoir energy. On the other hand, even if we ignore the economic aspects of the operations and continue indefinitely to supply and drive gas through the partially depleted reservoir rock, a limit of oil recovery mill be approached wherein the production will be maintained only by evaporation in the gas phase. Interfacial forces will prevent further mass displacement of the residual oil long before complete recovery is achieved. This limitation reflects, of course, the immiscibility of the displacing and displaced phases. I n water drive fields, the gross aspects of the oil expulsion processes are relatively simple. Here the water in the reservoir rock extending beyond the oil bearing section is induced to invade the latter by the pressure drop created by the oil withdran-als. This entry of water into the oil-producing formation serves to retard
Figure 1
they become trapped within the pores, since the pressure gradients driving the oil to the well bore do not suffice to distort the bubbles against the resisting interfacial forces so as t o permit them t o pass through the pore constrictions. During this phase of the producing history there is a substantially one-to-one equivalence between the volume of oil expelled from the rock and the volume of free gas developed. But as this process continues and the expansion tendency of the gas phase increases, due both to declining pressures and additional gas evolution, the trapped gas bubbles will break out of their confining pores and coalesce with those from nearby pores. Ultimately, when the composite free gas volume represents some 3 to 10% of the total pore space, it will form continuous free gas channels, fed by continued evolution from the immediately bounding oil and diffusion from gas pockets which are still isolated and trapped. Once these channels have been developed, the low. viscosity of the gas comes into play, and a flow of free gas into the well bore builds up rapidly. A schematic representation of a solution gas drive field is shown in Figure 1. Since the energy contained in the dissolved or free gas and its capacity to displace the oil are the basic elements giving rise to oil production by the solution gas drive mechanism, the escape of the gas becomes the controlling factor in maintaining the oil recovery operations. The continued removal of oil from the porous rock implies an expansion of the gas channels and increased gas mobility therethrough. At the same time, the reduction in residual oil content and interference by the gas phase results in a declining mobility for the oil. I n addition, the viscosity of the oil rises as the solution gas escapes. All these factors lead t o a rapidly increasing rate of depletion of the gas content as compared to the oil. I n fact, toward the later stages of production the
1402
Figure 2
the pressure decline and also to provide a mass displacement of the oil. While microscopically this may be visualized as a pistonlike process, the microscopic features are quite different. The water, generally being the wetting phase as compared to the oil, tends to surround the residual oil in the individual pores and pinch off the filaments being driven forward before they have completely escaped t o the pores ahead. These trapped oil droplets are left behind the advancing gross oil-water interface. I n addition, smaller pores still filled with oil may be bypassed by the water advancing through surrounding larger pores, leaving behind ad-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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*
ditional volumes of isolated residual oil. The gross geometry of the fluid distribution in a field producing by water drive may be visualized by reference t o Figure 2. The total of the undisplaced and unrecovered oil in water drive fields amounts, under favorable conditions, to 25 t o 40% of the original oil content. The amount recovered may approach 60 to 80% of that initially available for oil recovery. Here, too, supplementing a natural water drive by water injection does not change the basic physical displacement process, though such operations are often of great value in substantially increasing the amount of economically recoverable oil. Moreover, because, as already noted, displacement of oil by water is generally more effective than displacement by gas, the use of water injection as a scavenging or secondary recovery operation is often found to be a profitable follow-up after a primary production phase b y gas drive. Here, too, if the flushing of the rock with water be continued indefinitely, a minimum of residual oil will remain trapped by capillary and interfacial forces and evade this recovery mechanism except through the infinitesimally slow processea of solution in and diffusion through the water. Phase immiscibility is again the underlying cause. For completeness, mention should be made of still another type of oil-producing mechanism though it is not directly related to displacement processes. This is the mechanism of gravity drainage, which simply reflects the natural tendency for the oil at higher structural levels in the reservoir t o drain downstructure if not opposed b y counterbalancing pressure gradients or hydrostatic heads. I n practice, when the oil is produced by this mechanism the oil zone is generally overlain by a gas cap which expands to follow up and replace the oil flowing to the low structure wells, as shown in Figure 3. However, the replacement of the oil by the expanding gas phase is only an incidental part of this mechanism, which depends primarily on the body force of gravity rather than on displacement by an immiscible phase. While this mechanism will control the producing operations only under rather limited favorable conditions, it does offer, when these obtain, possibilities of oil recovery as high as any other presently established method. From a physical standpoint, the limit of recovery by gravity drainage is determined only by the oil saturation a t which the oil still retains mobility for continued downstructure drainage. This generally lies in the range of 15 to 30% of the pore space. The recoveries under practical conditions from reservoirs depleted by gravity drainage may be as high as 75% of the initial oil content. But once more failure t o recover the last 25% can be attributed to the fact that the capillary forces opposing drainage ultimately overcome the gravity force supporting the drainage. Possible Future Developments
,~,
This lengthy discussion of present practice in oil recovery operations has been presented for two reasons: h t , to provide some understanding of current concepts of the physical processes involved in oil recovery. Whether right or wrong these must, of course, constitute the background for whatever approach might be taken in attacking such unsolved problems as still remain. Secondly, an understanding of the degree of effectiveness of present methods of producing oil automatically points to the nature of the problem of increased recovery. From the previous discussion it will be seen that the major unsolved problem of oil recovery per se is simply that of increasing economically the presently achieved recoveries of 20 to 80% to values approaching loo%, as far as possible. Here we immediately encounter a peculiar paradox. If recovery alone were the goal there actually would be no problem; 100% recovery has been literally possible since the beginning of the industry, by the brute-force procedure of mining the oil-bearing rock and washing it clean of the entrained oil. Such oil-mining operations have actually been tried, notably in the July 1953
Pechelbronn Field in France. As an economic solution to the problem, however, oil mining is obviously out of the question, especially if it is t o be applied to fields of 5000, 10,000, and 15,000 feet itl depth. It is clear, therefore, that as a practical matter the term “oil recovery” must be visualized as referring t o operations in which the value of the oil recovered exceeds the cost of production. With this understanding, the possibilities for achieving 100% recovery, as suggested here, still do not represent practicable solutions at the present time, or they would already have been incorporated into current practices. Nevertheless, it is believed that the type of procedures outlined sufficiently approach limits of profitability, and that future research may possibly shift the balance between costs and return, so as to make them economically attractive, a t least under favorable conditions. In any case, they may serve to concentrate attention on the basic difficulties of solving the problem of achieving complete oil re covery and stimulate new types of approach.
_ - - .-_- - - - - ---- --
I
Figure
3
As was previously emphasized, gas drive and water drive producing mechanisms involve displacement processes-in one case the oil by gas and the other, oil by water. Moreover, the completeness of such displacement is ultimately limited by the immiscibility of t h e dkplacing and displaced phases. This immiscibility, in turn, implies the presence of fluid interfaces and interfacial forces which, because of the extensive internal surfaces in a porous reservoir rock, have a gross magnitude sufficient t o control the mobility and displacement processes. As previously noted, it is also these interfacial or capillary forces that set the limit on the oil mobility and recovery by gravity drainage, which otherwise is not directly dependent on displacement by an external phase. I n the light of this important, if not dominating role played b y phase immiscibility, a t least one approach to methods for achieving substantially higher recoveries than obtained by present methods will very likely be directed toward the elimination of immiscibility phenomena. Such a method, already proposed and which future research may develop into a n economically feasible operation in certain types of reservoirs, is that of vaporizing the liquid oil phase b y the addition of gas t o the formation and then displacing the oil-containing gas by dry gas. The injection of gas into producing oil reservoirs t o increase oil recovery, either as pressure maintenance during the primary producing life or as secondary recovery by gas repressuring, has been a long established practice. But these operations have served primarily to supplement or prolong the natural gas drive displacement of
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the reservoir oil beyond that provided by the gas originally dissolved in or associated with the oil. Their effectiveness has, accordingly, been limited ultimately by the effect of interfacial forces resulting from the immiscibi!ity of the phases involved, in quite the same way as the simple solution gas drive mechanism. The normal effect of adding natural gas to a crude oil is t o force the gas into solution and expand the oil phase. However, a6 excess gas is added beyond that which will go into solution and as the pressures are increased, the oil constituents develop an increasing tendency to vaporize. Ultimately, as this process is continued, the composite system of oil and gas will approach either the critical or dew point pressure, a t which the system as a whole will become a homogeneous phase. On still further increase of pressure to the cricondenbar the composite reservoir fluid becomes substantially equivalent to a gas miscible with the gas being added. This conversion will generally be materially facilitated and accelerated if the extraneous gas added to the system contains appreciable concentrations of the intermediate hydrocarbons such as the liquefied petroleum gases, propane, and butanes. This latter effect may be qualitatively visualized as due to t h e greater similarity and hence mutual solubility of the oil and gas when the latter is “wet” and may be semiquantitatively interpreted by reference t o the behavior of the critical loci in phase diagrams. Retrograde phase conversion has long been recognized as the controlling feature in the operation of condensate reservoirs Reservoirs of this type contain a gas phase, generally a t the dew point, from which liquid phase (condensate) condenses on pressure decline. Again, because of immiscibility and interfacial effects, the condensed liquid tends to remain trapped in the pore space and thus is lost to recovery if the field is allowed to decline in pressure. Accordingly, the methods of “cycling” has been developed, whereby through return t o the formation of the produced gas, after stripping it of its condensible hydrocarbons, the pressure decline is prevented or retarded, and retrograde condensation in the rock strata is avoided or minimized. Moreover, the dry gas, being substantially miscible with the reservoir gas phase, achieves virtually complete displacement of the latter. The total recovery of condensate is then limited primarily by the gross uniformity of the formation, well pattern effects, and related economic factors. This possibility for complete oil recovery by gas injection is essentially equivalent t o that of converting the composite gasliquid system into a reservoir gas miscible with the extraneous gas and then cycling, as in the case of condensate reservoirs ( 9 ) . Because of the relatively low volatility of crude oils the pressures and gas volumes required for vaporization of crudes will be considerably greater than would correspond to natural condensate reservoir fluids. While the use of “wet” gas will encourage a more rapid vaporization of the crude, it is to be expected that pressure build-ups to 7000 to 8000 pounds per square inch or greater will probably be necessary for substantially complete vaporization of moderate gravity crudes, thus immediately limiting the applicability of this method t o reservoirs a t depths of 7000 to 8000 feet or greater. Of course, even if otherwise physically feasible, such operations must stand the test of economic practicality in providing sufficiently increased oil recovery to counterbalance the investment costs of the injection and processing plants, of the gas to be purchased, and of the added operating expenses. It is doubtful if under present oil price and operating cost levels an operation of this type would be profitable except in uniquely favorable instances. However, through changes in economic factors and research developments improving the technical aspects of the process, it may well be that the future will see at least some fields yielding substantially 100% recovery by high pressure crude oil vaporization followed by cycling. T h e limitations in ultimate oil recovery due to phase immiscibility and interfacial forces can also be removed, in principle, by 1404
displacing the oil by another liquid which is miscible with it. Laboratory experiments ( 1 ) have demonstrated that complete removal of oil from a porous medium can readily be obtained by displacing it with the liquefied petroleum gases, propane, and butanes, which generally are completely miscible with crude oils. -4nd incidental confirmation has been obtained from field observations of stimulated oil recovery when liquefied petroleum gas (LPG) has been injected into partially depleted oil-producing formations for storage purposes. Here, again, i t is the economic phase of the problem which will determine the practicability of LPG flooding for increased oil recovery. Perhaps the major deterrent to immediate widespread application of this method is the tremendous investment required for the LPG, which would be needed to flood out fields of appreciable size, and the cost of auxiliary facilities such as storage and separation plants. Since the volume of LPG to be injected will probably be a t least of the same order of magnitude as that of the oil to be recovered, unless an effective method of slug flooding is developed, the investment in LPG alone will be comparable to or greater than the value of the oil produced. On the other hand, it should be possible to recover substantially all the LPG injected for re-use in other projects, and the long term cost would thereby be reduced. Only future research will finally determine the economic feasibility of this approach to complete oil recovery. Still another possibility for attacking effectively the problems of immiscibility and interfacial forces lies in the use of additives in the water injected in waterflooding operations. Early in the history of waterflooding practices various addition agents were tried as means for increasing waterflooding recovery. Soda was used in some of the first experiments, and was soon discarded when plugging effects developed. So-called wetting agents have been tested repeatedly in the laboratory and occasionally in the field. Many of the laboratory experiments have shown some improvement in recovery, but no conclusive evidence of success has been reported from actual field performance. Two factors were probably largely responsible for the failure of these early efforts. One was that the high degree of surface activity of the wetting agents was apparently due mainly to polar constituents which were very rapidly adsorbed on the internal rock surface, thus denuding the n-ater of its additive as i t progressed away from the injection well bore. Calculations indicated that the cost of maintaining the desired concentration of some of the more effective agents in the water would exceed the value of the increased oil recovery. The other factor was that, as the term (‘wetting agent” implies, the choice of the additive was generally based on the concept that the resultant solutions would have improved wettability for the rock, as indicated by its surface tension. Since most oil-producing sands are already water wet, and all in fact are partially filled with “connate water,” wettability with respect to the rock surface itself is probaPly not the dominant factor limiting the oil recovery. Insurance of perfect wettability-zero contact angle-over the whole roc&urface would undoubtedly still fail to yield complete oil displacement and recovery. As already pointed out, it is believed that interfacial interactions between the water and oil, in the presence of the pecuiiar internal surface area and structure of the porous rock, and their immiscibility control and limit oil recovery by water displacement. To the extent that the wetting agents previously used may also have served t o reduce the water-oil interfacial tensions some improvement in the microscopic displacement efficiencies might well have been accomplished, as has indeed been indicated in the laboratory experiments. But a more effective approach would appear to lie in concentrating on modifying the interfacial forces directly through the use of addition agents. I n the last several years, studies have been made of waterflooding additives differing in their ionic properties. Tf agents could be found or formulated which, when added to water, give vanishing inter-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Y
facial tensions with respect to the crude, and also do not suffer appreciable adsorption on the solid surfaceadmittedly a satisfaction of conflicting requirements-substantially increased if not complete recovery of the oil should be obtained. (For simplicity the effect of viscosity differences between the displacing and displaced phases has been ignored here. Actually, however, displacement efficiencies approaching 100% even between miscible phases will occur only if the displacing fluid has a viscosity equal to or exceeding that of the phase being displaced.) Of course, the additive must be subject t o relatively low manufacturing costs so that the operations will remain profitable after incurring the additional expense of treating the flood water. Though not related to the physical concepts or processes underlying the above-outlined methods for attempting to improve oil recovery, mention should be made, if only for completeness, of the possibility of distilling the oil in place. Burning the oil in place has been proposed many times in the past, and there have been claims of its trial in fields in the USSR. One of the basic problems has been and apparently still is, that of providing an oxygen supply to permit combusf;ion and also generate enough heat to bring the oil to its flash point. Recently a U.S. patent has been issued which describes a process in which coal beds interstratifying oil sands are burned through supply of an air stream, and the combustion vapors serve both to heat the oil, to increase its mobility, and to drive it t o the producing wells. It is conceivable that even when such beds are not present, it may be possible to generate sustained combustion processes in an oil sand, by the creation of air flow channels and properly distributing the air supply, and to recover substantially all the residual oil other than that used in supporting the combustion. Research on both the engineering techniques on maintaining the combustion and control of explosion hazards may well bear fruit, although i t may appear to be a long-shot gamble as compared t o other attacks on the problem of improved oil recovery. Conclusions
This discussion has been directed only to possibilities for achieving complete oil recovery. This limitation in subject does not imply t h a t such are or will be the only types of fruitful research on the problems of oil recovery: 100% recovery is of no unique virtue as compared to, say 90% recovery, unless the additional 10% can also be gained a t a profit. Moreover, even a process which may lead to 100% microscopic displacement efficiency may yield considerably lower than 100% recoveries in practice, when applied to the nonuniform reservoirs occurring in nature and under the economic limitations imposed by investment and operating costs. From a practical standpoint, therefore, complete recovery is to be considered as an ideal goal rather than as a sharp criterion of the efficiency of the oilzproducing industry. The possible methods for achieving complete oil recovery outlined here all represent processes which are already being con-
July 1953
sidered by the industry. No doubt, current but hitherto undisclosed research will reveal still other and probably more promising ways of attacking the problem. I n %fact,several of the methods discussed refer to applications in the form of secondary recovery operations-Le., after primary operations are virtually completed. As a general principle, however, the achievement of maximum ultimate recovery during the course of primary operations will be economically preferable in virtually all respects to the sequence of secondary recovery following the termination of primary recovery operations. And beyond the advances to be made in the physical processes of oil recovery itself, concurrent research on geological aspects of producing reservoirs, on drilling, on artificial lift methods, on well treatment, on corrosion control, etc., will certainly contribute substantially to the over-all economics of oil production and thereby extend the potentialities of recovery even by present techniques. Finally, it may be of interest to call attention to the stakes involved in research for increased oil recovery. As of January 1, 1952, the remaining proved oil reserves of the United States have been reported to be about 27.5 billion barrels. The cumulative production to January I, 1952, was 43 billion barrels, so that the total expected ultimate recovery from the fields discovered by the first of the year is about 70 billion barrels. If the gross average recovery factor from these fields by past and present operations is assumed to be 5070, probably a very optimistic assumption, it follows that some 70 billion barrels will still remain unrecovered in the originally productive formations by the time they are all depleted and abandoned. If the direct discoveryreplacement costs for undeveloped reserves be taken as of the order of $1.00 per barrel, the equivalent replacement cost by new discovery for the unrecovered oil, if converted to economically recoverable oil, would be $70 billion-an extremely conservative estimate in view of the certain continued rise in future discovery costs. Even a 1% increase in over-all recovery factor for the fields already discovered-Le., from 50 to 5l%-would thus have a future reserves discovery value of a t least $1.4 billion. It is such values that are a t stake in the gamble for a pay-out on research to improve current methods or develop new methods for oil recovery. While the achievement of 100% oil recovery is admittedly nothing more than a theoretical physical possibility and may forever remain an unsolved problem, any progresa made toward its solution will merit the gratitude both of the industry and of the nation. literature
Cited
(1) Henderson, J. H., Gove, N. D., Ledbetter, H. J., and Griffith, J. D., J. Petroleum Technol., in press; Kennedy, H. T., Oil Gus J., 51, 58-61, 69 (June 30, 1952). (2) Kat5, D. L., Trans. Am. Inst. Mining Met. Engp.s., 195, 175-82 (1952). RECEIVXD for review Ootobec 15, 1952.
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AoCEPTEDZFebruary 4, 1953.
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