Because of unfavorable wetting conditions much residual oil is left when a porous material is flushed by water. Methods suggested to change reservoir wetting to improve oil displacement efficiency are generally expensive. The present laboratory study was undertaken to gain all understanding of the factors which determine reservoir wettability, and to find out if oil displacement efficiency might be improved by a wettability change accomplished at low cost in an oil reservoir.
Contact angle measurements were made on mineral surfaces using several sets of reservoir oil and water samples. Results of the contact angle studies suggest that reservoir wettability may be primarily determined by natural surface-active substances present in the reservoir fluids. The effect of changing salinity and pH of the water phase was studied. The results suggest that gross changes in preferential wettability might be accomplished by injection of water containing simple chemicals to alter pH or salinity in the reservoir. Such treatment could be much less expensive than injection of commercial surface-active agents.
Waterflood tests have also been made using synthetic cores and oil and water having wetting characteristics similar to those of reservoir fluids. Cores initially oil-wet were flooded in such a way that they were made preferentially water-wet by the advancing flood water. This reversal in preferential wettability achieved greater oil displacement efficiency than when either oil-wet or waterwet conditions were maintained throughout the flood. For the systems studied, the higher the oil viscosity the greater the percentage improvement obtained over conventional waterflood recovery. This suggests that a flooding process making use of wettability-reversal may extend the oil viscosity range over which water flooding is attractive.
Because a precise adjustment of reservoir wettability does not seem to be required, and because altering the pH or salinity in some reservoirs may be inexpensive, it appears that a waterflooding process employing wettability- reversal could find successful field application.
This paper presents the results obtained after calculating matches of the observed pressure performance of five fields completed in a common aquifer. A general description of the Central Basin Platform area in West Texas in which the five fields, Andector, Embar, Martin, TXL, and Wheeler, are located is contained in the paper. The method of utilizing the electric analyzer to calculate simultaneously matches of the observed pressure performance of the five fields is outlined. The determination of boundaries of pressure communication is discussed and the extent of pressure interference between fields consistent with the configuration of the area aquifer is shown graphically.
Anomalies in the pressure performance of the Andector Ellenburger Field resulted in an investigation of pressure interference between several Ellenburger fields located on the Central Basin Platform in West Texas. Four fields, Embar, Martin, TXL, and Wheeler, were found to contribute significantly to the pressure drawdown at Andector.
An initial investigation of the pressure performance of the Andector Ellenburger Field revealed that the reservoir pressure could not be matched by the calculation technique usually applied to water drive reservoirs. In the past, many have advanced the opinion that fracturing in the Ellenburger was a local condition restricted to the immediate uplifted area around the field, in which case each field would in all probability be surrounded by its own relatively small aquifer. Following several unsuccessful attempts to account for the pressure performance at Andector, a system assuming a common aquifer involving several fields in the immediate vicinity of Andector was placed on the electric analyzer and the effect of inter field interference was noted. A refinement of the initial analyzer calculations involved the determination of the approximate boundaries of the aquifer, the calculation of detailed matches of the pressure performance of the interfering fields, and an evaluation of the degree of communication between fields.
Geology and Development
The fields considered in this work are located in the Central Basin Platform, a north.south trending structural feature in West Texas lying between the Delaware Basin to the west and the Midland Basin to the east. The locations of various individual Ellenburger fields in this general are shown in Fig. 1 with the five major fields which appear to be definitely intercommunicating colored in black and located in the circled portion of the figure.
The various factors considered in recommending the initiation of a gas injection project in the southern portion of the Cedar Lake Field are discussed. Performance history under gas injection operations is reviewed and these data are analyzed, utilizing both the material balance method and the fractional flow and frontal advance expressions.
Results of the analysis of the performance data indicate that the injected gas has contacted and affected at least 60 per cent of the reservoir and a substantial increase in ultimate recovery can reasonably be expected. By holding the reservoir pressure appreciably above the bubble point, the well productive capacities have been maintained substantially above the level predicted for primary operations.
The analysis of the Cedar Lake project suggests that in certain limestone reservoirs, at least, the probable success of gas injection cannot be predicted simply from observation of permeability distribution throughout the pay section, as indicated by core analysis data, on either one or a number of wells. Further, the performance of this particular project fails to indicate any basis for classifying carbonate reservoirs in general as being inherently unsuited to a dispersed type gas injection program, thus indicating that each reservoir should be considered on its own merits, regardless of the composition of the reservoir rock.
Early in the life of the Cedar Lake Field, an extensive data gathering program was initiated to provide an accurate record of reservoir performance characteristics. From the study of these data it was apparent that there was a critical need for supplementing the natural reservoir energy in order to maintain well productivities and obtain the maximum ultimate oil recovery. Accordingly, detailed engineering studies were made of the various methods of secondary recovery which might be applicable. As a result of these investigations, the decision was made to initiate a gas injection program of sufficient intensity to maintain reservoir pressure at approximately 600 psia, or some 274 lb above the bubble point pressure of 326 psia.
A mud-conditioning program found to be very effective for drilling andcompletion operations on routine field wells requiring relatively shortdrilling time involves a moderate alkaline-tannate-bentonite treatmentresulting in an ultimate filtration rate of 10.0 cc or less (API test). Mudweight schedules are planned from pressure information on completed wells inproducing reservoirs and drillstem test data obtained on other zones not beingproduced at present. In general, terminal mud viscosities average 45 sec(Marsh). This value has been found to be sufficient to remove cuttings from thewell bore on the average well, with the slush pumps in general use on therigs.
On field wells requiring drilling times in excess of approximately 30 days,an alkaline-tannate-lime-bentonite treatment has been effective in maintainingdesirable viscosities and filtration rates with a substantial reduction inchemical costs. This system has shown particular advantage on heavily weightedmuds and those with abnormal flow-line temperatures.
For the most part, the chemical treatment utilized in wildcat drillingfollows closely the program used on field wells, depending on depth andduration of drilling operations. Wildcat mud programs are planned frominformation available from various geologic and operational sources, takinginto consideration the possibility of encountering mud problems of a specialnature in the wildcat area. Careful planning of wildcat mud programs has provedto have definite value in avoiding most serious mud problems.
An analysis and recommended treatment are presented on several special mudproblems which have been encountered in the area in the past. Those problemsdiscussed are lost circulation, blowouts, sloughing shale, excessive chloridecontamination, sulphate contamination, and prevention and correction of cementcontamination.
Drilling-mud control in the Southwest Texas area, in general, does notinvolve the multiplicity of problems encountered in other coastal areas. Forinstance, it is rare that a mixture of problems such as lost circulation,abnormal pressure, severe sloughing shale, and others, occurs on one wellwithin close enough limits in vertical depth not to be taken care of by asensible casing program, together with a relatively clear-cut mud program.There are conditions such as the fairly widespread occurrence of abnormalpressure in the lower Frio and Vicksburg zones, and the tendency for sloughingshale in the Jackson section which make for rather expensive mud control.However, the availability of fairly accurate geologic and operationalinformation makes possible a reasonable degree of standardization on mudtreatment over a relatively large area. It may be said that, over most of thisarea where drilling is currently in progress, the natural mud made is very poorin quality, but responds readily to moderate chemical treatment to yield verygood wall-building characteristics.
Laboratory apparatus has been devised which permits study of the displacementof oil from cores by water and by gas. The cores used contained interstitialbrine as well as oil.
Experiments were run to determine the comparative effect of varying theproperties of the fluids used. No great effect was noted on the maximumdisplacement achieved. This observation made it unnecessary in initial work touse fluids in their exact reservoir conditions. Consequently, the displacementswere run at near-atmospheric pressure in Pyrex glass equipment, using strippedcrude oils.
Introduction and Theory
The chief object of this work has been to determine the efficiency of gasand water as primary agents for displacing oil from reservoir rock underlaboratory conditions in which capillary phenomena were predominant. To thisend the maximum displacement of oil from cores has been ascertained. Thismaximum displacement may not be equal to the maximum displacement from areservoir; but it will he a close approximation to it sometimes, and othertimes the laboratory information will be useful in reservoir engineeringpredictions. It is believed that the laboratory experimental maximum representsthe upper limit for the reservoir recovery.
The experiments were carried out by obtaining cores of interest from thereservoir, and filling the pores with interstitial brine and oil with therestored state technique. Then the oil was displaced from the core as describedlater, either by brine from below, or by gas from above. The former type ofdisplacement suggests analogy to production by water drive, but not to waterflooding, for reasons discussed below. The latter type of displacement isbelieved to simulate production by gas cap displacement.
The displacements were performed by what may be termed thecapillary-pressure method. The cores are placed in capillary contact with anoil-wetted membrane which has very small pores (about I micron in diameter).Pores of this size will transmit oil hut prevent the passage of gas or water,unless the pressures used are higher than the capillary pressures employed inthis work. Accordingly, use of the membrane makes it possible to apply acapillary pressure differential between the displacing phase and the oil in thecore.
This paper describes the sources of funds required by the petroleum industryto finance capital expenditures and also presents a discussion of the effect ofrising construction costs on these expenditures. The petroleum industry obtainsits capital funds from several sources: (1) internal, from retained cashearnings; and (2) external, from borrowings and the sale of securities to thepublic.
The upward trend of capital expenditures of the petroleum industry is causedin the main by the influence of two powerful factors: (1) the physical growthin the demand for oils; and (2) the rising cost of drilling wells andconstructing refineries, pipe lines, and other facilities. The segregation ofthese factors is accomplished by deflating the actual capital expenditures sothat they are shown in terms of 1939 costs and then subtracting the adjustedseries from the actual figures to yield a set of data representing theexpenditures made on account of higher costs.
Rising costs affect prices and the portion attributable to this factor hadto be generated from the cash earnings of the industry, which called for higheroil prices.
Capital may be defined as "wealth employed in or available forproduction." All production requires capital. Expanding industries requiremore capital than static ones, and technological industries employ more capitalthan those in which little equipment is needed. The petroleum industry is bothrapidly growing and highly technological, and, being a large industry, itscapital requirements are prodigious, amounting to about one-seventh of thetotal of all American business, excluding agriculture.
Capital formation may be defined as the method by which the wealth orcapital needed in the productive processes is created. There are various waysin which capital funds may be obtained but there is only one way in whichcapital can be created - out of production in excess of consumption, that is,savings. The physical realities are simple, but the monetary concepts arecomplicated because the mechanism of credit can draw upon future savings.
The volumetric behavior of five mixtures of black oil and natural gas and oftwo mixtures of condensate and natural gas from a field in the San JoaquinValley was experimentally established. This work was carried out at 100?, 190?and 250?F in the pressure interval between 400 and 5000 psi. The viscosity ofthe liquid phase of four mixtures of black oil and natural gas wasexperimentally measured in the above-indicated temperature and pressureintervals. The effect of methane upon the precipitation of bitumen from theblack oil was studied at 230?F.
The solution of a number of the economical and technological problems whicharise in connection with petroleum production is dependent upon theavailability of the necessary quantitative information covering the materialsinvolved. This requires data concerning physical properties of naturallyoccurring hydrocarbon mixtures at the pressures and temperatures characteristicof underground petroleum reservoirs. The experimental determination of thedesired data for each situation encountered is relatively time-consuming.However, it is considered to yield results sufficiently more reliable thanthose obtained from correlations to justify the effort. For these reasons therewas undertaken a laboratory investigation of the viscosity of the liquid phaseand the volumetric and phase behavior of mixtures of oil and gas samplesobtained from wells in a field located in the San Joaquin Valley.
The nomenclature of petroleum engineering is not at present entirelystandardized. Many commonly used technical terms are not everywhere given thesame meanings. For the purposes of this paper the following definitions will beadopted:
Black oil refers to the dark-colored, hydrocarbon liquid obtained when theproducing horizon of the well lies partly in the condensed hydrocarbon phase ofthe reservoir. Condensate is the term applied to the relatively volatile, paleyellow or amber-colored hydrocarbon liquid that is often obtained from thesurface separator system when the producing horizon of the well lies in the gascap of the reservoir. The gas obtained from the surface separator system iscalled natural gas regardless of whether the well is producing black oil orcondensate.
As might be expected from the multicomponent character of naturallyoccurring hydrocarbon materials, the compositions of black oil, condensate andnatural gas are subject to considerable variation depending upon thetemperatures and pressures prevailing in the surface separator system as wellas upon the overall composition of the material produced from thereservoir.
The Louisiana Continental Shelf is a submarine area extending offshore asmuch as one hundred miles. The Gulf bottom in this region varies considerablyin extent, profile and composition and consists largely of sedimentarydeposits, predominantly those of the Mississippi River. It is very young in ageological sense.
Up to the present time few major structures have been erected offshore inthe Gulf and no comprehensive preliminary tests have been carried out. Resultsof pile-driving tests are of little value as actual bearing capacities farexceed the dynamic resistance to driving as measured by conventional formulas.Soils encountered are, with few exceptions, cohesive and piling consequentlydevelop their supporting power through surface friction.
Because of the time required and relatively high costs, no complete loadtests have been made. The results of those tests which have been made, andinformation obtained from pulling piling in one structure, indicate that designloadings could be considerably increased, and it is contended thatcomprehensive preliminary investigations including an actual load test,combined with borings and examination of materials encountered by competentsoil experts, would effect economies in design far in excess of their cost.
The Louisiana Continental Shelf - General
The Louisiana continental shelf is a submarine area which extends seawardfor as great a distance as one hundred miles off the present shoreline. TheGulf bottom in this region is characterized by low slopes and its outer marginis outlined by the fifty fathom (300 ft.) contour. South of the margin of theshelf the slopes steepen abruptly in the continental slope zone and plunge intothe deeps of the Gulf of Mexico.
The engineer who is called upon to design a structure on this shelf isconcerned only with the uppermost portion to a depth generally not to exceed100 to 15? ft. This layer is very young in a geological sense, and it has thecharacteristics usually associated with infancy. A few of these are lack ofstrength, lack of uniformity, and high water content. It consists of thesedimentary deposits of the various rivers and streams entering the Gulf. Thesestreams are all heavily burdened in quantities sufficient to fill the areas attheir mouths, were it not for the fact that continual subsidence, equal orgreater in amount and acting directly opposite, prevail over the sedimentaryworkings of the streams. This subsidence has been estimated by geologists to beat the rate of at least one foot per century.
Influence of Mississippi River
The Mississippi River, which has been the predominant factor in theformation of practically all the Louisiana continental shelf, is one of tworivers flowing into the Gulf that have been able to build protruding deltas.The Mississippi has done so because of its enormous size and load. It depositssediments in amounts sufficient to replace and actually gain on the amount ofsubsidence.
Experimentation which measures differences in pressure across the interfacesof immiscible fluids in the interstitial spaces of porous media may be termed"capillary pressure experimentation". In the literature of petroleumtechnology since 1941 there have appeared twelve papers popularizing capillarypressure concepts and developing applications of these concepts for thesolution of problems of practical reservoir engineering importance, such as thedetermination of connate water, fluid distribution in transition zones,reservoir rock textural properties, and oil recovery from petroleumreservoirs.
Although a review of the literature would be justified at this time in order toreconcile the conflicting viewpoints which are contained in the cited papers,the object of this note will be of more limited scope. It is believed that therecent paper of Muskat calls sufficient attention to many of the uncertaintieswhich arise upon examination of the published literature, such that untilsatisfactory counter-proposals can be made it will serve our purpose only tomention some other like uncertainties. This will emphasize further the needconfronting petroleum technologists to reconsider and reformulate theapplication possibilities of capillary pressure experimentation. In particular,I suggest that we examine the thesis that capillary pressure experimentation asabove defined can lead to measurements reflecting the recovery of oil frompetroleum reservoirs, as was proposed first by Amyx and Yuster and was firstreduced to practice by the experimentation of Welge. In fact, I shall dealexclusively with the Welge papers since it is the principle one on this subjectwhich has appeared to date. I shall attempt to show that the possibility ofdescribing oil recovery features in terms of capillary pressure phenomena hasnot been established entirely.
The Welge paper was a pioneering effort to evaluate this recovery applicationpossibility. P. P. Reichertz comments on the Welge paper state that non-wettingphase discontinuities are developed occasionally in the Welge experiment, andrestate Muskat?s argument that discontinuous fluid elements (partiallysaturating the interstices of porous media which are elsewhere saturated withsome other immiscible phase or phases) are not subject to the requirements ofhydrostatic equilibrium. It is the purpose of my comment to show that it isunnecessary to make Reichertz' postulation regarding nonwetting phasediscontinuities (however valid) when it is desired to criticize Welge'stheoretical treatment of his problem. It can be shown that Welge's method ofmeasuring values for "capillary pressure" is invalid in many instancesof application, such that his reported curves of capillary pressure versusfluid saturation often have no physical meaning, even throughout the intervalof fluid saturation where it might be otherwise suspected that no phasediscontinuities occur.
In this paper, the theory of elasticity has been applied to the rock about adeep well. It is assumed that the rock has a modulus of elasticity and aPoisson's ratio and that the theory of elasticity applies. It is necessary toknow or assume the state of stress existing in the rock before it is penetratedby the well drill.
The application of this theory indicates that stress concentration of shear,tensions, and compressions about the bore hole are of a high order. This isparticularly true when a horizontal compressive stress exists in one directiononly in the formation before drilling. If such an initial state of stressexists before drilling, then the rock will have stress concentrations of bothtension and compression at the same elevation and of such magnitude thatfailure of the rock is likely. Accompanying these is a shearing stress of largeproportion which is likely to produce spalling of the well walls. Internalpressure applied to the well bore will relieve the extreme compression but notthe tension and has little effect upon the shear. Plastic deformation of therock through a geological time tends to mitigate the stress concentrations.
It has long been known that the stresses about holes and re-entrant cornersof elastic solids under the influence of loads are different and generally moreintense than those imposed upon the body elsewhere. Stress concentration at are-entrant corner can be reduced by increasing the radius of the fillet at thecorner. Stress concentration at the end of a crack in a plate undergoing eithertension or compression can be relieved appreciably by drilling a hole at thevery end of the crack. Also, stresses can be increased in an elastic body intension or compression by making a hole in it. The stress concentration isgreatest at the edge of the hole.
Considerable knowledge of the stresses which exist about the bore hole of adeep well may be had by applying our knowledge of engineering mechanics; ormore particularly the theory of elasticity. It is, of course, necessary to makethe assumption that the rock is elastic and behaves as an elastic solid, thatis, that it obeys Hooke's law, has a modulus of elasticity, and a Poisson'sratio. It is also necessary to know, or assume, the state of stress whichexists in the rock prior to the penetration of the drill. With this knowledgeit is possible to compute the stresses about the bore hole.
The problem can be simplified by considering several simple cases separatelyand then by applying the principle of superposition to solve the more complexcases which are made up of the simpler ones.