US2402588A - Method of oil recovery - Google Patents

Description

lime 25, 1946 n K H. ANnRl-:sz-:N A 2,402,538

vMETHD 0F OIL RECOVERY Fuga sept. 14, 1940 NVENTR ,Kuri Hnruresen BY* A Patented `)'une 25, 1946 Kurt H. Andresen, Bradford, Pa.. 355181101', by

mesne assignments. to Essex Royalty Corporation, New York, N. Y., a corporation of Dela- Application September 14,- 1940, Serial No. 356,736

Claims.

l This invention relates to a method of recovering oil from subterranean oil-bearing sand strata of different permeabiiities'by injecting a fluid under pressure into an input well tc force oil Vthrough the strata into an output well from which oil is recovered, such oil producing being 'commonly known as production by secondary recovery methods, and more particularly the invention relates to a method of increasing the eiiiciency of the producing operations by -decreasing the fluid conductivity of more permeable non-oll-producing strata adjacent to less permeable oil-producing strata.

ne method employed to produce oil from oilbearlng sands is to force a fluid, such as water or a gas, into an input weil and through the oilbearing strata to force oil to flow through the strata to an output or producing well from which oil or a mixture of oil and uld is taken. In accordance with this process a series of input wells approximately equidistant from a producing wel] are'drilled and exposed to the same oil-bearing strata. Fluid is forced down the inputwells and out into the oil-bearing strata under sufficient pressure to drive the oil through the strata into the output or producing well.

In carrying out this method of oil. recovery serious diiiicultles have been encountered when a stratum or strata permit now of driving duid to the output well without forcing a suilicient amount oi oil therefrom into the output well. Oil-bearing strata having different permeabilities and so different degrees of uid conductivity may lie closely adjacent one another and be exposed to the same input weil opening. Under these circumstances the driving fluid will drive the recoverable oil from the more permeable strata before the recoverable oil is driven from adjacent less permeable strata leaving substantial quantitles of recoverable oil in the less permeable strata. As more permeable strata are emptied of recoverable oli they become more permeable to the less viscous fluid of the fluid drive and act to short-circuit the driving fluid operation. Thus, it becomes necessary to pump in more and more pressure fluid for each barrel of oil produced at the output well and a condition may be reached where the cost of pumping and handling the quantity of driving fluid necessary to produce a barrel oi oil is greater than the value of the oil thus recovered.

Also` since the rate of oil production under fluid drive is a function of the conductivity of the sands and the pressure drop across them, it requires a higher pressure to maintain desired rate 4 of production from the tighter or less permeable olproducing sands. But if after the more permeable strata are depleted the pressure of the driving fluid is increased to increase the rate of production from the tighter sands the ilow oi driving fluid through the loose sand becomes so great as to render the production unprofltable.

The process of the present invention provides in general that when the ratio of driving fluid to oil produced at the output well becomes unduly high due to depletion of more permeable strata. adjacent less'permeable recoverable oil-bearing strata, the depleted more permeable strata are so selectively sealed or their conductivity is so reduced that when injection of driving fluid is resumed a greater proportion of the driving uid will now through the non-depleted oil-producing strata. Further, the seal provided is such that the pressure under which the pressure fluid is injected into the wells may be appreclably increased without increasing the driving fluid oil ratio. Thus, the rate of oii production from remaining tighter (less permeable) oil-producing sands is increased. l

The problems involved in selectively sealing sand strata located hundreds of feet below the surface of the ground are diiilcult. Methods of selective sealing utilizing the relative positions of the adjacent depleted and non-depleted strata and their respective depths below the ground surface are relatively unsatisfactory where the wells are deep and possibilities of error in the depth measurements are increased. Yet the selectivity of the sealing is necessary to prevent sealing of producing strata. Also, to oiIset the necessity for successive treatments the dam formed by the sealing process must be permanent and able to withstand high pressures when the uid drive is carried out at such high pressures.

Another characteristic of the well that makes selective sealing of the strata diilicult is that the walls of the well exposed to the producing strata are "shot" by setting oli a high explosive which ruptures, breaks. and produces fissures in the sand extending undetermlnable distances in from the well hole proper and also increasing the effective diameter of parts of the well hole undeterminable amounts.

In the present embodiment o! the invention these problems are solved by injecting into the input weil a solution of a precipitatable material which is caused to flow preferentially into the pores of the more permeable strata and by a subsequent and controlled chemical action the preclpitatable material is precipitated from the solution in the pores to decrease the conductivity of the strata into which the solution is iiowed. Ihe precipitation is accomplished by injecting into the pores of the solution-containing depleted stratum under controlled conditions a chemically active material capable of yielding a reagent or ion which reacts to precipitate the precipitatable material from the solution in the pores of the depleted stratum. The chemically active material and precipitatable material and 'their concentrations are so chosen that the precipitate is formed at a relatively slow rate.

More speciilcalhf, an alkaline sodium silicate solution is first flowed into the input well in such manner that it selectively enters the pores of the depleted more permeable strata and the alkalinity of the solution is then gradually reduced to cause a water insoluble gelatinous precipitate containing SiO: to form slowly in the pores. In the present embodiment the alkalinity of the so` lution is reduced by a reagent that is in effect a weak acid and so only` slowly yields hydrogen ions to bring about the precipitation, Carbonio acid is satisfactory and the process is carried out by flowing carbon dioxide gas into the weil in such manner that the gas selectively enters the v' pores which have already been penetrated by the solution. The carbon dioxide dissolves in the solution to form carbonio acid and to reduce the alkalinity of the solution. The dissolving action is increased by the alkalinity of the solution, by the pressure under which it is introduced and by the very large exposed surface of the solution in the pores exposed to the well hole,

Sometime after the carbon dioxide has dissolved the precipitate starts forming in the form oi' a gel having a viscosity far greater than that of waterso great that even when subjected to a high pressure drop across the gel in the pores it does not move or migrate through the pores because of its high viscosity. The eil'ect of placing in the pores a gel which, although having a small tensile strength, has a high viscosity. ls shown by the equation of Darcys law, which is as follows:

KA (P, P2) Q uL where Q=iow in unit volume per unit of time =Darcys constant (permeability) A=unit area Pi-Pz=pressure drop along unit distance u=coeiiclent of viscosity L=1ength Whereas the relative viscosity of water in centipolses may be considered as unity. the viscosity of the gel in centipoises would be in the order of millions. Thus, since the flow through the sand varies directly with the pressure drop across it and inversely with the coeillcient of viscosity, by making the coeilicient of viscosity very large the tlow through may be made negligible. Thus, the gel acts as a dam and is as effective as though it were rigid.

With regard to the question of the concentration of the alkaline sodium silicate solution that is introduced, the following factors, among others, require consideration. The more concentrated the solution the more viscous the gel subsequently formed from it, and therefore a concentration is picked suiliciently high to obtain a suiiiciently viscous gel. But the concentration of the solution is kept low enough that the viscosity of the solution as it is introduced is not so high as to require too high a pressure to force it luto the pores of the more permeable strata. In this connection, however, it is noted that the viscosity of the solution being higher than that of water has a greater selectivity to enter the more permeable pores of the strata under a given pressure than would a less viscous liquid because the higher vthe viscosity of a liquid the more sensitive it is tov differences in permeability.

But since it is desirable to have the gel form in the pores to some depth from the exposed surface of the sand the concentration of the solution is also kept suillciently low that the formation of the gel by the carbon dioxide treatment is retarded, If the concentration is increased beyond a certain point the gel may so form near or at the surface at which the carbon dioxide is introduced as to prevent penetration of the carbon dioxide into the solution in the pores further removed trom the exposed surface of the sand.

For the same reason the medium or chemical used to bring Yabout the gelling action is selected to cause the gelllng to take place slowly. Carbon dioxide has such a favorable action because in dissolving in the solution the acid radical that it yields can develop only to a limited concentration. Thus, since the carbon dioxide gas is readily soluble ln the solution, and since the acid that it yields is relatively weak, it permits the gelling reaction to take place at the desired slow rate.

One solution which gives satisfactory results has approximately the following composition:

Per cent NazO .39 SiO: 1.25 H2O 98.36

Figure 2 shows an arrangement of apparatus for supplying the chemically activegas to the oil-bearing strata.

Referring to the drawing, and particularly to Figure l, the numeral i indicates a Water input Well whichextends from the surface 5 of the ground to oil-bearing strata 2, 3 and l. As suggested above, a number of these input wells are usually drilled around a producing or output well at spaced intervals approximately equidistant from the output well. Each of the input wells l is provided with a. pipe line 6 extending from the surface of the ground through a packer l to the oil-bearing strata. Driving iiuid such as water is pumped into the well and is caused to now out only into the strata 2, 3 and l by the presence of the packer l which may be of any desired type and which prevents the water from passing ul wardly through the input well.

The water from a suitable source (not shown) is injected into the well through a branch line 8 and flows through the pipe E into the strata 2. 3 and 4, forcing oil therein towards the producing well from which the entering oil and water is taken either by pumping or by flow under the impetus of the fluid drive. The rate of flow ol water is measured by a water meter 9 and isregulated by valve Il.

Stratum 1 is represented in the drawing as being the more permeable. and hence as the water flooding operation proceeds this stratum will be depleted of recevable oil beiore the strata I and l are depleted. Since the viscosity of water is less than that of oil, the rate of iiow of water through the stratum 2 increases as the stratum is depleted of recoverable loil and the ratio ot driving fluid to oil produced may increase to an imeconomical point. e

.In accordance withv one embodiment oi the method of the present inventionl when stratum 2 is thus depleted, as indicated by a substantial increase in the wateroil ratio of the iluid produced at the producing well, the well is treated in the-following manner. The supply o! water is shut ofi. A tank I3 containing a measured quantity of sodium silicate solution is connected to line 'l and the solution is caused to dow into the well. Tank I l may be provided as shown with a vent valve Il, pressure gage I5, air supply line it, and air control valve i1.

The solution normally ilows under hydrostatic pressure down through the pipe 6 and the hydrostatic pressure may be lowerl than the pressure oi' the water drive under which the well had been operating. Thus, the solution forced down has a selective tendency to enter only the more Apermeable strata under Vthe reduced 4pressure at which it is introduced. As above pointed out, this selectivity is further enhancedv by the fact that the viscosity of the solution is greater than that of water.

Whereas in most cases thehydrostatic pressure of the fluid provides suilicient head to force the solution into the more permeable strata, w'hensuch hydrostatic pressure is not suillcient it may be aided by supplying air under pressure to the tank through the line i8 and controlling such air flow to obtain the desired rate oi' flow of the solution into the well to maintain the selectivity as above described.

I'he quantity of solution used is sumcient to fill the pores ci the stratum to a desired distance from the well hole to obtain the desired piugging. One of the factors influencing the quantity of solution used is the thickness of the Astrata to be sealed. The thickness of the strata may have been at least roughly determined by core analyses made during the drilling of the well or of a well in the area of the well being treated.

After the solution has been introduced the pipe t is cleared of solution by forcing air therethrough and the level of solution is forced down to Just below the more permeable strata. which in the present disclosure is stratum 2. Pipe 6 is disconnected from branch line l at. for example, union IB` As shown in Figure 2, a pipe i9 of smaller diameter than the pipe i and provided with a packer 2li is inserted within the pipe 6. Carbon dioxide gas is then forced through the pipe i9 into the stratum 2 to precipitate the precipitatable material of the solution. The advantages of using the inner, smaller pipe are twofold. since the volume of the inner pipe is small there will be relatively little dilution by air of the chemically active gas as it passes through the pipe. Furthermore, the inner pipe prevents the gas from coming in contact with residual solution which adheres to the inner surface of the pipe 5 and so avoids the formation of quantitles of precipitate in the pipe 8 which might be later tlushed into the well to clog the pores of the faces of the strata l and l when water injection is resumed.

vout driving the solution ahead of it.

It has beenfound desirable to supply the gas to the input well at a relatively low rate. In one case where satisfactory results were obtained. gas was supplied at the rate of cubic feet per hour (based on atmospheric pressure) over a period of ten hours. Such a slow rate not only makes the gas flow more selective but also permits it to seep through and dissolve in th'e solution in the pores and to penetrate the pores with- Using such a dilute solution of sodium silicate forms a gel that has a Suiliciently high viscosity so that the dam formed is substantially permanent, and has an advantage that the time for gellng varies with the variation in the alkalinity (the pH) so long as the pH is above or below a narrow range. Thus, as the pH is reduced by the absorption of the CO: gas, the setting time for the gelling is correspondingly reduced. In the narrow range the change of pH causes practically no change in the setting time of the gel. Below this narrow range, as the pH is further reduced. the setting time correspondingly decreases.

With higher concentrations of sodium silicate the range above-mentioned is wider and in this range variation of the pH does not vary the setting time of the gel. Above this wide range there is a narrow range of pH in which a relatively small change in pH causes a relatively large change in setting time, and above this latter range variation in the pH has no eilect upon the setting time.

Another advantage of using such a dilute sodium silicate solution is that as the pH of the solution in the pores is reduced to bring about the gelling action relatively little change in the viscosity of the solution takes place during the early stages of the gelling action, after which the gellng action proceeds at a relativelyrapid rate. In more concentrated solutions, however. as the pH is reduced to bring about gelling action the viscosity of the solution starts increasing almost simultaneously with the reduction of the pH. In other words, with the concentrated solutions the viscosity is more nearly a linear function of time. Thus. by using such a dilute solution oi' sodium silicate and avoiding Increase in viscosity of the solution as the CO1 gas is flowed into the solution in the pores. the penetration, dissolving and ditluslon of the C01 throughout the solution in the pores is improved.

The solution treatment leaves a liquid body 2i of the solution in the pit of the well. To prevent the incoming gas i'rom forming a precipitate with the liquid body 2i, which precipitate might later be deposited on the faces of the non-depleted strata 3 and 4 or even ll the well hole with the precipitate and prevent ilow of flooding water thereinto. a strong alkali may be injected into the liquid body 2i to render the solution so alkaline that no precipitate will remain in it. The alkaline treatment may' be carried out by dropping sticks of sodium hydroxide into the pit containing the liquid body 2i.

After the gas treatment and caustic treatment, a period of time is allowed to permit the precipitation to complete itself and the precipitate to set before the fluid drive is resumed. A period of sixty hours was permitted to elapse in the present example alter gas treatment in order to make sure that the reaction was complete before resuming water injectionY The optimum rate of gas supply and the time required for completing the reaction will, of course. vary with the type of reagent used, the concentration of the solution used. and the type of formation to which the solution is supplied.

I'he use of a gaseous precipitating medium having inherently a much lower viscosity than a liquid medium insures penetration of the precipitating agent into the pores so that the precipitation is caused to take place throughout the solution.

As above noted the concentration of the treating solution is preferably such that the viscosity of the solution is kept relatively low so that it readily enters the permeable strata. under the relatively low pressure used. Also it is kept sudlciently low that the reaction rate between the solution and the'subsequently dissolved gas isrso slow thatit does not interfere with the absorption of the gas and the migration of the dissolved gas to the solution farthest from the well hole. This permits the subsequent precipitation to take place throughout the solution in the pores. Yet the concentration is kept sutilciently high to form within the pores a gel that has a high enough viscosity to provide a permanent seal, i. e. one which will not break under a high pressure fluid drive if such is used.

Other materials than sodium silicate that may be used in carrying out the method of the present invention are, for example, solutions of aluminates and titanium salts. The kind of gas used will depend upon the nature of the solute used in the impregnating solution. If the solute used depends for its solubility upon the alkalinity of the solution, as sodium silicate does, then an acid forming gas may be used. such as carbon dioxide illustratively mentioned above, or one of the oxides of sulphur, or the hydrogen halides, H28. or the oxides of nitrogen. Where the maintenance of the iprecipitatable material in solution depends upon factors other than alkalinity, other appropriate precipitant gases would be used.

Referring now to another embodiment of the method of the invention by which the use of the inner pipe i9 for theintroductlon of the gas is eliminated, the solution is introduced in several stages and the gas treatment is likewise made in several stages. Thus, after a part of the solution has been introduced through p/lpe line 6, it is immediately followed by a CO2 treatment, which forces solution remaining in line 6 into the pores nd then itself enters the solution in the pores.

en another portion of the solution is flowed In. `forcing ahead of it the solution previously introduced and gas treated. Then follows a second gas treatment. These treatments are repeated as desired and follow one another at sutliciently short intervals that the gel does not form until after the last treatment. After the last gas treatment, the solution with the dissolved gas is allowed to stand as above described until the seal is complete.

The following example is given of the manner in which the above procedure may be carried out on a water intake well where twenty-three barrels of the solution were estimated as. the correct quantity: The supply of flooding water to the intake well is turned olf. The tank i3 is then connected up to the line 8 and fourteen barrels of the solution are allowed to iiow in under hydrostatic pressure. This first batch of solution sweeps the flooding water from the line 6 and flushes it out of the pores of the oil depleted permeable strata and nils the pores around the well hole full of undiluted solution. With this first step that part of the solution diluted by the 8 flooding water in the well at the start of the treatment may be ineffective but the amount of solution used for the rst step is large enough to leave undiluted solution in the well and pores immediately adjacent the well hole.

Two hundred pounds of CO2 may now be introduced at a slow rate, forcing the solution left in the line E into the pores and subsequently itself dissolving in the solution in the pores. The remaining nine of the twenty-three barrels of solution are now introduced in two treatments of 4.5 barrels each, and each followed by 200 pounds of CO2 as described. After the last CO2 treatment, sodium hydroxide sticks are dropped into the well hole to render the solution in the well hole so alkaline as to prevent any subsequent precipitation in the well hole during the time allowed for the prf :ipitation to take place in the pores.

An advantage of this embodiment is that the solution and gas are mixed in sections and the gas does not have to diffuse through the s0- lution to reach the solution farthest from the well hole.

The method of the invention may also be used to selectively seal the more permeable non-oilproducing strata of sands from which oil has been produced by means of a gas (air) drive. But in instances where the more permeable strata of gas intake wells are sealed the procedure is modified so that when the gas drive is resumed there is no chance for any of the solution to enter the remaining oil-producing strata. This precaution is taken because water in the presence of the gas drive acts to seal or plug the sand.

l'n practising the invention on a gas intake well, therefore, in accordance with one embodiment of the invention, after the gas input has been shut off and before the sodium silicate solution is injected, an organic liquid is introduced into the well approximately to the level of the loose or oil depleted sand. This liquid preferably has a specific gravity higher than that of the aqueous silicate solution, is immiscible with water and has a vapor pressure suiliciently high at well temperature so that evaporation can take place. One liquid satisfactory for performing this function is carbon tetrachloride or a mixture of carbon tetrachloride and gasoline.

After this liquid is injected 4into the Well the sodium silicate-carbon dioxide treatment is carried out as above described. After the reaction period has passed and the seal formation ls com- Dleted the excess liquid in the well is driven ofi. This excess consists predominantly of the protecting liquid initially introduced.

The protecting liquid having a specific gravity heavier than that oi' the solution serves to prevent the sodium silicate solution from penetrating the well below the level of the loose sand which is intended to be plugged and the immisclbllity of the solution with the protecting liquid also serves this purpose.

The use of a protecting liquid having a high vapor pressure permits evaporation of any liquid remaining after the well is pumped so that after the Kas repressurlng is resumed, any liquid remaining would cause no permanent plugging as, lor example, water might.

When injection of water. to the inlet well is resumed, the water-oil ratio at the producing well is substantially reduced and oil production by flooding is again economically feasible. Also the pressure under which the fluid drive is opersteps of flowing 'solution in place steps o! flowing into ated may be increased to increase the production rate if the remaining strata are much tighter than the plugged strata. -Thus the method o! the present invention makes available oil present in the less permeable strata which would not otherwise be practically available. High selectivity and a deep penetration ot the precipitate is obtained by using a gaseous precipitant and a relatively low impregnation pressure.

i. The method of selectively sealing more permeable undesirable fluid-passing porous sand strata exposed to an input well o! an articial iiuid drive oil producing system without sealing desirable fluid-passing less permeable strata exposed to the well, comprising the steps o! stopping the fluid drive into said input well. iniectq ing into said input well a dilute alkaline solution of sodium silicate under such reduced pressure that the solution preferentially flows into the pores of the more permeable undesirable iiuidpassing strata and not into the pores of the less permeable desirable fiui strata. subsequently injecting a gas into said input well at such rate and pressure that it selectively enters the solution-filled pores and dissolves in the solution to reduce the alkalinity o! the solution and slowly cause the solution to gel in lowing time for the precipitating place. and making the concentration ot the solution such that it contains approximately 1.25% SiO: by weight.

2. The method of selectively sealing oi! the now oi uuid through undesirable fluid-passing porous rigid sand formationsexposed to an oil well without plussinsr desirable porous oil-producing strata exposed to the well. comprising the into the well a relatively dilute solution oi sodium silicate (approximately 1.25% SiO: by weight) under conditions of such slow controlled iiow that the solution enters the more permeable strata to be plugged to the exclusion lthe less permeable strata, owing carbon dide gas into said well under such slow now that it selectively enters the pores o! the more permeable strata illled with the solution to reduce the pH thereof, and maintaining said treated in said pores until the solution iorms in the pores a uld silicic acid gel o! suiiicient viscosity and depth to act as a permanent barrier to the ow. o! duid through said pores.

3. The method ol selectively sealing oi! the how of nuid through undesirable duid-passing porous rigid sand formations exposed to an oil well without plugging desirable porous oil-pro-r ducing strata exposed to the well. comprising the the well a relatively dilute solution of sodium silicate (approximately 1.25% BiO: by weight) under conditions ot such slow controlled new that the solutionententhemoro wr permeable strata to be plugged to the exclusion o! the less permeable strata, iiowing carbon di. oxide gas into said well under such slowr now that it selectively enters the pores o! the more per` meable strata nlled with the solution to reduce the pH thereof. repeating said solution and gastreating operations successively until the desired amount of solution has been iiowed into the pores. and maintaining said treated solution in place in said pores until the solution forms in the pores a iluid silicio acid gel of sumcient viscosity and depth to act as a permanent barrier to the flow of duid through said pores.

4. The method of selectively sealing oi! the flow of fluid through undesirable iluid-passing porous rigid sand formations exposed to an oil well without plugging desirable porous oil-producing strata exposed to the weil, comprising the steps of flowing into the well a solution of sodium silicate of such dilute concentration that variation oi the pH of the solution through practically the entire range causes a variation in the setting time of the silicio acid gel under conditions of such slow controlled ilow that the solution enters the more permeable strata to be plugged to the exclusion of the less permeable strata. iiowing carbon dioxide gas into said well under such slow flow that it selectively enters the pores oi the more permeable strata lled with the Vsolution to reduce the pH thereof. and maintaining said treated solution in place in said pores until the solution forms in the pores a iiuid silicio acid gel oi sumcient viscosity and depth to act as a permanent barrier to the now of fluid through said pores.

5. The method of selectively sealing oil the flow of uuid through undesirable fluid-passing porous rigid sand formations exposed to an oil well without plug desirable porous oil-pro-n ducing strata expos to the well. comprising the steps o! llowing into the well a solution oi sodium silicate of such dilute concentration that variation of the pH of the solution through practicaily the entire range causes a variation in the setting timeoi the silicio acid gel under conditions oi such slow controlled iiow that the solution enters the more permeable strata to be Plugged to the exclusion o! the less permeable strata, ilowing carbon dioxide gas into said well under such slow ilow that it selectively enters the strata iilled with Y repeatins saidporco.

KURIER..`

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