US3139929A - Secondary recovery by miscible fluid displacement - Google Patents

Description

Jul 7. 1964 8. HABERMANN 3,139,929

SECONDARY RECOVERY BY HISCIBLE FLUID DISPLACEMENT Filed Nov. 30, 1959 2 Sheets-Sheet l 7" A .aua EXA M1045 I (aw/Vs INVENTOR.

- rron 5y y 1964 I B. HABERMANN 3,139,929

SECONDARY RECOVERY-BY MISCIBLE FLUID DISPLACEMENT Filed Nov. 30, 1959 4 2 Sheets-Sheet 2 IN V EN TOR.

A 77 ORA/E Y reservoir gas.

I fined by the followingequationi I where nited Su tesv This invention relates to secondary recovery of petroleum from subterranean reservoirs. Specifically, it relates to a miscible fluid-displacement method of secondary recovery having improved areal sweep efliciency, and comprises simultaneous injection of water with the miscible fluid and the drive gas. I I I In attempts to recover petroleum'from subterranean I reservoirs by secondary recovery, much attention has recently been focused on the miscible fluid slug displacement method. In this method, a fluid (herein termed a fdisplacernent fluid' miscible with the reservoir crude oil is injected into the formation from an injection well area:

3,139,929 Patented July 7, 33964! ice placing fluid to its permeability with respect to the displaced fluid, k lk In-addition to their adverse 'eflect on'areal sweep efliv ciencies, high mobility ratios between the fluids cause fingering, resulting in. a high degree of mixing between the fluids. When'this occurs, the displacement fluid be a herent characteristics of the formation. Generally, thisi ratio is greater than one and (is hence adverse to high and is forced through the formation to drive or carry-- the crude oil towards a suitablylocated production well.

Examples of suitable-displacement fluids are LPG, propane, butane, isobutane, pentane, isopentane and petroleum distillates. Since such fluids are relatively expen-' sive, they are in turn forced through the formation by a less expensive driving fluid which issoluble therein-or miscible therewith, e.g., naturalgas;methane,.ethane or Usually, thedriving fluid is gaseous at sweep efliciency) because the permeability of the formation with-respect tothe reservoircil is lowered by the presence of gas bubbles in the reservoir oil. This condition, referred to as free gas saturation, occurs whenever the reservoir pressure is below the. bubblepoint pressure of the reservoir fluids. This can occur naturally but is usually the result of pressure depletion during primary recovery. i

Previously, attempts to improve the mobility ratio between the displacement fluidv and the reservoir oil have centered about methods for reducing-the ratio of the viscosity reservoir oil to that or thedisplacement. Thus, it has been proposed that the. viscosity of the-displacement fluid be increased by the-addition thereto of crude oil orcrude oil distillates. A refinement of this is the reservoir conditions. The displacement fluid is thus employed in the form of a slug interposed between the I driving fluid and the crude petroleum, and is provided only in an amount suflicient to insure separation of the driving fluid and the petroleum. I

Although miscible'fluid displacement provides high oil recovery from the swept area, much difliculty has beenexperienced in obtaining a satisfactory areal sweep of the reservoir. The poor areal sweepof the displacement fluid slug has been found to be related to high mobility. ratios between the displacement fluid and the oil, between the displacement fluid and the'drive' fluid, and between the oil and the drive fluid. -The mobility'ratio is dek ,k =permeability of the formation with respect to the displacing fluid and displaced fluid, respectively.

Laboratory tests on miscible displacement in a simu-- lated five spot pattern over a wide range of mobility ratios have shown that the maximum sweep efficiencies (before'breakthrough'of the miscible fluid) are achieved.

at the lowest mobility ratios; ratios of one or less being necessary to sweepat least 70 percent of the core 7 area, with ratios between 1 to 10 sweepingonly 70 to,

43 percent of the area. From this test work, it can be seen that the areal sweep efficiency of a miscible displace ment operation is proportional to the mobility ratios in the system. The preceding equation shows that these mobilityratios are inturn directly proportional to the ratio of the displaced fluid viscosity-to the displacing fluid 7 viscosity, [Ll F2, and inversely proportional to the ratio I ,.of the formation permeability with respect. to the dis techniqueof adding aviscous oil to the slug of displacement fluid in -varied amounts to obtain a slug having I graduated bands of successively lower viscosity from its leading to trailing edge, thereby avoiding any single sharp transition in vscosity between the oil andslug or between the slug and the drive fluid. These methods, however, are beset .by many problems, among which is that addition of crude oil or distillates to'the miscible slug destroys its miscibility with the drive fluid, and true misci ble displacement is lost.-- When thisoccurs, the drive fluid tends to linger through the slug into the oil phase towards the producing well rather than advance as a.

smooth front, and much of the sing and oil is bypassed.

'Another factor to be considered in anycommercial processis the impracticality of adding the necessary amount of oil to the displacement fluid to increase its viscosity.

sufliciently. If relatively expensive distillates are not employed and a portion of the produced crude oil is used as the viscosity additive, it is apparent that the viscosity of the displacement fluid cannot be as great as the reservoir oil viscosity; as a practical limit, the viscosity of the displacement fluid is much less than that of the reservoir oil. Yet, if the most favorable mobility I ratios are to be obtained, the viscosity of'the displacement fluid must be made at least as great as that of the reservoir oil, and in those cases where the formation has a low relative permeability to the-oil due to free gas saturation, the viscosity of the displacement fluid must be made greater than that of the reservoir oil. Actual recovery dataobtained by laboratory displacements in core samples simulating reservoir conditions have shown that substantially no improvement in areal sweep effl- I ciency can be achieved by the technique of increasing the viscosity of only the displacement fluid slug.

It has also been proposed that the mobility of the drive fluid be reduced so as to decrease the mobility ratio between the displacement fluid and the drive fluid. 1

In practice, it has been found that although the areal sweep of the drive fluid is thus improved, the displace ment fluid slug fingers severely into the reservoir oil so that it no longer separates the drive fluid from the reservoir oil. When this occurs, miscibility is lost and only a fraction of the oil is recovered from the swept area.

It is the general purpose of this invention to improve the areal sweep efficiency of a miscible fluid displacement process and to prevent fingering and bypassingof the reservoir oil by the displacement fluid.

The aforementioned purpose is achieved in accordance with the invention by admixing water with both the displacement fiuid and the drive fluid. The presence of This results in; a-greatly improvedareal moth-acrylic acids, methacrylarnide, styrene, vinyl acetate, acrylonitrile, methacrylonitrile, vinyl alltyl ethers, vinyl chloride, vinylidene chloride, etc. In addition to the aforementioned limitation on the extent of hydrolysis, the suitable acrylamide polymers are of sufficiently high molecular weight that an 0.5 percent by weight aqueous solution thereof has a. viscosity of at least 4 centipoises Ostwald at 21.5 C. A commercially available polymer meeting these requirements is marketed by The Dow Chemical Company under the. name Separan. The

above-mentioned. polymers are preferred since they form .-.stable solutions and'do not easily precipitate when heated or whenin the-presence.of mineral anions and cations.

sweep efficiency for the processfl} 3; However; the usegof any other water-soluble material The invention i de ib d i detail in th following" which forms a stable viscous aqueous solution is also paragraphs and examples which'rfer tothe figuresrof h? Scope -p Invention; P of which: I p such materials includepolyvinyl alcohol,'carboxymethy1 FIGURES and 2 illustrate theare'alsweep-pattins s t-fi r.P y y d t Wh n obtainable'by the inventiom-u ploying an aqueous solution'of these polymers incre- FIGURES 3 and 4 ill tr t th e lsweep pattern mentalextent to'whichthe mobility is decreased is proobtained in miscible fluid displacements embodying two slugs of fluids having successively lower viscosities;

FIGURES 5 and 6 illustrate the areal sweep pattern typical of that obtained in a miscible fluid displacement obtained by miscible displacement employing three slugsof successively lower viscosity miscible liquids and a gas drive. p

The amount of water to be added to each of the displacement'and drive fluids canbe calculated for each reservoir from the water. and fluid viscosities, the relative portional to the diflerence between the viscosity of the polymer solution and the viscosity. of water. Generally, this can result in reducing the mobilities;from 2 to 10 times more than the reduction possible by-injection of only water with the displacement ancl'driving fluid, so

that the mobilities of eachof the injected fluids can be made equal to or less than that of the reservoir oil.

The preceding discussion has been. based onthe assumption that .the mixtures of .water' and displacement anddriving -fluids-will remainfliomogeneous Actually,

a certain amount-."of"'gravity separation' will occur, and the water willi'tend to settle to the lower strata. Accordthe amount of water-to be'injected'with the displacement permeability, of the formation to water and the fluids, i y Fl- Prefefredmode 9 096135011, the Effects Of gf the water saturation in. eachfluid, 'and the amount of y, m wulllelffid y injecting 1116 Water mm interstitial water present in the'-reservoirhitfienerally, .SSW PP strata; h and injecting the placement and-di'iving tfluidsvinto lower lying strata.

fluid is between about 0.5. and 10 -.barrels pen-barrel measured atstandard conditions and theiamount of water to beadmixed with the drive fluid'is between about 0.75 to 15 barrels per thousand cubic fetf'at standard conditions. Conveniently, the. mixtures are obtained by 7 simultaneously injectingzthe. water and the-fluid in" ques tion into the reservoir, preferablyat equal-velocities.

The admixture oi waterwith a: given fluid has a stri ing effect on the mobilitypf that fluid. for instance, 1

. 1.28 barrels ofwater are injected intoa typical oil-bean" ing formation along with eacli'barrel of a propane-rich displacement slug, the mobility of the'slug under a typical reservoir condition jis reduced-from about 1700 millidarcies/centipoise' to about l00 'millidarcies/centipoise.

Similarly, the injection of 2.08 barrels of water'alohg with each thousand cubic'feet of a drive fluid consisting of reservoir gas reduces the gas mobility from about 2900 millidarcies/centipoise to about 125 millidarcies/ To attain equivalent reductions in mobilities by increasing'only the viscosities of the respective fluids I would require a 1700 percent increase in the viscosity of the propane-rich slug and a 2320 percent increase in centipoise.

the gas viscosity. It is, of course, apparent that such increases are not possible by the heretofore suggested methods of adding hydrocarbon oil fractions to the fluids.

Even greater decreases in mobility of the injected water-fluid mixtures can be-obtained by increasing the viscosity of the waterbydissolving a viscosity-increasing additive therein. Preferably, the additive is 'a watersoluble partially hydrolyzed acrylamide'polymer- The latter material is a water-soluble acrylamide polymer Position:

miscible displacement -in-.a,=reservoir having a temperature of 2 F., a-pressure of 2,000 p.s.i., and 16 percent 40 .free-:gas sa turation in ,the oil phase. The viscosity of the 'oila-in the reservoir is 0.66 centipoise and the relative permeability of the formation to the oil is 83 percent of its permeabilityto oil with no gas saturation.

7 EXAMPLE I The invention as applied to this reservoir comprises the simultaneous injection of water with the displace ment slug and with the drive fluid. The displacement fluid comprises a mixture of 80 volume percent propane and 20 volume percent'reservoir gas, ;=--while the drive fluid comprises a methane-rich reservoir gas. To obtain equal water and displacement fluid velocity, 1.28 barrels of Water are'injected Iwith eachbarrel of displacement fluid, and to obtain equal-water and-gas velocities in the drive phase, 2:081barrels 'of-water are injected per thousand cubic feet of reservoir gas ,The following 'mobility ratios are thereby-obtained? Table 1;, p

v Mobility ratio Overall, between theoil and water-' -drive fluid 1.99

Between the oil and water-{displacement slug 1.59

Between the water+displacement slug and the water-l-drive fluid 125 To determine the flow pattern achieved underthese conditions, a laboratory study was made displacing fluids .from a consolidated sandstone-plate sealed between clear sheets of plastic and representative of a quarter section of a five spot pattern. Inlet and outlet holes corresponding to inlet well I and production well P, were provided at opposite corners of-the assembly, and suitable connections permitted liquids to be forced into the sandstone plate and through the same towards the outlet connection. This assembly was supported in a horizontal plane above a light source, and a camerawas positioned di rectlyabove the assembly to photograph the flow of The invention will.now be described by reference to Position:

D fluids, colored for identification, as they were passed from the inlet to the outlet connection. Cooled air was blown across the bottom surface of the assembly to prevent the hot light source from heating the assembly during a test run. Color slides or color movies were made during the tests to evaluate the flow patterns, and the outlet flow was measured to determine the amount of oil recovered. By' this method a continuous record of the EXAMPLE n The aqueous polymer embodiment of the invention as applied to this reservoir comprises the simultaneous in- I jection of a polymer solution with the displacement and drive fluids. A typical polymer solution having a viscosity of 0.5 centipoise at reservoir conditions, or only twice that of water alone, is employed at aninjection ratio of 1.60 barrels per barrel of the displacement fluid and at 2.59 barrels per thousand cubic feet of the drive fluid at standard conditions. ratios are thereby obtained:

Table '2 The following mobility Mobility ratio Overall, between the oil and aqueous polymer-j-drive fluid 1.06 Between the oil and aqueous polymer-l-displacement slug 0.93 Between the aqueous polymer+displacement slug and the aqueouspolymer+drive fluid 1.14

A laboratory test duplicating the aforementioned mobility rates in a sandstone core similar to that employed in Example I provided a 71 percent recovery before 7 breakthrough of the displacement fluid into the produc-- ing well. v

. I EXAMPLE III For comparison purposes, the flow pattern obtained by a miscible displacement processtypical of conventional practice was studied in the laboratory.' This typical prior art method comprises injection of a 10 percent pore volume slug of 80 percent propane mixed with 20' percent reservoir gas, followed by a reservoir gas drive. The following mobility ratios are obtained between the phases: t

. Table 3 m Position: Mobility ratio Overall, between the oil and gas drive fluid 46.8 Between the oil and displacement slug 27.4 Between the displacement slug and drive fluid 1.7

The laboratory test -was on a sandstone core model similar to that employed inExample I and duplicated the aforementioned mobility ratios. The results from this test are depicted by FIGURES 3 and 4 which respec tively show the flow patterns obtained after 20 pore volof the invention.

EXAMPLE IV Again, for comparison to the invention and to evaluate.

the flow pattern such as might be obtained by using a low viscosity propane slug and a water drive fluid in atypical reservolr, another test was made. The following mobility ratios are typical of what would be encountered under reservoir conditions:

Table 4 Position: Mobility ratio Overall, between the oil and water-l-drive fluid 1.0 Between the oil and the propane slug 38.0 Betweenthe propane slug and the water-{- drive fluid 0.0265

A laboratory test was conducted duplicating the mobility ratios of Table 4. The oil recovery at breakthrough was 23.6 percent of the oil in place, or only about 31 percent of the recovery attainable with the invention. The flow patterns at 20 percent'pore volume of fluid injected and at breakthrough are shown by FIGURES 5 and 6, respectively. FIGURES 5 and 6 show extreme fingering of the slug phase through the oil with poor.

areal sweep of the miscible slug. Continued displacement in this case showed a relatively high areal sweep of the drive fluid because the mobility ratio between the oil and drive is equal to 1. However, because the miscible slug fingered ahead of the drive andwas not retained as a hand between the drive and oil phases substantial amounts of oil was bypassed and left behind as residual oil in the swept area. This loss of'miscible fluid between the phases can be seen in the right-hand side of FIGURE 6.

EXAMPLE V To study another miscible slug technique typical of prior art practice for comparison to the invention a laboratory test was conducted duplicating the mobility ratios obtained under reservoir conditions when employing a miscible fluid slug gradated into three bands of successively lower viscosity. Such a slug is obtained by adding varied amounts of a high viscosity crude to separate bands of a propane ich miscible slug. The slug is usually driven with a low viscosity reservoir gas and.

the following mobility ratios prevail:

Table 5 Description: Mobility ratio Overall, oil to drive 75.0 Between oil and slug 1 3.0 Between slug 1 and slug 2 2.9 Between slug 2 and slug 3 3.0 Between slug 3 and drive 2.9

. The flow pattern obtained in the laboratory test duplicat- 'water or an aqueous polymer solution with a miscible displacement fluid and a drive fluid permits the use of low mobility ratios and thereby attains high reservoir oil recovery. This improvement is shown to be attainable only with the invention, and other methods directed to increasing the viscosity of the miscible slug or to increasing the viscosity of the gas drive do not achieve any comparable results; in fact, the results tend to show that these methods are adverse to high oil recovery.

Having disclosed and illustrated my invention, I therefore claim:

l. The method of obtaining a high areal sweep efliciency in a displacement process for the recovery of oil from a subterranean reservoir which is penetrated by at least an injection well and a withdrawal well, which method comprises: simultaneously injecting into said said oil under reservoir conditions, the amount of said water being between about 0.5 and about 10 barrels per barrel-oi said hydrocarbon, controlling the amount of water-so. injected to provide a velocity of water through said reservoir about equal to the velocity of said hydrocarbon liquid through said reservoir; forcing said slug of water and said slug of hydrocarbon through said reservoir towards said withdrawal well by simultaneously injecting into said injection well water and a drive fluid miscible with said hydrocarbon liquid, the amount of said water injected with said drive fluid being between about 0.75 and about 15 barrels per thousand cubic feet of said drive fluid, controlling the amount of water so injected with said drive fluid to provide a water velocity in said reservoir about equal to the velocity of said drive fluid therethrough, and withdrawing displaced oil from said reservoir through said withdrawal well.

2. The method of claim 1 wherein the said hydrocarbon liquid is selected from the group consisting of LPG, propane, butane, isobutane, pentane, isopentane',

crude oil and crude oil distillates.

3. The method of claim 1 wherein said drive fluid is gaseous at reservoir conditions.

4. The method of claim 3 wherein said drive fluid is selected from the group consisting of natural gas, methane, ethane and reservoir gas.

5. The method of claim 1 wherein the said water contains a dissolved viscosity-increasing additive.

6. The method of claim 5 wherein said'viscosity-increasing additive is a. water soluble acrylamide polymer which has been hydrolyzed to the extent that between about 0.8 and percent of the amide groups have been converted to carboxyl groups.

7. The method of obtaining high areal sweep efficiency 1 in a displacement process for the recovery of oil from a subterranean reservoir which is penetrated by at least an injection well and a withdrawal well which comprises injecting water into the upper portion of said reservoir through said injection well while simultaneouslyinjecting into the lower portion of said reservoir through said injection well a hydrocarbon which is liquid and miscible of water so injected to provide a velocity of water through said reservoir about equal to the velmity of said hydrocarbon liquid throughsaid reservoir; forcing said water and said hydrocarbon through said reservoir towards said withdrawal well by continuing to inject said water into the upper region of said reservoir through said injection well and simultaneously injecting into the lower portion of said reservoir a drive fluid which is miscible with said hydrocarbon liquid, the amount of said water injected with said drive fluid being between about 0.75 and about 15 barrels per thousand cubic feet of said drive fluid, controlling the amount of; water so injected with said drive fluid to provide a water velocity in said reservoir about equal to the velocity of said drive fluid therethrough, and withdrawing displaced oil from said reservoir through said withdrawal well.

8. The method of claim 7 wherein the said hydro carbon liquid is selected from the group consisting of LPG, propane, butane, isobutane, pentane, isopentane, crude oil and crude oil distillates, and said drive fluid is selected from the group consisting of natural gas,

methane, ethane, carbon dioxide, air and nitrogen.

9. The method of claim '7 wherein said viscosity-'in- Y creasing additive is a water soluble acrylamide poiymer which has been hydrolyzed to the extent that between about 0.8 and 10 percent of the amide groups have been converted'to carboxyl groups. 1

References Cited the file of this patent.

UNITED STATES PATENTS Great Britain Mar. 23, 1955

Claims (1)

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