ACID SOLVENTS FOR OIL WELLS Physicochemical Properties Adapted to Various Production Conditions
L. C. CHAMBERLAIN, JR., AND R. F. BOYER The Dow Chemical Company,
Midland, Mich.
3. Failure to obtain good penetration and failure of the well to purge itself after acidizing. 4. Failure of acid to penetrate oil film on rock. 5. Emulsions formed during and after acidizing. 6. Incomplete reaction with the rock. 7. Failure of retreatments or failure of acid to react in proper portion of formation.
HE acidizing of wells is an established, progressive force in the business of oil production, whose background and history are now generally known. Since the composition of the solution used to treat a well has a vital effect upon the results obtained and because each producing sand is different, the treating solution-must be adapted to fit the particular need. So much change and development have taken place in the art that it was thought desirable to bring the data on the solutions used up to date. The 15 per cent hydrochloric acid generally used in treating wells dissolves some 1800 pounds or 11 cubic feet of limestone per 1000 gallons of acid (1,4). These acids are used in predominately limestone formations, of which a representative analysis might be the following (6):
T
Silica Iron oxide Aluminum oxide Calcium carbonate Magnesium oarbonate Sulfates
These points have been proved or suspected as the chief contributors chemically to what failures or difficulties are met in acidizing wells. Each of them can be overcome by a change in the composition of the acidizing mixture.
Inhibitors and Demulsifiers to Prevent Metal Corrosion Solution of iron in mineral acids takes place very readily a t p H values below 3. At 100” F., 15 per cent hydrochloric acid dissolves iron a t the rate of 0.4 mg. per square inch per minute. The solution rate increases rapidly with increased temperature. This is comparatively slow in comparison to its rate on limestone, but acid does not eat iron away uniformly. Frequently this average rate per square inch is concentrated on 0.01 of that area, and thus an iron pipe can be punctured. Agents are known and used which reduce this solution of iron M in acid to less than one per cent of the usual rate. T of- them act as films which cover the metal surface and pre-
l-lO% 0.1-1 0.1-2 50-100 0-40
Some
The soluble portions of the rock are the calcium and magnesium carbonates and the iron and aluminum oxides; the residue consists mainly of sulfates, shale, and silica as sand or fine silt (8). Of considerable consequence also is the chemical reaction of the acid with the metallic equipment of the well; this reaction takes place a t 0,001 to 0.01 of the rate with which the acid acts on limestone, and a t 0.01 to 0.04 of the rate with which it acts on dolomite. I n 75 to 80 per cent of the treatments made, the use of the ordinary 15 per cent hydrochloric acid satisfactorily reacts with the formation, and increases its permeability and the oil production from it to a profitable degree. I n the difficult remaining 20 to 25 per cent, however, a wide variety of problems has been met; many of them have been solved by changing the chemical and physical properties of the acidizing solution. Each small change in the acid must be carefully tested ( 2 ) for its action on the constituents of producing formations. In benefiting one of the functions of the acidizing solution we must be sure not to detract from any other beneficial action or to add some other unexpected obstacle to successful treatment. I n this way the compounding of the acidizing solutions is like the design of a tonic. One of the goals of research on acidizing is to make well tonics out of acid-in other words, to develop a n acid that will produce optimum results in any well and harm none. The best approach to finding the “universal acid” for oil well treating is probably made by first considering those difficulties that have arisen from the use of ordinary acid:
Laboratory and field data are presented to illustrate those chemical and physical properties of acidizing solutions which affect their attack on metallic parts of the well, their flow into and reaction on the porous rock, the formation of emulsions and secondary precipitates, their return from the well, and their subsequent separation from the produced oil. Special attention is given to reaction rates as applied to determining the distance which acid travels in pores before spending itself, and the resultant shape of the pore after acidixing. These data, in conjunction with calculations on the optimum distribution of permeability near a well bore, lead to a method for choosing the acid which is most desirable for given field conditions.
1. Corrosion of the metal equipment from the bottom of the well all the way through the refinery. 2. Secondary precipitation of metals dissolved by the fresh acid, which the spent acid is unable t o hold in solution. 400
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At first the remedy attempted for metal hydroxide precipitation was to withdraw the acid as soon as possible from the well so that the metal hydroxides would not be fully precipitated. Although a proper move, this step was not sufficient, and intensive laboratory work was instituted to produce a foolproof method. The chemistry of secondary precipitation of metal hydroxides is best shown with neutralization or titration curves of the acids. When ferric iron is dissolved in the acid, it acts as a weak acid. It shows on the curve a t a p H of 2.5 and CUBIC CENTIMETERS OF IN SODIUM HYDROXIDE precipitates a t a p H of 2.7. Aluminum salts are similar but precipitate a t a FIGURE1 (left), NEUTRALIZATION CURVEOF HYDROCHLORIC ACIDCONTAINING IRON AND ALUMINUMCHLORIDES slightly higher pH. A mixture of the 100 cc. of normal hydrochloric acid, 1 gram of aluminum chloride, and 1 gram of ferric chloride two will affect the neutralization of acid; as shown in Figure 1, both the iron and FIGURE 2 (right). NEUTRALIZATION CVRVEOF STABILIZED HYDROCHLORIC ACID aluminum hydroxides precipitate before CONTAINING IRON CHLORIDE the spent pH of 5 is reached. 100 coo of normal hydrochloric acid, 1 gram of ferric chloride, and 3 cc. of stabilizer If the DreciDitated iron hvdroxide is allowed tb settle for several days, it vent acid from reaching the metal (6). They also prevent sinks to a minimum volume of a t least forty times the space the escape of hydrogen from the metal surface, which stops it occupied as the oxide of the metal. Thus a rock containthe action of any acid that may diffuse through to the metal. ing only 0.1 per cent of its volume as iron oxide and 20 per However, it is possible to obtain corrosion even through cent of its volume as pore space will generate by precipitation enough iron hydroxide to fill one fifth of its pores, or these coating agents if electrolytic potentials are placed between two portions of metal. Since dissimilar metals generrather one fifth of the pore space of that rock existing beate between them an electrolytic potential when immersed hind the rock dissolved. Since shortened time of contact was not a cure for this in aqueous solutions, it is necessary to remove any special metal equipment and leave only the common iron equipment precipitation, the research was turned in search of agents that in the well during acidizing. prevented the precipitation altogether. These were found in Corrosion in refineries running oil from acidized wells has certain organic compounds such as acetic and citric acids, been traced in practically all cases to the calcium and magwhich actually prevented precipitation of the hydroxides a t nesium chlorides present in the cut oil. This sometimes is the their usual pH. The neutralization curve of the metal connatural brine accompanying the oil and is sometimes spent taining acid after treatment with these stabilizing agents was acid, either or both of which may be present in the form of then similar to that of metal-free acids up to the spent p H (Figure 2 ) . It was found that each stabilizer was capable of stable emulsions. Since natural brines have a pH of 6 to 8 and spent acid from carrying a definite amount of metal hydroxide such that from 4 to 5, there will be a difference in the so-called free acid de1 to 3 per cent stabilizer was necessary to carry 1 per cent iron termination, depending upon the end-point indicator used in as chloride. By including such stabilizers in the iron-conthe titration. Determination of the p H of the bottoms, taining acid, cores acidiz:d normally and all the iron came settlings, and water, or of the water extract of the oil is a more through in the spent acid. reliable indication of the corrosiveness of the oil than is titraAfter core testing, the stabilizers were put into the field. tion to a fixed end point, especially when the end point chosen The field work has been going on for several months, and the is of the order of p H 7 to 10. amount of metal compounds being removed with the spent Whether it is brine, acid, or spent acid, the cure for the acid has been surprising. difficulty is the elimination of emulsions (9). This can be done a t the refineries by washing, treating with chemical Surface Tension Control to Overcome Difficult emulsion breakers, or electrical precipitation; but in so far Injection and Incomplete Purging as acidizing practice goes, it is best accomplished in the proper design of the acidizing solution itself', so that it prevents the Gas blocking or Jamin effect is a factor in acidizing during formation of the emulsions in the well. and after treatment. Some wells making 25 to 100 barrels of oil daily under moderate rock pressures will not take 25 barrels of acid in 2 days a t pressures five to ten times their Stabilizers to Reduce Secondary Precipitation of formation pressure. Others become choked with the charge Metals of spent acid and carbon dioxide gas, and either cannot reThe possibility of secondary precipitation during acidizing sume production or do so slowly and incompletely. The resistance to flow attributed to Jamin chains arises was recognized by most of the early writers ( l l ) ,and the cause was attributed usually to iron and aluminum compounds from the fact that the pressure necessary to move a globule of and t o carbonate precipitation on release of carbon dioxide liquid along a pore filled with gas is a definite quantity, depressure. As is true of most theories concerning phenomena pending mainly on the size of the pore and the surface tension observed in the field, it has not been possible to prove defiof the liquid to the gas. The pressure effect of the globules is nitely the extent of the influence of iron or aluminum content additive; therefore a multitude of them strung out along a on the results of acidizing, but it has been shown in the laborapore system can build up a considerable back pressure. T o tory that acid containing small amounts of dissolved iron will demonstrate the Jamin effect, experiments have been run in effectively plug producing pores. capillaries and in sands.
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T o anyone experimenting with chains of liquid and air segments in a capillary, it is immediately apparent that the chief factor determining the pressure (3) necessary to induce flow is that of wetting. If the liquid wets the tube thoroughly, the chain of liquid segments is interconnected by a film of its own substance, and movement of the liquid takes place under very slight pressures. In a system, therefore, that is completely wetted by the aqueous phase, the minimum pressure necessary to start and maintain flow will be small, and the surface tension of aqueous acidizing solutions will be of minor importance in regard to the purging of the well. W h e r e complete wetting does not obtain, the pressure necessary to start b rectly flow varies with surdi-
“‘mi
iI
z5
face tension and the surface tension of t h e treating solution will determine to a large extent what pores ill be entered by the treating solution and what pores will purge themFIGURE 3. EFFECTOF SURFACE TENs e l v e s of t h e SION ON JAMIN PRESSURE OF LIQUIDS spent solution Measurements in unwet glass capillary, 0.8 mm. in diameter and resume 1320duction. Figure 3 shows that the pressure necessary to flow a chain of bubbles in an unwetted capillary is gradually reduced with decreasing surface tension until, a t about 30 dynes per cm., the pressure decreases extremely rapidly; this denotes that the tube has become perfectly wet with the liquid. The diameter of the pore also affects the pressure necessary to purge it, the pressure increasing as the size of the pore grows smaller. Since it is a large factor in the resistance to flow in a Jamin chain, the surface tension of every solution used in wells should be studied and modified as necessary. Laboratory tests may be converted more satisfactorily to actual Jamin pressures that may be encountered in wells by expanding results from actual core tests rather than from glass capillary measurements. A McCloskey core one inch long and one inch in diameter was filled with ordinary acid which was 90 per cent spent, and was allowed to stand until the acid had entirely spent itself. Then air pressure was gradually applied until one of the pores on the surface began to bubble. This core had an original permeability of 60 millidarcies and required 2.5 pounds per square inch pressure to start one pore producing after treatment. A similar core with a permeability of 31 millidarcies required 4.5 pounds per square inch to start flow after impregnation with acid. After impregnation of the 31-millidarcy core with an acid of reduced surface tension, only 0.6 pound per square inch was required to start flow. At 4.5 pounds per square inch approximately 500 pounds would be needed to flow a 10-foot section, and the substitution of the reduced surface tension acid would reduce the necessary pressure to 67 pounds. Laboratory core tests and many field applications with acidizing solutions containing agents to lower the surface tension of the solution have shown that: (a) the acid is injected faster and with less back pressure, (b) the acid treats tight sections not entered by ordinary acid, and (e) the spent acid is more easily and more completely purged from the well.
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Wetting Agents to Start Acid Reaction through Oil Film In some formations the oil wets the rock so thoroughly that acid may stand in contact with the oil-saturated rock for hours without reacting. Treatments frequently return unspent acid and production increases are below normal. Experiments were made with oil-covered rocks and 15 per cent hydrochloric acid, which contained many agents having both acid and oil solubility, until some were found that started reaction immediately. Agents which reduced the inhibition period from 8 hours to a few seconds were phenols, cresols, and xylenols. They were applied to the acid used in the areas where the acid was not spending itself and were successful.
Demulsifying Acids Acidizing brings together aqueous acid, brine, or spent acid, crude oil, carbon dioxide gas, and the fine silt residue left after the solution of the rock. These are ideal conditions for the formation of stable emulsions, and therefore much of the oil produced immediately after acidation with ordinary solutions of hydrochloric acid must be treated before it is run into the pipe line. Since spent acid is different in composition from the naturally occurring brines, the emulsions formed from acidizing have not always responded to the same chemical agents or mechanical treatment as were already in use in those fields. The chief difference lies in the fact that the pH of spent acidizing solution lies between 4 and 5 , while that of naturally occurring brines ranges from 6 to 8. The p H has considerable effect on the stability of emulsions and on the choice of the most effective demulsifier. The high calcium content of the spent acid is also a factor in choosing demulsifiers, since it reduces the solubility of many of the organic compounds commonly used.
FIGURE 4. EFFECT OF ACIDIZINGON EMULSIONS IN THE CRUDE PRODUCED
By including demulsifiers in the acidizing solution itself , emulsions are prevented in many fields to such an extent that the oil produced even immediately after the acidizing may be run without treatment. Another advantage of including demulsifiers in the acidizing solution is that they usually lower the surface tension of the solution, with the attendant advantage of affording easier injection and more complete purging. Figure 4 shows the emulsion content of a well in Richland County, Ill., before and after acidation with a demulsifying acid. The well produced about 130 barrels per day before treatment and 250 barrels after treatment. With refineries and pipe line companies growing more and more particular about the brine content of oil handled, the prevention of emulsions during acidation is increasing in importance. The most direct cure is to render the acidizing solution itself less likely to emulsify with the oil in the well.
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Intensifiers to Increase Reaction with the Rock Since hydroch~oricacid will dissolve all the important limestone constituents except sand, shale, and calcium sulfate, it requires the addition of a solvent for one or more of these materials in order to increase the proportion of rock dissolved, By increasing the proportion of rock dissolved, greater p e r m e a b i l i t y and, what is perhaps important, less insoluble residue are the advantages to be gained. The best solvent for sand and shale is hydroflueric acid, and ammonium salts increase the solubility of calcium sulfate, both as the anhyon &.ite and as the basis of these factS,ammonium bifluoride was added to the ordinary acidizing solution early in the history of the practice (7); one TEMPERPTURE, OF routine test that is FIGURE5 . EFPECTOF TEnfways made on PERATURE ON REACTION RATE pies from the field is the OF ACIDIZIKGSOLUTIONS comparison of the solu4 N (13.7per cent) hydrochloric aoid at atmospheric pressure bility of the rock in ordinary hydrochloric acid and in hydroch]oric acid containing a quantity of ammonium bifluoride, which has been called “intensified acid.” Many rocks will show either no increase in solubility in intensified acid or only a small amount. However, some fields increases only when ammonium bifluoride is give used in the acid, such as in the Sherman Field, Isabella County, Mi&. Ordinary acid gives 50 to 75 barrels increase per day, while intensified acid gives from 200 to 700 barrels increase. Also in this field the bifluoride acid has been successfully used after unsuccessful treatments on the same wells with ordinary acid. Some rocks will not react even to the extent of their total calcium and magnesium carbonate content in ordinary acid, whereas intensified acid almost invariably dissolves all of the acid-soluble constituents. Acidizing solutions containing ammonium bifluoride are also faster acting on dolomite. Laboratory tests show it to be approximately 36 per cent faster on some types of dolomite a t atmospheric pressure and 60 per cent faster on the same dolomite a t 500 pounds pressure. The effect on the rate of solution of limestone is negligible.
403
tirely consumed in enlarging the well bore; whereas another much less vigorous acid would extend its action to greater distances from the well bore and enlarge the latter only slightly Or not at Although the Same quantities Of acid with the same total capacity for dissolving rock are used in each case, they have entirely different effects on the permeability variation of the stratum near the well bore and hence on the increase in production of the well. Somewhere between these two extremes there exists a permeability gradient which, for the same amount of acid injected, will give the greatest production increase. In order to be able to control the rate of reaction of the solutions, it is first necessary to know what physical laws affect them. To make such a study of factors affecting solution rate, it was necessary to use materials available in uniform, dense pieces of known composition. White marble and white crystalline dolomite are ideal, and so the first work undertaken was that of comparing their characteristics with core samples from producing formations. The effect of temperature, pressure, velocity of the acid past the limestone surface, concentration of the acid, and the degree of spending of the acid (similar to the effect of the concentration of the acid, except for the addition of calcium and magnesium salts) were studied. The effect of addition agents to hydrochloric acid and the results of using the more poorly dissociated acids such as acetic were also investigated. The greatest difference in reaction rate is determined by the characteristics of the rock-that is, whether it is limestone or dolomite (Figures 5 and 6). The temperature and pressure charts both show the reaction rate on dolomite to vary between 5 and 10 per cent of that on limestone. Next in importance is the temperature of the formation and of the treating solution (Figure 5), velocity (Figure 7), concentration of the acid (Figure 8)) and to Some extent the Pressure (Figure 6). A rise in temperature of 20-30’ F. doubles the reaction rate a t atmospheric Pressure; approximately 40’ F. is netessarY a t 500 Pounds Per square inch Pressure. A threefold increase in velocity is required to double the reaction rate. Thus, Some extension of the acid into the formation may be had by increasing the Pumping rate. For instance, if we assume linear flow in a single pore, then a n acid which is ordinarily spent after 10 minutes and a t 10 feet Of penetration will, if the pumping rate is increased three times, be spent in
Controlled Permeability Gradient to Give Proper Reaction Rate The reaction rate is of special importance because, along with the pumping rate, it determines where the acid consumes itself most as it penetrates radially from the well bore. In effect, the acid increases the permeability of the stratum by enlarging or cleaning out the many individual pores and capillaries which comprise the porous rock. But since these are enlarged less and less with increasing distance from the well, there arises what might be termed a “permeability gradient,’’ which plays a decisive role in subsequent well production. For example, we can think of a vigorous acid being introduced into the well a t such a rate that it is en-
0
100
200
300 400 500 600 700 800 900 1000 1500 GAGE PRESSURE, POUNDS PER SQUARE INCH
2000
FIGURE6. EFFECTOF PRESSURE ON REACTION RATESOF ACIDIZIWG SOLUTIONS 13.7 per cent hydrochloric acid
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800
1200
1600
2000
2400
2800
VELOCITY, REVOLUTIONS PER MINUTE
FIGURE 7. EFFECT OF VELOCITY ON REACTION RATEOF ACIDIZING SOLUTIONS 0.1 One-inch N hydrochloric cylinder ofacid marble, a t 70 F. inch andlong, 50 pounds rotatedper in square inch pressure
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indicate the ideal law of permeability variation near the well bore. A number of laws of permeability variation extending out to‘ thirty-two well radii were examined, and the amounts of acid required to produce them were calculated. The production rates corresponding to these various laws of permeability were next determined. Instead of doing all of this by direct computation, which would require the derivation of numerous formulas as well as considerable arithmetical work, recourse was had to a hydrodynamic well model which automatically yields production data. This material is summarized in the typical permeability curves of Figure 10. Table I illustrates the production rates realized in a number of different cases, along with the amounts of acid required. All entries marked” were calculated by formulas given by Muskat (10). The last column in the table gives the acidizing index (i?), the ratio of percentage increase in production rate to amount of acid required, which is considered a measure of the efficiency of the acidizing process. The table is arranged in order of decreasing acidizing index and brings out the fact that the same increase in production rate may arise from several different acid treatments which require vastly different quantities of acid. Of the cases considered, the highest acidizing index is for doubling the size of the well bore, but since this leads to only a small increase in well production, it is not of practical interest. Besides
5 minutes and a t a point 15 feet into the pore. The effects of concentration and of “per cent spent” (Figure 9) are both linear; that is, the rate of reaction is halved when the acid is diluted with an equal volume of water or when it is half spent. The first hundred pounds of pressure applied to the acid exerts a large effect on the rate of reaction, but subsequent increases of pressure haveless and less effect, until the z line becomes very flat around h 0.2 z 800 to 1000 pounds. Thermo? dynamic calculations indicate s that approximately lo8 atmos$1 pheres are necessary to stop the 4 reaction of hydrochloric acid on limestones. This is far above ACID CONCOVTRATITION, PER CENT any pressure to be found in Oil 0 10 20 30 40 50 60 70 80 90 100 FIGURE 8. EFFECTOF CONCENPER CENT SPENT wells. TRATION ON REACTION RATE OF ACIDON REACFIGURE 9. EFFECTOF SPENDING There are very few addition ACIDIZING SOLUTIONS TION RATEOF ACIDIZING SmurIom agents for hydrochloric acid Rutland white marble, hydrochloric 13.7 per cent hydrochloriq acid, 104’ F., 500 pounds per acid, 74’ F., atmospheric pressure that will speed up its reaction square inch pressure other than the ammonium bifluoride already mentioned. An increase in its concentration, in its temperature, or in the velocity of injection are the available methods for accelerating its reaction with the formation. T o obtain a slower acid action, the foregoing 5 90 factors should be reversed; better still, addition agents, of which there are many, are used in the hydrochloric acid; or the poorly dissociated acids are substituted for this strong mineral acid. The question naturally arises as to what permeability variation following acid treatment gives rise to a maximum increase in production for equal quantities of acid consumed. Once this is ascertained, it should be possible to add another term in the prescription for the acid to be used in any given well. Although a purely theoretical calculation is impossible, since no relation exists between the permeability of a porous structure and its other 0 20 40 60 80 100 120 140 I60 180 200 220 240 260 280 300 320 physical constants such as porosity, grain size, RADIAL DISTANCE, CM. etc., yet various computations can be made FIGURE 10. THEORETICAL VARIATION IN PERMEABILITY NEAR WELL BORE on an artificial radial flow system so as to FOLLOWING ACIDATION
2
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permeable and free from sand, capillaries 1/*4 inch in diameter and several inches in length could be readily drilled. Weak acids (0.1to 1per cent) were passed through these capillaries at relatively low flow rates, and the variation in size of the capillaries was carefully followed. Stronger acids caused the capillaries to enlarge too rapidly for accurate measurement. At first the experiments were performed a t atmospheric pressure, but the presence of carbon dioxide bubbles in
FIGURE 12. ACIDIZING OF CAPILLARIES WITH 0.1 PER CENTHYDROCHLORIC ACID Total pressure, 30 pounds per square inch; constant flow rate: run 19 = 18 cc. per minute, run 20 = 55 GO.; length of capillaries, 3 am.
the capillary would give rise to Jamin action and in general interfere with the results. Eventually the runs were made in a regular core-acidizing apparatus a t 30 pounds per square inch gage pressure. This equipment is shown in Figure 11. Some typical results for 0.1 per cent hydrochloric acid are FIGURE 11. HIGH-PRESSURE-CAPILLARY ACIDIZING APPARATUS shown in Figure 12. The square of the capillary radius plotted against time gives an almost linear relation, which is 90 per cent of this increase could be obtained with 50 per cent convenient since the amount of limestone removed from the as much acid. Case XI1 (enlarging the well bore to 32 times walls of the capillary, and hence the amount of acid consumed. its initial size) yields a significant increase of 83 per cent in is proportional t o production and requires 195 units of acid. But case VIII, a t h e change in uniform 32 fold increase in permeability to 32 well radii, gives radius squared. 95 per cent as much increase as XI1 with only 5 per cent as The data thus far much acid; case IV, a hyperbolic decrease in permeability, obtained from yields 83 per cent as much increase as case XI1 with only 1.4 capillary studies per cent as much acid. In other words, case XI1 represents a indicate that the very inefficient acidizing process. The problem is to find and r a t e of g r o w t h use a n acid which will have high efficiency as indicated by a varies linearly, large value of the acidizing index. b o t h with acid concentration and with velocity of TABLEI. THEORETICAL ACID REQUIREMENTS FOR VARIOUS flow. PRODUCTION INCREASES It is possible to % ’ Grams use the slopes of Increase Acid AcidEffect of Reizing Acidizing on i n Prothese straight lines quired Index No. Case Permeability0 duction to calculate how R = 2Ro 10b 0.186 53.7 Enlarging well bore I 1.35 42.8 K = b/r 58 Curve 2 (Fig. 10) I1 far the same type K = CR-dr 1.80 32.8 59 Exponential 111 K = a/rl.e 2.69 of acid will travel 25.7 69 Curve 1 (Fig. 10) IV 2.14 52 b K = 4Ko 24.3 Uniform increase V in a uniform capil66 b 3.90 16.9 K = 8Ka Uniform increase VI 14.6 Linear 5.20 76 Curve 3 (Fig. 10) VI1 lary before it is 9.99 K = 32Ko 7.91 Uniform increase 79 b VI11 2.86 22 b completely spent, R = 4Ro 7.85 Enlarging well bore IX 12.2 3.12 38b R = 8Ro X Enlarging well bore and what t h e 48.8 1.17 R = 16Ro 57 b XI Enlarging well bore 195 83 b R = 32Ro 0.43 XI1 Enlarging well bore resulting shape of FIGURE 13. APPARATUS FOR COMPARRo, R = initial and final radius of well bore; Ka, K = initial permeaING THE ACTION OF Two ACIDS OF THE the capillary will bility and permeability of stratum after acidizing, respectively. SAMESTRENGTH b Calculated by Muskat’s formulas (IO). be. For example, i n a caDillarv sufficiently long so that the flow of acid through it Eemaini substantially constant during treatment, one can show that Admitting that it is desirable to control the permeability the normality of the acid after traveling distance L is given by variation near the well bore, in contrast merely to pumping acid into the well and hoping for the best, one may ask how this variation can be accomplished in actual formations with existing acids or with acids modified by addition agents. where N O = original normality As a start on this phase of the problem, recourse was again p = density of limestone had to single capillaries, but this time they were actually Q = flow rate, cc. per minute drilled from limestone cores and subjected to acidizing under S = b/No, where b is slope of ra us. t curve (Figure 12) e = 2.718; ?r = 3.1416 various conditions. By using pure limestone, which was im-
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careful attempts to machine all scratches to the same initial width and depth. The use of lactic acid on a similar limestone block gave a very uniform increase in scratch width. These studies are as yet incomplete; but when full data are finally obtained on various types of acid, various coneentrations, and B range of flow rates, i t is felt that an accurate prescription can finally be made for the exact couditions necessary to produce tlie desired type of permeability variation. It can he objected that the action of acid in a single capillary is entirely different from its behavior in a porous body, and that channeling in an actual well will always lead to unpredictable results. This is true, but the general trend of hehavior should he the same; and since each acid is tested on cores from different localities, eventually a complete correlation will be available. Some field work on fitting the reaction rate to the conditions existing in tho formation has been in progress for some time in various fields and was recently applied to the oolitic Mc-
FIGURE15. ACIDIZED RADIALS C R A T C SYSTEM ~ SIIOWING CAANNEIANG
From the data of Figure 12, run 19, a t a constant flow rate of 18 cc. per minute, for sliich S = 2.9 X lo-&, the following values mere eoiiiniited: .L--Cm. 0 10
20 60 100
200
Radius ol
Bt. 0
0.35 0.67 I .84 8.28 6.50
N
9% Spent
cepi1iary a h r 20 M i a , Cm.
0.030 0.021
0 80
0.04G
0.0i7 0.008 0.002 0.0005
48
78 99 100
0.040
0,087 0.029
0.022 0.020
Pernieahility variatioii for this capiila,ry, as calculated from the radii in the last column, is s h o w in Figure 10 and agrees i n shape with ctirve 2, a hyperbolic decrease with increasing distance frorn the well, or K = ah.. It is readily appreciated from the above formulas that low values of the slope function, S, which means a slowacting acid, and high values of flow rate, Q, ivill enable the acid to travel farther before becoiriirig spent. 1,nboratory data i d everit,uallg yield classified values for all types of acids. With tlie exception of several factors such as tiirbuleiice, oiliness, and presence of foreign materials, these 1al)oratory results can be trni~slatmlto tile cnse of a uniform capillary situated in a well. In addition to the studies on c:ipillni.ies, several expcrimei,ts have been made by passiiig acid through narrow scrat,cher on the surface of limestone blocks, to which pieces of plate glass are cemented with a trarisparcnt viscoiis niaterid These experiments are especially interesting since they give a visual picture of tho plienomerra of acidiaing; in addition, they yield niaiiy qualitatire d a h . A typical amarigernerit for comparing tile action of two acids of the same strei!gth is sliown in Figure 13. Figure 14 represents a scratch system tlmt has been acidized. One acid, 0.04 N hydrocliloric, yields a high degree of taper.; t h e otlier, 0.04 IV lactic, gives an almost uniform change in sernt,cli width. Figure 15 represents another typical scratch design which simulates ai1 actual radial flou~system. This pmticulaisystem, acidieecl with 1 per eeiit hydrochloric acid a t 10 pounds pressure, resulted in niarked clianrieling in spite of
Five Lhousood lidions of seid used
ilk
earh well
Closkey in tlie Illinois basin. This formation is almost ideally uniform in porosity and permeability, and has a reaction rate about double that of ordinary limestone. Experimontal acidizing was done with acids whose reactioii rates are from 20--70 per cent that of ordinary solutions. At first i t was thought that little, if any, improvement resulted from the change, but decline curves kept on these wells showed in nearly every case a flat.ter, longer lived increase in production. Figure 1G is typical of the comparative decline curves of adjoining wells treated with retarded and uriretarded acid.
Literature Cited (1) Aeidiaer (trade pub. of Dowell. Ino.), No. 4, Nov.. 1936. (2) Chamberlain, L C.. Oil Weekly, 88,20-34 (1938). (3) Chamberlain. L. C . . U. S.Patent 2,024,718 (Deo. 17. 1939). (4) Chapman, M. E., Oil Gas J . . 32 (21). 10 (1933). ( 5 ) Egioff. Guatsv, Nelson, E. S.,Mnautov, C. I).. and Wirth. C., Ibid., 36 123). 06-73 11937). (6) Grobe, J. J., and Sanford, R. T., U. S.Patent 1,577,004 (Sept. 13. 1932) (7) Flcath. Sheldon, and Fry, \Villiam. Zbid.. 2,011,579 (AuE. 20. 1035). (8) Howard, W. V., and David, M.W., Dull. .4n.Assoc. Petroleum Oeol., 20, 1380-1412 (l93ti). (9) Love, 1 7 W., nnd Fitacerald. P. E., Ibid.. 21, 616-20 (19Si). (10) Muskat, Morris, "Flow of Iiomogencous Fluids through F o i o u ~ Medin," 1 s t ed.. Chap. VII, pp. 400-2!), New Ywk. McGrawIIiIi Book Go.. 1937. (11) Nercomhe, R. B., Oil Weekly. 69. 19-20 (1933).
