The importance of transverse diffusion on the fznger development in a miscible-phase displacement at an adve.rse mobility ratio of three was studied in a porous plate and slow rates (1. 6 ftlD) were used to determine t~e effect of residence time on the geometry of the fzngers. The shape of the fingers was observed directly by use of the X-ray technique. At fast rates numerous narrow fingers were observed, but at slow rates a single somewhat bulging finger was produ~ed. !he amount of material moved transversely by dlffuslOn across the plate was sufficient to modify the finger geometry in the slow-rate run because of the long residence time. These results are in contradiction to some of the postulates in the literature.The composition of the effluent stream, however, was not affected by the flow rate. This result is not inconsistent with the observed change in the shape of the finger in a short model, but it seems likely that a short model does not offer adequate and proper scaling of the reservoir. The model used was probably a valid one for studying the effect of transverse diffusion on the finger geometry, but .a longer model would be needed for proper scalzng of the effect of the change in the finger shape on the efficiency of displacement as measured by the composition of the effluent stream.
The displacement of oil by water in typical porous media has been studied, using new equipment developed to handle flooding experiments on 1-in core plugs with a precision of about one pore per cent. Long core floods using cores up to 10 ft in length were used to supplement the short core studies. From the results of several thousand flooding experiments on a variety of porous media, we have found that the wettability is the single most important variable affecting the recovery history of a water flood. Cores are, therefore, usefully described as being (1) water-wet, (2) oil-wet, or of (3) intermediate wettability. This classification scheme has provided a sound basis for the interpretation of laboratory waterflooding data. For water-wet cores it can be concluded:1-in core plugs show the same recovery history as long cores when flooded under the same conditions of rate, viscosity and wettability;recovery is practically independent of rate of flooding from reservoir rates up to 40 ft/day;at extremely high rates (over 100 ft/day) significant recovery increases were observed;displacement by imbibition alone displaces the same quantity of oil as water drives do with rates up to 40 ft/day;lowering the interfacial tension will reduce residual oil if the lowered value is maintained at the flood front. An explanation for these conclusions based on the interplay of the viscous and capillary forces at the flood front (called "Viscap" concept) is presented, and from this we may conclude that laboratory floods of water-wet core plugs offer no particular scaling problems. The intermediate wettability case is more complicated, and because of limitations in measuring the wettability of cores and the reservoir, this group needs more study. The behavior of oil-wet cores is markedly different from the water-wet case. Generally, oil-wet cores show a considerable amount of oil production after water breakthrough even for a viscosity ratio of unity. Also, they exhibit a saturation gradient at the flood front (sometimes called an end effect). This end effect invalidates laboratory results unless the floods are scaled using a relationship involving length, viscosity, rate, interfacial tension and contact angle. Introduction The need for secondary recovery procedures to reduce the fraction of oil left in the reservoir is generally accepted by the oil industry. One of the most effective of these secondary recovery methods is water flooding.
The centrifuge has been found to be an extremely useful tool for determiningcapillary pressure curves and for establishing connate water and residual oilin small core plugs. The use of the centrifuge for determining the propertiesof small core plugs has been discussed in the literature, but practically noexperimental evidence has been presented justifying the use of this method inpreference to other methods (mainly disc method) now in use. Data have beenaccumulated and are now presented demonstrating the advantages of thecentrifugal method, some of which are:rapid establishment of equilibrium,excellent precision yielding very reproducible results,availability ofhigh pressure differences between phases,simple operational procedure, andability to establish connate water, residual oil, or to obtain completecapillary pressure curve in one day or less. A commercially availablecentrifuge capable of speeds up to 3,800 rpm, and a high speed attachmentproviding a maximum rate of rotation of 18,000 rpm were used in this work inconjunction with core holding tubes designed specifically for thisapplication. In view of the outstanding advantages associated with the use of thecentrifuge the conclusion has been drawn that this method should be used inplace of the more tedious, slower, and less reliable disc method fordetermining capillary pressure curves and for establishing connate water andresidual oil in small core plugs. Introduction The need for accurate values of connate water, residual oil, and capillarypressure curves in reservoir engineering has been recognized for many years, and the extent of the effort devoted toward solving these problems is reflectedin the large number of papers which have appeared in the literature during thepast decade. Most of the methods discussed in the recent literature employ aporous disc as a semi-permeable membrane to separate two immiscible phases andmake possible the application of an explicit difference in pressure between twophases in the core. Thus, in the determination of the capillary pressure curvethe wetting phase (water) is displaced by the non-wetting phase (air oroil). Several investigators have used centrifugal forces to develop pressuredifferences between phases. As a result of the first work in this field theconclusion was reached that "the more rapid centrifugal method formeasuring irreducible saturation may have equal value, (compared to use ofdiscs) but more work is required to establish this point." This work wasfollowed in 1945 by a more detailed study in which the concepts of thisexperimental work were more clearly enunciated. A method for calculatingsaturation for any given speed of rotation based on the observed volume ofwater displaced was developed and a wider application of the method wassuggested. T.P. 3052
This study of the effect of gravity segregation was carried out in a vertical core of unconsolidated sand, 4 ft long and 2 in. in diameter. Fluids of varying viscosity and density were injected into the top of the core and moved at constant rates of 25, 50 or 100 ft/day by use of a positive displacement pump. The composition of the efflux was analyzed by measurement of the refractive index. The main variables in this study were in viscosity ratio, the density difference between in-place and displacing phases, and the rate of flow. The experiments fell into four logical groups:favorable viscosity ratio and favorable density difference;favorable viscosity ratio and unfavorable density difference;unfavorable viscosity ratio and favorable density difference; andunfavorable viscosity ratio and unfavorable density difference. The behavior of these four systems at various rates of flow was determined by measuring the length of the mixing or transition zone which developed between the displaced and displacing phases. The data indicated that gravity segregation could act to shorten this mixing zone when the displacing material was the less dense phase, and lengthen the zone for unfavorable density differences. The magnitude of the effect was most marked at slow rates when sufficient residence time existed to allow significant flow to occur. The change in the length of the mixing zone with density difference, rate and viscosity ratio was plotted, and from these graphs it became evident that the length of the mixing zone was really dependent upon the ratio of the viscous forces to the gravity forces. If the ratio of these quantities is used as a parameter, we find that the plots of mixing zone length vs the dimensionless quantity V/Vc, yields a greatly simplified presentation of the data. Such plots confirm the importance of the ratio of the viscous forces to the gravity forces in analyzing the flow behavior in vertical systems. Introduction The idea of using miscible phase displacements to increase recovery of crude oil has been under active study for more than 15 years. Most of this work has been carried out in the research laboratories of the industry and in universities. In such studies a great deal of attention has been directed toward determining the amount of miscible material required to recover all of the oil from a porous medium. The nature and extent of the mixing zone which develops between the two miscible materials has also been observed for many situations, including those where the fluids used have different densities. In many of the cases where density differences exist, however, the effect of gravity segregation on the mixing zone has not been explicitly studied. It seems quite likely that serious errors might be present in such experimental work. That is, the literature presents certain data which suggest changes in efficiency of the process as a result of some explicit change in a variable, when it may well be that the change in the efficiency results from gravity effects. The objective of this study, therefore is to determine the effect of density differences of miscible fluids on the observed efficiency of the displacement process. Since such density differences cause selective movement of fluids within the porous medium, the study becomes vitally concerned with gravity segregation. Moreover, since the amount of gravity segregation which can occur for a given set of fluids depends upon residence time, the rate of flow in these experiments also becomes an important variable. Finally, since the actual distribution of the fluids within the system (fingers, etc.) also affects the results, the viscosity ratio is another pertinent variable. Hence, this study may be characterized as a determination of the nature of the mixing zone between miscible phases as a function of density differences, viscosity ratio, and rate of flow. Fortunately these variables are inter-related. SPEJ P. 1ˆ
An earlier publication has discussed three methods for obtaining relativepermeability data on small core samples and the apparatus and technique for thecapillary pressure displacement method. This paper describes the apparatus andtechnique for the solution gas and the dynamic displacement methods andpresents a routine procedure for obtaining oil-gas and water-oil relativepermeability data. Theoretical and experimental considerations are presented to show that theend effect commonly associated with the dynamic flow mechanism is extremelysmall where constant rates are employed in the flowing phase. An effect of flowrate on relative permeabilities obtained by the dynamic system is found onlywhen gas is one of the flowing phases and this effect is ascribed to a form ofchanneling in the capillary system. The apparatus and procedures used to obtain relative permeability data withthree phases flowing are described and some preliminary results of the use ofthis method are shown. Introduction In a previous publication from this laboratory there appeared a briefdiscussion of the concepts behind three basic methods for obtaining relativepermeability data. These three methods were called the capillary pressuredisplacement method, the solution gas displacement method, and the dynamicdisplacement method - the names being suggestive of the type of process usedfor obtaining the desired saturation prior to making the permeabilitymeasurements. In the same publication, the apparatus and technique for thecapillary pressure displacement method were described and some typical resultsobtained by this method were presented. This paper will present:The routine procedure used to obtain permeability data on small coresamples flowing;The experimental techniques for the solution gas and dynamic displacementmethods for obtaining relative permeability data;The results of some studies on the mechanism of fluid flow throughconsolidated porous media; and,The preliminary results on the determination of relative permeability for asystem in which three phases are flowing. T.P. 3056
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