US3074481A

US3074481A - Method for the improvement of areal sweep during secondary recovery

Info

Publication number: US3074481A
Application number: US842414A
Authority: US
Inventors: Habermann Benzion
Original assignee: Union Oil Company of California
Current assignee: Union Oil Company of California
Priority date: 1959-09-25
Filing date: 1959-09-25
Publication date: 1963-01-22
Anticipated expiration: 1980-01-22

Description

Jan- 22, 1953 B. HABERMANN I METHOD PoR THE IMPROVEMENT 0E AREAL swEEP DURING SECONDARY RECOVERY med sept. 2s, 1959 3 Sheets-Sheet l afMz/cw HAJERAMNA/ A er gli A l Jan. 22, 1963 B. HABERMANN METHOD FOR THE IMPROVEMENT OF AREAL SHEEP DURING SECONDARY RECOVERY s sheets-511581 2 Filed Sept.' 25, 1959 Erfra- INVENTOR. ENZ/0N HAaERMA/VA/ ,Jj MW" ,4rromvfy Jan. 22, 1963- a. HABRMANN 3,074,481

METHQD FOR THE IMPROVEMENT OF REAL SWEEP Filed Sept. 25. 1959 DURING SECONDARY RECOVERY 3 Sheets-Sheet 3 l r INVEVTOR.

- 'Nz/QA/ HA ERMANA/ This invention relates to secondary recovery of petroleum from subterranean reservoirs. Specifically, this invention relates to a method for obtaining a high sweep efficiency of a displacing fluid through the reservoir.

In secondary recovery of petroleum from a reservoir,

a Widely accepted practicehas been the injection of a fluid into the reservoir under suliicient pressure to force it through the reservoir to a producing well, thereby sweeping oil from the reservoir. The choice of injection and withdrawal wells is widely varied and depends on the type of secondary'recovery process as well as the geometry of l United States Patent O the reservoir. Among the techniques which have been employed are pattern drives, eg., 5 spot, 'L spot, etc., which are commonly used with a liquid drive fluid such as water, and line or staggered line drives which are commonly employed with a gaseous drive fluid. Other techniques, such as edge and crestal injection are used when needed to suit the reservoir geometry.

A widevariety of fluids has been used or proposed for use as the displacing fluid, and these fluids may be con` venicntly grouped into a gas, liquid, or combined gasliquid classification; the term gas" as used herein being generic to those materials which normally exist in the gas phase under atmospheric conditions as well as those fluids which eXistin the vapor state at the reservoir conditions. Examples of gas drive fluids are air, nitrogen, carbon dioxide, and various hydrocarbon gases, such as methane, ethane, and natural gas. Liquid'displacement fluids, i.e., materials which are liquid or are in a critical phase above both their critical temperature and pressure at reservoir conditions, can be grouped into aqueous and non-aqueous liquids, examples of the latter including hydrocarbons such as liqueed petroleum gas (LPG), propane, butane, isobutane, pentane, isopentane, hexane, isohexane, crude oil, kerosene, gasoline, petroleum ether, xylene, toluene, benzene, coal tar oils, etc.; oxygenated compounds such as glycerol, phenol, methyl, ethyl, propyl, and isopropyl alcohols acetone, acetaldchyde, propanol, etc. Examples of suitable aqueous drive fluids are water, brine, and aqueous solutions of various wetting or surface active agents and/or thickening agents, eg.. fatty acid soaps, alkyl pyridinium compounds. metal alginates, sodium lauryl sulfate, etc., as well as wafer soluble polymers such as polyvinyl or polyallyl alcohols, carboxymethyl cellulose. and partially hydrolyzed polyacrylamide. Because of their miscibility with crude oil, hydrocarbon drive fluids provide a more complete displacement of the reservoir oil than is obtainable with aqueous solutions; however, their relatively high cost offsets their efficient displacement,

and water, with or without wetting, surface active, or

- thickening agents, is predominantly employed as a liquid drive uid.

Combined gas-liquid and miscible liquid systems have also been used or suggested for use. Among such systems is the mixed gas and water phase drive which is achieved by the simultaneous injection of water and natural gas into a reservoir, and the various gas-hydrocarbon liquid drives which have recently received much attention. The latter displacement, frequently referred to as miseible phase or miscible slug displacement, nvolves first injecting into the reservoir a hydrocarbon which is either a liquid or in a critical phase above its critical temperature and pressure at the reservoir condi- Patented Jan. 22, 1953 ice tions and which is miscible with'the reservoir oil, and then driving the injected hydrocarbon slug through the reservoir with a gas or a combined gas-water mixture which is also miscible with the liquid slug. A modification of. this process employs a slug with a gradated concentration of hydrocarbon liquid and drive gas to avoid a sharp?" interface between the drive gas phase and the slug phase.

Hydrocarbon liquids employed in these processes have v been LPG, butane, crude oil, and petroleum distillates.

A third type of miscible fluid displacement process herein considered as a liquid drive system is the high pressure condensing gas method wherein a gas, such as natural gas, is injected at pressures in excess of about 3,000 p.s.i., into the reservoir. The advancing front of gas acts as a solvent to dissolve light fractions of the reservoir oil and thereby form, in situ, a liquid slug phase which is between the remaining gas drive and the reservoir oil and is miscible with each.

Another type of miscible fluid displacement involves the injection of a liquid which is miscible with the reservoir oil and with water which is employed as the drive fluid. Various alcohols, such as methyl, ethyl, propyl, isopropyl, and butyl alcohol, phenol, glycerol, etc.; ketones, such as acetone, butanone, etc.; aldehydes, such as acetaldchyde, propionaldehyde, ete; and various other solvents such as carbon tetrachloride. methyl chloride, dimethylether, sulfur dioxide, and dichloro-difiuoromethane can suitably be used. Because many of these solvents freA quently have a viscosity which is less than that of thel FIGURE 2 is a map of actual tlow conditions of a fluid displacement process in a laboratory model as described in Example l,

FIGURES 3, 4 and 5 are maps of actual flow conditions of the fluid displacement process with application of the invention as described in Example II, and

FIGURE 6 illustrates a ilow pattern typical of the in',- ventiori as applied to a line drive fluid displacement procL ess. v

Common to all" these types of secondary recovery is the problem of obtaining satisfactory distribution of the injected fluid throughout the reservoir, since the latter is not a homogenous mass having the same structure throughout, but, rather, contains zones having different permeabilities and fissures, cracks, or channels having relatively high permeability to the injected fluid. These irregu larities tend to direct the flow of the injected fluid into narrow lingers or channels through the reservoir rather than as a smoothly advancing front, lay-passingmuch of the reservoir oil which is never recovered. This fingering tendency of the injected fluid is also caused by the indiscriminate choice of fluids having very low viscosities relative to the reservoir oil since it is related to the mobility below with reference to the.

points-7, 8 and 9 on the central circular arc.

on the advancing front lies directly between injection', well 2 and production well 5 and is closer to well 5 thany 3 where as'. a1=viscosity of displacing huid and displaced lud,

respectively; and Kg, Kl=perrncability of the reservoir with respect to the displacing lluid and displaced liuid, respectively;

circular. arcs'around injection wells 1, 2 and3 and in its final position by arcs'a and-bv between wells 4-,and5 and Sjand 6, while the shadedfarea representsthat portion v of thev reservoir not swept by the injected uid.' TheV reasonfor this anomalous iow distribution is apparent` from ,aA study of the geometry of the system -shownby Point 8,

any other pointon the .fluid front, Such as 7 and 9. The

I'resistance to iluid how from point ,8 to well 5 is proportional to-distance d which can be seen to be less than" distances c or e for points 7 and 9, respecntively.v Accordingly, point 8 advances ,more rapidly to well 5 than do points7 or-9,v resultingA in the final iront assumingk the shape-:shown by lines a and b.

Fromthexforegoingyit is apparent' that satisfactory flowr distribution of a displacement fluid..through a reservoir is di'lcult to achieve .and that, even with additiorrof viscous additives to thefdisplacing iluidto achieve favorable mobility ratios; a complete-areal sweep vof the. i

il'uidfis. not. possible with customary secondary recovery techniques..

- ItI is purpose of 'this invention to improvethefsween efficiency in secondary -lrecoveryf by theA iluid'fdisplacement rnethod;v v

'This purpose'. is achieved-"b-yf thenovel injection and:

displacement method of' this invention which, in. itsv lluid` into the'reservoir through a first. well' communicate .ing` with: thefreservoir. The displacement fluid can. be. anyof thenurnerous aforementioned -lluids or combina.-

tions tlziereof-whichy have been. used oit-suggested forlise.

, simplest form; comprises: first; injecting a displacement L and'various other solvents such. as acetonitrile, acrylonis front it does not slow down this portion. 'The cilcct of this injection, therefore, is to cvcn thc 'front by increasing the rate of advance of the trailing portions relative to the elongated portions. .The viscosity and quantity of the viscous lluid which is injected into the second well can be readily controlled toproduce a zone of any volume and viscosity,l for instance, a relatively expanded zone o moderate viscosity, or a concentrated zone with a high viscosity dependingxon the'size and degree of elongation ofthe leading portions lof the front. As the front advances, the injected viscous liquidtends to distribute itself across the'frontwith the greatest conv centration at the most .-forward'point of the front as it is slowly, forced towards the producing well. VAfter sucient viscous liquid has been added to retard lingering v of the displacing fluid, the second well can be closed olf or alternatively switched to injection of the displacing huid in combination with, or in substitution for, the injectionvo the 4displacing fluid through the rst well.- After-the fluid front has reached the third well and again becomes deformed with elongated-portions at each `well site, the procedure lis repeated anda fourth row of 'wells is put on production to withdraw the displaced oil. This procedure is repeated as many times as may he desired' or as may be necessaryl to'sweep the entire reservoir. j

The viscous liquid'which isinjected into the reservoir can suitably comprise any cfa number of vscous liquidswhich do not plugor seal olf the formation and which are more viscous than eitherithe reservoiroil or the displacement duid: Generally, a liquid isemployed which has a viscosity 11A. to 20 times greater than either the reservoir oil or displacement iluid viscosity, whichever is greater, and preferably a/liquid which has a viscosity Z'to 10 times greater.

Liquids suitable foruse asl the invention can be for instance; water; brine; hydrocarbons, such as aromatics, e.g., benzene, toluene, xylene,` etc., crude oil and fractions thereof, e.g'.,'.distl1ates, reduced crude, tar, pitches, and waxes (the latter substances .aire to be used with caution toavoid plugging the formationy.v oxygenated compounds,l such as -.alcohols,.e.g., methyl,l

ethyl, normal and iso propyl, butyl, arnyl alcohol, phenol, glycol, glycerol, etc., ethers, eg., methy ethyl ether, d-

methyl diethyl ether, etc., ketones, eg., acetone, butanone,

etc'., and aldehydes, e.g., acetaldehyde, propionaldehydei A tlrile, lcarbon tetrachloride,v methyl. chloride, sulfurdisecondaryreeovery; Concurrently witlrzthisv 1'rtjec:l

tiontorf'if ldesired at'aclater timeafter the reservoirhas been" pressurized, withdrawal of oil is initiated from a.

seconda well spaced apar-tf from said. firstwells' "Ihese:v yvellswillv usually, but` not necessarily, be;well`s which# werel drilledjduring .theA previous primaryl production fromI thev reservoir;v For the ideal case withha mobility ratio-of the displacing .uidl to the oil.. phase-of one'or less and withy a reservoi'rfree from zones having-,zdiler'fentpermeabilitiesvand'-extensiveA fracturesor fissures.

thel displacingfluid willcadvance-uniormly as a smooth.

.front until it nears thegwithdrawal well where the local-- izedwithdrawal of the'oil at the Well will elongate that portion or' the frontA lying directly between thewithdrawal andA injection well. At this point in the operation, production: from -the second well is stopped,l and a viscous uid. is-.injected into .the reservoirthrough the second well yand-,withdrawal of'the oilfrom the'reservoir is initiated; at a third well spaced apart from said second well and so located that the'displacing fluid will continue 1 toi'm'ove inthe same general direction. The, viscous ,liquidA is thus injected into. the reservoir at the most extendedor elongated portion of'the advancing fluid front.' vBecause of its high viscosity, this liquid elec,

tively slows-or retards the` advance of the leading orv elongated region of the iront, but since the viscous liquid is notinthe ow path of the trailing portion of the oxide; and dichiaro-diuoromethane. The` purpose of the invention can be achievedwhethet" or not` the viscous liquid is lmisciblewith. the reservoir oil; however, it is preferred to. employ a liquid which is miscble with both the reservoir oil andthe displacement fluid to obtain a more'complete recovery of oil'. When water orA a hydrocarbon gas 'is employed as the displacementflluid,

such miscibility can be achieved by use of solvents mis-4 cible-with water or the hydrocarbon'gas and reservoir' oil, e.g.,any of the aforementioned' oxygenatedl come pounds,` `and other solvents as acetonitrile, a'crylonitrle. carbon tetrachloride, methylchloride, `sulfur dioxide, and dchloro-ditluoromethane. relatively low viscosity, various viscosity additives must' be employed in admixture withthem, eg., sugars, gums, resins. and glucosides, can be added ,to increase the solv'. j ent viscosity. Examples of gum and resin .solutionsare-r gum benzoin, rosin. ester gum, or sandarak in acetone.

gum benzoin, elemi, ester gum, gum mastic, kauri, 'rosinz or sandarak in phenol, normal and sobutyl alcoholfn-l butyl glycol, cyclohexanone. ethyl ether, ethylglycol. methyl alcohol, methyl glycol, methylene chloride, and* An example of'a crudel sugar in a solvent is fructose in alcohol or acetone, while normal and isopropyl alcohol.

various glucosides, suchas absenthin, aesculin, antiarin,

cerberin, convallamarin, coriantyrtin, curangin, digitalim. v

digitonin, digitoxin, glucogallin, glycovanillin, glycyphyl- "viscous liquid "of the Because Ythese liquids have 7 and thevetin can be used in an alcohol or ether solution,

such as methyl, ethyl, normal and isopropyl alcohol, or dimethyl ether, diethyl ether, etc.

The invention can also be practiced by use of water as the viscous liquid when the reservoir oil and th: displacement lluid have a viscosity' less than the injected water. Frequently, however, the reservoir oil has a viscosity greater than water and in these cases as well as those cases in which water is used as the displacement lluid, it is within the scope ot this invention lo add various thickening agents to form a viscous aqueous solution for use as the viscous liquid. Suitable agents are crude sugars, sucrose, fructose, galactose, etc.; the afore- -mentioned glucosides; fatty acid soaps and metal alginatcs; as well as various water-soluble polymers such as carboxymethyl cellulose, polyvinyl and polyallyl alcohols, and partially hydrolyzed polyacrylamides.

Suitable partially hydrolyzed acrylamide polymers are water-soluble acrylamide polymers which have been hy- -drolyzed to such an extent that between about 0.8 and l0 percent of the amide groups have been converted to carboxyl groups. As herein employed, the term "acrylamide polymer is inclusive ot' the homopolymers of acrylamide, i.e., polyacrylamide, and water-soluble copolymers. of acrylamide with up to about l5. percent by .weight of other polymerizable vinyl compounds; such as the alkyl esters of acrylic andmethacrylic acids, methacrylamide, styrene, vinyl acetate, acrylonitrile, methacrylonitrile, vinyl alkyl ethers, vinyl chloride, vinylidene chloride. etc.. In addition to the aforementioned limitation on the extent of hydrolysis, thc suitable acrylamide polymers are of sulliciently` high molecular weight that a 0.5 percent by weight. aqueous solution thereof has a-.viscosity-of atleast 4 centipoiscsOstwald at 21.5 C. A commercially available-polymer meeting these-requirements is marketed by'The Dow Chemical Company under the name Scparan." The abovementioned polymers are preferred for an aqueous viscous liquid, since they form stable solutions and do not easily v precipitate. when heated. or when in the presence of i mineral anions and cations; however, .the use of any other [n] :KMa where K and aare constants for a given series of polymers; M equals the polymer molecular weight; n is the intrinsic viscosity and is obtained by extrapolating to zero concentration a plot of:

versus. concentration, where C=concentration; n=viscosity of solution; fr0-:viscosity of solvent.

The values of K and a are reported in available literature on polymers, e.g., Synthetic Rubber, by G. S. Whitby,

Any long-chain oil-soluble polymer exhibiting a high solution viscosity is appropriate 'for use in accordance with the invention. Examples of suitable polymers are:

j polymers' of butadiene, isobutylene, chloroprene, co-

U polymers of butadiene and styrene, methyl-mcthacrylate, or acrylonitrile, copolymers of isobutylene and isoprenc, natural rubber, and polyvinyl chloride plasticized with tricresyl phosphate. The above formulas show that the iireaseimviscosity of a polymer solution is a direct function of the polymer molecular weight. Expressed in another form, the formulas show that the greater thc polymer molecular weight, the lower the concentration of the polymer needed to achieve any given solution viscosity. The maximum molecular weight of polymers useful in this invention is limited only by-the solubility of the polymer in the miscibledisplacement lluid. Although the lower molecular weight polymers-are more soluble, their use is disadvantageous since relatively large quantities of the polymer must be added to achieve a satisfactory viscosity increase. The preferred polymers are those which elfcct a substantial increase in the viscosity of the solution at concentration less than about l percent by weight, and which have molecular weights greater than about 10,000 and preferably between about 500,000 and 1,500,000. Suicient polymer is added vto the hydrocarbon liquid to cause more than a l() percent increase in viscosity. The amount of polymer added varies for each polymer but generally between 0.0l and l0 percent by weight, and preferably from 0.05 to 5 percent b vweight. The hydrocarbon solvent can be any liquid in which the polymer can be dissolved and which preferably,` but not necessarily, is -miseible with the reservoir oil and the displacement fluid. Examples of solvents include petroleum distillatcs, crude oil, petroleum ether, paratlin hydrocarbons such as propane, butanc. pentane, hexane, heptane, LPG. etc.; amm-aries. such as xylcne toluene, benzene, coal tar oils` cyclohexane, or mixtures thereof. Preferred fluids are LPG,

' propane, butano, isobutane, pentane. isopentane. hexane,

isohcxane. gasoline and kerosene. The optimum combination of polymer and solvent will` ofcourse, be dictated by reservoir characteristics, economic considerations and extent of elongation of the displacing uid front.

The invention has particular advantage when applied to a miscible uid displacement process employing a miscible slug comprising a hydrocarbon-polymer solution. ln this process, a long chain oil-soluble polymer, for example. polyisobutylene. is dissolved in a low molecular weight hydrocarbon solvent such as pentanc, and

a suflicient amount of the solution is injected into the reservoir to insure separation between the reservoir Oil and a subsequently injected gaseous drive uid. Ordinarily, the use of low molecular weight hydrocarbons in this manner-.is somewhat unsatisfactory from the standpoint of cost. since the suitable hydrocarbons are either relatively expensive per se or must be used with a.

relatively expensive sccondarv solvent in order to dissolve an effective amount of the polymer.

The process of the present invention. however. permits the use of inexpensive lower members of the aliphatic hydrocarbon series` eg., LPG. propane. or butano, as the miscible hydrocarbon liquid without the need of an expensive secondary solvent to dissolve the polymer. This is accomplished in accordance with the following procedue: A suicient quantity of LPG is iniected into the reservoir to insure separation of the reservoir oil and a subsequently injected. gaseous drive uid, e.g. methane. This reouires from about 3 to l5 volume percent of the reservoir pore volume. and is dependent upon the mobility ratio between the LPG and reservoir oil, being about 3 percent for a mobility ratio of one or less and about l0 percent for a mobility ratio of about 50.

. 7- dncing well itwill have been enriched with the reservoir oil. Although the LPG slug originally injected into the reservoir had very little solubility for a viscous long chain polymer, the slug as it reaches the producing well, enriched with oil, has a high solubility for such a polymer.

A concentrated solution of polyisobutylene in a solvent such as pentane, benzene, or crude oil can, therefore, bc injected into this well before it is closed and the polymer will dissolve in the -slug to provide a vcry viscous region adjacent the tingcr. A third weil is then opened to continue the advancement of the slug in thc same general direction, and after the slug has completely swept past the closed second well, this well can be switched to injection of methane, and the first well can be closed. To insure that the polymer solution is injected from the second well directly into the finger, the trird well can be opened to production and the second well temporarily closed prior to polymer injection so as to draw the nger beneath the second well. Thereafter, the second well is opened and the polymer solution injected directly into the nger. Because of the local high viscosity of the slug at its most advanced or fingered position, the fingering tendency is effectively corrected and the miscible uid advances towards the third well more evenly. This same procedure can be followed along a continuous row of wells in a field, i.e., a rst well or row of wells can be employed for injection, and a second, third, fourth, etc., row of wells are then employed in the following sequence of steps: (l) withdrawal; (2) polymer solution injection; (3) closed in; and (4) gas drive injection; until the miscible slug phase has swept through the entire reservoir.

The invention will be further described by the following examples. which are for illustration only and are not to be considered as limiting the invention:

EXAMPLE I -A consolidated sandstone plate, with a porosity of about 30 percent, one-eighth inch thick, 2l inches Wide and 7 inches long, was sealed between two plates of Lucire. Two rows of 7 holes each were drilled into one of the Lucite plates, equally spaced along the length of the plate adjacent each of the longer edges. A third row of similarly spaced holes was drilled along the mid-line of the plate to divide the plate into twelve squares, three and one-half by three and one-half inches. These squares are shown by the faint grid lines of FIG-

URES

2, 3, 4.and 5. which also show the numbering of the holes from left to right, from the upper row tothe lower row. -Tubing connections were inserted into the holes to permit passage or" lluids through the sandstone model. The latter was then saturated with clear keroserie having a viscosity of 1.76 centipoises at room temperature (75 E). The modei was held in a horizontal plane above a light source to permit observance of the ow pattern during the run. A run typical of conventional practice was commenced by injecting a red displacing iluid, consisting principally of hexane and having a viscosity of 0.32 ccntipoises at roorn temperature, into well 18 whileY withdrawing clear kerosene from well 19. This injection was continued until a red displacing tluid nger broke through to well 19, whereupon wells 18 and 19' were closed and the iiow pattern was traced onto a sheet of clear plastic placed over the model. This tracing is shown in FIGURE 2, as fiuid front F-1. The run was continued by repeating the injection of the displacing tluid into well 18 (well 19 being closed) and withdrawing from well 20 until the displacing duid broke through to well 20. The ow pattern was traced and is shown in FIGURE 2 by iluid front F-2. This procedute was repeated, employing wells 21 and. 22 successively as withdrawal wells and continuing injection into well 18. The fluid fronts at breakthrough to

wells

21 and 22 are shown by F-S and F-4, respectively. During the run the displaced kerosene volume was measlCmivcntional lisplaceuient-volunie kerosene displaced at breakthrough 1 Well: Total, milliliters 19 2.6 20 7.6 21 13.2A

EXAMPLE n The sandstone model was tlushcd clean and again saturated with clear kerosene, and the procedure of Example l was repeated. again employing red hexane as a displacing fluid. Viscosities of the kerosene and hexane were the same as in Example'l. The displacing iluid front at breakthrough to well 19 is shown as F-S and at break through to well 20 as F-6 in FIGURE 3. At this point the invention was applied by the injection of a mixture of oil andikerosene having a viscosity of 5 centipoises into well 20 with

wells

13 and 27 open to withdraw the displaced kerosene. A blue dye was added to this viscous liquid to identify it during the test.l The injected slug of viscous liquid after injection is shown by the shaded area surrounding wcll 20. During injection of this viscous slug, no noticeable shifting of tluid front, F-6, occurred. After two milliliters of the viscous uid had been injected into well 20,

wells

13, 20 and 27 were closed, well 18 was reopened to injection of the hexane displacing fluid, and well 21 was opened to withdrawal of kerosene. After breakthrough to well 21 of the viscous liquid slug, wells 18 and 21 were closed and the tlow patterns were traced to obtain FIGURE 4 which shows the displacing iuid front as F-7 and the viscous liquid slug as the shaded area. Thereafter, injection into well 18 was repeated, with well l22 opened for withdrawal of kerosene.` The displacing uid and viscous liquid slug both lingered into well 22, at which point wells 1S and 22 were closed, and the how pattern was traced to obtain FIGURE-5. The uid front of the displacing uid is shown as F-S and the viscous liquid slug as the shaded area in' FIGURE 5. The displacing tluid and viscous liquid mixed along their interface, this being indicated in FIGURE 5 by the less dense shading of the trailing portion of the viscous slug. As in Example I, the volume of displaced kerosene was measured and recorded at breakthrough to each well. This data appearsAv in the following table:

Table 2 [Typical of the invention-volume kerosene displaced at breakthrough] Well: l Total, milliliters 19 2.4

*Does no t include the two milliliters withdrawn fromx wells 13 nnu 2T during injection of the viscous liquid slug.

A visual'comparison of FIGURES 2 and 5 shows that the total area swept by the displacing duid and viscous slug in FIGURE 5 is much greater than obtained without use of viscous liquid injection as shown in FIG- URE 2. In FIGURE 2 it can'be seen that the tiuid front rapidly narrowed into a'tinger extending

past wells

20 and 21, to well 22. A comparison or' the amount of kerosene displaced shows that even when disregarding the two milliliters displaced by injection of the viscous liquid slug, the total recovery of Example II is 28.9

percent greater than that of Example I. The actual recovery, including the two milliliters produced through

wells

13 and 27, is 40 percent greater.

The invention has been illustrated in the foregoing examples under ideal reservoir conditions, i.e., with a reservoir having a uniform permeability throughout, and

with a fluid mobility ratio of displacing hexane to kerosene of 1.76/0.32, or 5.5, a value usually exceeded under actual reservoir conditions. 'ihe effect of these conditionshas been to minimize fingering in the model so as to obtain tlow patterns which may be readily evaluated. Under actual reservoir conditions, the displacing uid ngering will be more pronounced and, therefore, the irnprovement of the sweep eciency of the displacing fluid when the viscous liquid slug of the invention fis employed will exceed that observed in the laboratory studies.

Although the invention has been describedwith. .regard to the horizontal sweep eciency ofthe displace ment, in actual use it. can also be applied to obtain a satisfactory vertical sweep efficiency. In this use, the vertical location of the linger is located by any suitable method, such as by sealing off various heights of the producing borehole until the region producing the displacement fluid is located. This zone is then sealed oi with packers and the viscous liquid slug is injected into this zone, thereby insuring that the greatest mass of viscous liquid will be lconcentrated at the exact spacial location of the nger.

The invention has. been described by reference to a displacement process between a single injection well` and successive withdrawal wells. This was to illustrate the invention in its simplest form; in field use, a plurality of wells can be 'employed for injection and/or withdrawal. A type dow pattern which can be expected when the invention is applied to one of .the conventional drive techniques, i.e., a line drive, is shown in FIGURE 6.

y f FIGURE. 6 illustrates a typical dow lpattern in which f la displacing uid which has been injected :into the reservoirv through a-rst row lof injection wells, 35, has the intermediate front shown by the dashed lines at. .breakthrough' into a -second parallel row-of withdrawal wells, 49. When the lingering of the duid is pronounced as shown by FIGURE 6, slugs of viscous liquid are injected into each producing well. During this injection of viscous slugs, the displaced reservoir oil is lwithdrawn from the nexty succeeding parallel row of wellsorxrom. adjacent-wellsl in the same row as the -well used to inject the viscous slug. Alternatively, when itis desired to repressurize the reservoir, no withdrawal of reservoir oil is made. When the wells are closely spaced and the injected liquid is not. extremely viscous, the shape of the injected viscous slug can readily be controlled byfproper choice Lof withdrawal vwells during injection. This is l illustrated in FIGURE 6 bythe .various shaped areas surrounding wells 41, -42and 43. The slug injected into well 4l illustrates a zone having a circularhorizontal cross section, typical of vinjection into a reservoir when withdrawal is from a well or wells spaced a substantial distance .from that employed vto inject the viscous slug and/or when an extremely viscous liquid is injected into a. reservoir, or whenY no withdrawal of reservoir oil .is made. The elliptical shaped slug at well 42 represents injection of a relatively low viscosity. liquid into the reservoir while the displaced vreservoir oil is withdrawn from adjacent wells 41 and 43. This method is preferred since it distributes the greatest portion of the slug transverse to the direction of lingering. Well 43 is shown with a slug having a circular horizontal cross section obtained by viscous liquid injection without any withdrawal of reservoir oil. The amount of slug injected into each well is directly Vproportional to the extent of lingering occurring at each well site as determined, for instance, by

the relative production from each well. It is, of course;

apparent that in the extremely rare instance where no ingerhas formed adjacent a withdrawal well, no viscous slug need be injected.

After the viscous slugs have been injected into the wells of row 40, these wells are closedand reservoir oil is withdrawn from the wells in row 50. After the displacing uid or viscous liquid reaches the wells in this row, they are closed to'production and reservoir oil is withdrawn from the next succeeding row 60, or if the extent of fingering is again pronounced, additional slugs of viscous liquid are injected through the wells in row 50. FIGURE 6 shows the iluid front as it reaches the withdrawal wells in row and it-can be seen that substantially all the area between

rows

35, 40 and 50 has been swept by the fluid.

In another modification of the line drive method of the invention, vnot all the wells of a single row are employed for injection of the viscous slug. For instance, every second or third well in row 40 can be employed for slug injection, such as injection through wells 4l and 43. In the succeeding parallel row of wells 50, viscous slugs are injected through wells opposite those wells of the preceding row which were not employed for vis'- cous slug injection, in this case, well 52. This modification can suitably be used when it is desired to elongate the slug in a direction at right angles to the fluid ow direction, since wells adjacent the well employed for slug injection are available for withdrawal of displaced oil.

From the foregoing illustrations, it is obvious that the number and location of injection and/ or withdrawal wells can be varied so as to obtain substantially any desired ow pattern or sweep in a secondary recovery process.

Having clearly, completely and concisely described my invention, l therefore claim:

1. The method of recovering oil from a subterranean reservoir pierced by at least three wells consisting of an input well, a second well spaced a substantial distance therefrom and a third well located between said input and second wells which comprises: injecting a displacement huid into said input well; withdrawing said oil from said reservoir through said third well until the presence of said displacement lluid is detected in the eluent from said third well; thereafter, ceasing said withdrawal of said oil from said third well and injecting a slug of viscous liquid into said reservoir through said third well, said viscous liquid having a viscosity at reservoir conditions greater than the viscosity of the most viscous of said first displacement fluid and said oil, the viscosity and amount of said viscous liquid being suicent to substantially retard the advance of said displacement fluid in the vicinity of said third well bore; thereafter shutting in said third well and withdrawing said oil from said second well while continuing to inject said displacement fluid into said input well.

2. The method of claim l wherein said displacement fluid is an aqueous liquid.

3. The method of claim 2 wherein said viscous liquid is an aqueous solution of a water-soluble thickening ,agent selected from the group consisting of: crude sugars, glucosides, fatty acid soaps, alginates, and water-soluble polymers.

4. The method of claim 2 wherein said viscous liquid comprises a liquid which is miscible with said reservoir oil and with water and contains a thickening agent selected from the group consisting of crude sugars, gums, resins and glucosides.

5. The method of claim 2 :1s-applied to a miscible displacement process wherein said displacement uid com`-` 8. The method of claim o wherein said viscous liquid comprises a liquid which is miscible with said oil,

9. The method of claim 8 wherein said viscous liquid contains a thickening agent selected from the group consisting ot' long chain oil-soluble polymers, crude oil, crude oil distillates. and mixtures thereof'.y

10. The method of c1aim.9 as applied-to@ miscible displacement process wherein said displacementiluid comprises a slug of hydrocarbon which is liquid at reservoir conditions and which is first injected into said input .well and drive gas which is thereafter injected into said input well.

ll. The method of obtaining oil from a subterranean reservoir pierced by a plurality of wells substantially arranged to form at least first. second and third consecutive parallel rows oi weils which comprises: injecting a displacement lluid into said reservoir through wells in said iirst' row; withdrawing said oil from said reservoir throughwells in said second row; ceasing said withdrawal at each o said wells in said second row when said displacement uid is present in the effluent therefrom; in-

' jecting a viscous liquid into each of said wells of said second row from which withdrawal has ceased; said viscous liquid being more viscous than most viscous of said displacement tiuid and said oil. the viscosity and amount of said viscous liquid being suiticient to substantially retard the advance of said displacement uid in the vicinity of said third well bore; closing said wells in said second row, withdrawing oil from wells in said third row and continuing to inject said displacement fluid through 30 said wells in said first row.

l2. The method of claim 11'? wherein said-viscous liquid is injected into said reservoir throughv alternate wells in said second row, while withdrawal' of displaced uids from said reservoir is conducted from the wells adjacent said alternate wells in said second Arow during the injection of said slug of viscous liquid.

13. The method of claim 11 wherein withdrawal of said oil through wells in said'third row is initiated simultaneously with said injection of said slug of said viscous liquid into said reservoir through said wellsin said second row. i

14. The method of claim 11 wherein consecutive parallei rows of wells are successively employed in the sequence of: first, withdrawal of displaced oil; second, injection of viscous liquid; third, closed.

References Cited in the le of this patent UNITED STATES PATENTS 2,771,138 Beeson Nov. 20, 1956 2,798,556 Binder et al. July'9, 1957 2,827,964 Sandiford et a1v Mar. 25, 1958 2,842,492 Engelhardt et al. July 8, 1958 2,920,041 Meadors Jan. 5, 1960 2,924,276 Heilman et al. Feb. 9, v1960 OTHER REFERENCES Fagin, K. M.: Effect of Well Spacing on Water Flood Operations and Economics, The Petroleum Engineer, June, 1946. pages 64-72.

Claims

1. THE METHOD OF RECOVERING OIL FROM A SUBTERRANEAN RESERVOIR PIERCED BY AT LEAST THREE WELLS CONSISTING OF AN INPUT WELL, A SECOND WELL SPACED A SUBSTANTIAL DISTANCE THEREFROM AND A THIRD WELL LOCATED BETWEEN SAID INPUT AND SECOND WELLS WHICH COMPRISES: INJECTING A DISPLACEMENT FLUID INTO SAID INPUT WELL; WITHDRAWING SAID OIL FROM SAID RESERVOIR THROUGH SAID THIRD WELL UNTIL THE PRESENCE OF SAID DISPLACEMENT FLUID IS DETECTED IN THE EFFLUENT FROM SAID THIRD WELL; THEREAFTER, CEASING SAID WITHDRAWAL OF SAID OIL FROM SAID THIRD WELL AND INJECTING A SLUG OF VISCOUS LIQUID INTO SAID RESERVOIR THROUGH SAID THIRD WELL, SAID VISCOUS LIQUID HAVING A VISCOSITY AT RESERVOIR CONDITIONS GREATER THAN THE VISCOSITY OF THE MOST VISCOUS OF SAID FIRST DISPLACEMENT FLUID AND SAID OIL, THE VISCOSITY AND AMOUNT OF SAID VISCOUS LIQUID BEING SUFFICIENT TO SUBSTANTIALLY RETARD THE ADVANCE OF SAID DISPLACEMENT FLUID IN THE VICINITY OF SAID THIRD WELL BORE; THEREAFTER SHUTTING IN SAID THIRD WELL AND WITHDRAWING SAID OIL FROM SAID SECOND WELL WHILE CONTINUING TO INJECT SAID DISPLACEMENT FLUID INTO SAID INPUT WELL.