Selective Acidizing and Permeability Determination by an Electrical Method

Transcription

1 .- AMERICAN INSTITUTE OF MINING ASD METALLURGICAL ESGISEERS Technical Publication No (CLASS G, PETROLEUM DIVISION. NO. 19a) DISCUSSION OF THIS PAPER IS INVITED. Discussion in writing (a cnpies) may be sent to the Secretary. American Institute of Mining and Metallurgical Engineers, 29 West 39th Street. New York. N. Y. Unless special arrangement is made. discussion of this paper will close Sept Any discussion offered thereafter ~ - should preferably be in the form of a new paper.. Selective Acidizing and Permeability Determination by an Electrical Method BY DAXA G. HEFLEY* AND P. E. FITZGERALD,' MEMBER A.I.M.E. (New York Meeting. February 1943) ABSTRACT basic Carr technique were devised to accomplish this result. AN apparatus has been developed which permits selective acidizing of producing forma- In order to overcome the possibility of tions and determination of the relative perme- the acid entering an undesirable section at ability of a formation by fluid injection. This the bottom of the well, organic gel plugs2 apparatus, known as the "Electric Pilot," is an were introduced into the bottom of the well, electrical fluid-interface locator, the electrical which sewed to halt the entrance of the circuit of which is completed when the elec- treating solution into these zones.3 These trodes in the well are in contact with a conduc- chemical later liquefied because of tive fluid, but is not completed when immersed bacterial and were removed from in a nonconductive fluid. Results are given for a the well with the produced fluid. number of typical acidizing applications in In fields where a gas cap existed, as in the which, the interface locator was used to control the acid. Description of the method used in Goldsmith field of Ector County, Texas, it making a permeability survey and the data was imperative that a treating technique be obtained are also included. evolved that would confine the acid to the oil-producing zone below the gas cap. By INTRODUCTION so doing the oil-productive capacity of the Early in the development of the acidizing wells could be improved without greatly industry it was recognized that one of the increasing the gas production and this most important problems to be solved in would lower the gas-oil ratio of the well. the chemical treatment of any well was the The technique employed4 to achieve this determination of what part of the exposed effect utilized two pumps, one for pumping section in the well should be treated and the the chemicals down the tubing, the other to selection of the technique that would insure simultaneousl~ force the treatment of the desired section. space to confine the acid to the oil pay, one of the first techniques adopted for thus preventing the chemicals from enterassisting in proper placement of the acid, ing the upper gas-bearing which is still widely used, is the method of Experience has indicated that gas wells confining the acid to the producing section and certain types of oil weus respond better by means of a column of oil, which is known to "stage treating." This technique utilizes as the carri method of treatment, named the introduction of a certain volume of acid for its inventor. In the Carr method it is not and then the to UP" possible to selectively acidize a specific before introduction of the next and usually horizon in a multiple pay zone, so many successively larger stages. Where such improvements and modifications of the methods are employed steps must be taken Manuscript received to keep a disproportionate share of the acid at the of the r Institute Jan from entering the most permeable and * Dowel1 Incorporated, Tulsa. Oklahoma. 1 References are at the end of the paper. previously treated A special tool -. Copyright by the American Institute of Mining and Metallurgical Engineers. Inc. Pe~RoLEurn TECHNOLOGY. July Printed in U. S. A.

2 2 SELECTIVE ACIDIZING AND PERMEABILITY DETERMINATION called the "jet gun" is often used. This device is attached to the bottom of the tubing through which the. acid is introduced. It contains many small jets, which treated were inaccurately located or the data were so meager that no definite conclusions could be reached in regard to the location of the permeable, porous zones that contained the oil. cause high-velocity streams of acid to be impinged upon the sections of lower permeabilities. All of these selective acidizing methods depend for their success upon the completeness and accuracy of well data provided by the geologist and engineer representing the well owner. Often it has been learned, too late in many instances, that the zones to be Recently there has been placed in field service an apparatus known to the trade as the Electric Pilot, which can be used (I) to determine the relative permeability of various zones in a well bore and (2) to introduce the treating chemicals into the desired zones. Relative permeability is determined by measuring the fluid-injection rate into all parts of the exposed formation, and acidizing control is effected by introducing acid and oil into various parts of the formation at such rates that the top or bottom level of the acid in the borehole, as indicated by the electric pilot, is held at the desired level. Briefly, the instrument is an electrical fluid-interface locator, the electrical circuit of which is completed when the electrodes in the well are in contact with a conductive fluid, such as acid or salt water, but is not completed when immersed in a nonconductive fluid such as oil. The electrode unit is run into the well on an insulated singleconductor cable, the insulated copper core and the external steel wires forming the two conductors required for the electric circuit from the surface to the bottom of the well. A diagrammatic sketch of the electrode unit is given in Fig. I. Essentially, the instrument consists of a steel tube several feet long, on the outside of which are mounted two electrodes, completely insulated from the steel tube. The insulated conductor wire of the cable is connected to the upper electrode, and the lower electrode is electrically connected to the upper by means of an insulated wire having a resistance of several hundred ohms. In order to complete the electric circuit in the well, it is necessary to establish electric connection

3 DANA G. HEFLEY AND P. E. FITZGERALD 3 between the body of the pilot and one or both of the electrodes. In some applications the interface locator is run inside the tubing and in others it is with respect to the electrodes at the bottom of the well. On the commercial units for this service, all the equipment is mounted in a light panel truck. LLEY 1 STUFFING BOX MASURE METER R DRIVEN REEL INSULATED C R AND ON 110 VOLT AC GENERAT TRANSFORMR run in the casing and open hole with no tubing in the well. The seating ring shown near the top of the electrode in Fig. I is required only when it is run inside the tubing. The seating ring seats in a companion piece previously installed on the bottom of the tubing, thus causing any fluid introduced into the tubing to go into the fluid passages of the electrode unit and out the bottom of the unit. The surface equipment used with the interface locating unit is shown in Fig. 2. This consists of a power-driven reel for spooling the insulated cable, with a measure meter and weight indicator installed on the line near the reel. The insulated copper conductors in the cable and the cable sheath are connected to slip rings, mour.ted at one end of the reel. This permits electrical readings to be taken while the cable is in motion. Operating current for the unit is supplied by a small alternating-current generator connected to a transformer. An ammeter in the circuit shows the amount of current flowing, and this is a direct indication of the location of the conductive fluid Actual operation of the interface locator is most easily understood by reference to Fig. 3, in which the electrodes are shown completely immersed in oil, immersed half in acid and half in oil, and completely immersed in acid. The illustrations show the electrode unit connected with a battery (B) and an ammeter (A), and also the ammeter readings that can be expected under each of the three conditions possible at the bottom of the hole during use. In the left-hand illustration of Fig. 3, only oil is in contact with the electrodes, and as oil is a nonconductor the electric circuit between the electrodes and the body of the pilot is not completed. No current flows in the circuit and the ammeter registers zero. In the center illustration, acid is in contact with the lower half of the steel body of the electrode unit, and oil is in contact with the upper electrode and the upper part of the steel body. Acid is a conductor of electricity, therefore the electric circuit is completed between the lower electrode and the body of the unit. Current now flows in

4 4 SELECTIVE ACIDIZING AND PERMEABILITY DETERMINATION the circuit from the battery to the top electrode, through the resistance, to the bottom electrode, through the acid to the body of the unit, to the ammeter and back operator observes an electrical instrument, and when this instrument reads zero it means that the acid or salt water is below the bottom electrode. When the instrument ELECTRODES ELECTRODES IN OIL AND ACID IN ACID 1 FIG. 3.-OPI;RATION OF IXTERFACE LOCATOR. No current flows when electrode unit is immersed in oil but a definite current flows when one or both electrodes are in acid. to the battery. A definite amount of current is now flowing in the circuit and the ammeter will show about a half scale deflection. In the right-hand drawing of Fig. 3, only acid is in contact with the electrode unit. In this case the electric circuit is completed between the lower and the upper electrodes and the body of the unit. The current now flows in the circuit from the battery to the top electrode, through the acid to the body of the unit, to the ammeter and back to the battery. As the resistance between the two electrodes has now been shorted out, considerably more current will flow than was possible in the previous illustration. The ammeter will now indicate practically a full-scale deflection. Although these illustrations have assumed acid and oil to be the two fluids in contact with the electrodes, other conductive anti nonconductive fluids will work equally well. In making permeability surveys it is customary to use salt water insteacl of acid. The illustrations in Fig. 3 are analogous to the actual happenings in a ell. The gives a half-scale deflection it means that the interface between acid or salt water and oil is between the bottom and top electrodes. When the instrument gives a fullscale deflection it means that the conductive fluid is at or above the top electrode. Location of the interface between a conductive and a nonconductive fluid in a well makes possible two distinct types of well applications: (I) control of fluid-injection rates so as to maintain the interface at the desired level, and (2) surveying wells by following the movement of the interface under controlled conditions. Under the first classification the most important commercially to date has been selective acidizing of the lower pay in a zone containing two or more pays, although successful acidizing of an upper pay has also been accomplished using an electrode having an electrical connection practically the reverse of that described previously. The most important commercial application of the moving electrode technique is the determination of the relative permeability of a formation by observing the rate of the oil-

5 DANA G. HEFLEY AND P. E. FITZGERALD 5 water interface while fluid is being injected into the formation. ACIDIZING OF SELECTED ZONE The electric interface locator has proved of great value in the acidizing of many ously, oil is pumped into the annulus between the tubing and casing. By this means an acid-oil interface is obtained that is relatively free from turbulence and from the possibility of emulsion formation. The relative pumping rates of acid and oil INJECTION RATE DATA PRESSURE PSI PERMEABILITY WRDSITY : 4940 c' m 5020 TIME FROM START RELATNE PERMEABILITY OF SURVEY. MINUTES RATE OF FLOW, FT/MIN FIG. 4.-PERMEABILITY BEFORE ACIDIZING. Before acidiiing, the formation had two permeable zones with a relative permeability of nearly seven in the upper zone and three in the lower zone. wells with high gas-oil ratios, high water-oil rat,ios, two or more pay zones, sands exposed above or below limestone, leaky casing, or wells that have been deepened. In such wells it is desirable that acid be placed in a selected zone in order to produce best results. To acidize the lower pay of a multiplezone well the electrode seating nipple is installed on the lower end of the tubing and sufficient tubing is run in the well to place the electrode seat 2 ft. above the top level to which acid should rise during the treatment. The electrode unit is then run into the tubing and seated in the bottom of the tubing. As the electrode seating ring is near the top end of the unit, this permits the two electrodes to project out below the bottom of the open-end tubing, with the mid-point between the electrodes at the top acid level. During the treatment, acid is pumped down the tubing and discharged into the well below the electrodes, and simultane- employed during the treatment are then determined by the readings on the electric meter at the surface. When the meter gives a full-scale deflection, indicating that the acid level is too high, either the acidintroduction rate is slowed down or the oil rate speeded up. If the acid level is too low, as indicated by a zero reading on the instrument, the acid introduction rate is speeded up or the oil slowed down. It is generally found that the ratio of oil to acid required to hold the top of the acid at the desired point is much less toward the end of the acid treatment than at the beginning. This is exactly what would be expected, as it means that the permeability of the lower zone is increased by the acid while the permeability of the upper zone, which received no acid, remains substantially unchanged. Fig. 5 shows the injection rates for acid and oil used in treatment of a well in the West Texas area where the electric interface locator was used to confine

6 6 SELECTIVE ACIDIZING AND PERMEABILITY DETERMINATION the acid to the bottom pay in a well having two pay zones. At the start of this treatment the acid-injection rate was approximately half the oil-injection rate, while at of wells consists of the introduction of sufficient salt water into the bottom of the well to cover completely all of the section to be surveyed and the forcing of this salt KID TREATMENT, FLUID INTER- FACE WNTAINED AT 4990 F T FIG. 5.-ACID-TREATMENT DATA. During treatment of the lower pay, the casing pressure remained nearly constant, but the rate of introduction of acid increased as the treatment progressed. the end of the treatment, acid and oil were beingpumpedat substantiallythesame rate. Without some means of continuously determining the location of the top of the acid throughout the treatment, selective acidizing by pumping acid down the tubing and oil down the annulus is merely guesswork. The electric interface locator makes this method of selective acidizing an accurate and reliable procedure. General well data and results obtained from a number of selective acidizing treatments in which the electric interface locator was used are shown in Table I. This gives an idea of some of the various types of wells and different techniques to which the electric interface locator has been found applicable. Various techniques have been used for permeability surveys, but the one that seems to give the best results in a majority water into the formation by introduction of oil into the well. The rate of fall of the salt water-oil interface is determined, by means of the interface locator, during the injection of salt water into the formation. From these data the relative permeability of various sections of the formation can be calculated. A good example of the technique employed and the data obtained is given for a well in the Slaughter pool of Hockley County, West Texas, on which a permeability survey was made before and after a selective acidizing treatment. This well had an original total depth of 5028 ft. with 5%-in. casing set at 4720 ft. On completion of the well in 1939, it was given a 5000-gal. acid treatment but no attempt was made to direct the acid to any particular zone. Production history of the well over a 3-yr. period indicated that selective re-acidizing of the less permeable zones should result in increased production. In making the permeability survey before re-acidizing, sufficient salt water was

7 DANA G. IIEFLEY AND P. E. FITZGERALD 7 introduced into the bottom of the well to fill approximately to the casing point. The well was then filled with oil, and oil was pumped into the well at a rate of 10 gal. per min. throughout the survey. The pressure at the surface is shown on the curve marked "Casing Pressure" on Fig. 4. This shows that the pressure was constant at 295 lb. until after the interface passed the first pick the permeable sections from the wellcutting data. The survey showed that thc upper permeable zone at 4975 ft. had twice the permeability of the lower zone at 5020 ft., so the lower zone was acidized with 5000 gal. of acid, using the interface locator to control the injection of acid. The electrode unit was seated in the lower end of the TABLE I.-Data on Selective Acidizing Well Location.Before After Creek County. Oklahoma Arbuckle 8 ft Arbuckle and Bartles- Rogers County, Okla- Mississippi homa. a Greenwood County. Kansas City Kansas. I Ector County. Texas.... Permian permeable zone, and that after this the pressure rose rather rapidly. The location of the salt water-oil interface was followed with the electric interface locator, and a graph of interface location against time was made from these data (Fig. 4). For comparison with this injection permeability data, a "relative porosity" log, derived from microscopic determinations run on well cuttings, is also shown in Fig. 4. There seems to be some general agreement between the two sets of data but in this case it might have been difficult to tubing and so located that the acid would be held below 4990 ft. Oil was pumped down the casing and acid down the tubing in the quantities required to maintain the interface at the desired point. Fig. 5 shows the pumping rates for oil and acid; also the casing and tubing pressures used throughout the treatment. After acidizing, the well was put on production until practically all the spent acid had been produced frnm the formation. An after-treatment permeability survey was then conducted, using a technique

8 8 SELECTIVE ACIDIZING AND PERMEABILITY DETERMINATION similar to that used on the pretreatment survey. However, it was found that this survey could be conducted with the hydrostatic head of a well full of oil, so no surface 49W INJECTION RATE DATA PRESSURC,PSI equivalent to gallons per minute, since a hole drilled with a 4%-in. bit holds approximately I gal. per foot. The flow rate into a given zone is obtained by taking the differ- PERMEABILITY f n 49.. MOO JON) TIME FROM START RELATIVE PERMEABILITY OF YIRVCY, MINUTES RATE OF FLOW, FT/MIN. FIG. 6.-PERMEABILITY AFTER ACIDIZMC. The formation exhibits the same permeable zones as before acidizing but the permeability of the lower zone has been increased by a factor of three. pressure was put on the well except near the end of the survey. This lower pressure on the formation caused the injection rates into all zones to be proportionately less than on the first survey. The data obtained on the after-treatment permeability survey are shown in Fig. 6, and a compilation of the results of both surveys is given in Table 2. Time TABLE 2.-Results ence in the injection rates above and below the permeable zone. The relative permeabilities before acidizing, as shown in Table 2, are numerically the same as the flow rates into the respective zones. In calculating the relative permeabilities after acidizing it was assumed that the section at 4975 ft. had the same permeability before and after the of Permeability Survey Injection Rates, Ft. per Flow Rates into Permeable Relative Permeability Min. Zone, Ft. per Min. Before a++$ng Mteraalmng I :::: I : : 1.69 The injection-rate data given in Table 2 acidizing treatment, since no acid entered are calculated from the slopes of the curves this section. Using 6.96 to represent the in Figs. 4 and 6, which represent the permeability of the section at 4975 ft., the quantity of fluid entering all permeable relative permeability of the section at 5020 zones below the interface. These data are ft. after acidizing will be 8.11, and the given in feet per minute, which is nearly section at 5026 will be Thus, the

9 .DANA G. HEFLEY AND P. E. FITZGERALD 9 permeability of the zones at 5020 and below were approximately 3.2 times as great after acidizing as before. The electric interface locator or "Electric Pilot" has been used in numerous types of wells in several states. The results obtained to date indicate that by the use of this instrument new economies in well treating may be secured through resultant improvement in acidizing techniques. Improved acidizing techniques, or the introduction of the chemicals into the proper zones in a well, should result in better wells and increased recoveries. The use of this apparatus in surveying wells for the location and relative permeabilities of porous sections in all kinds of wells, including those used in secondaryrecovery projects, will provide data of much value in determining completion and workover practices. I. R. H. Carr: U. S. Patent (1932). 2. J. J. Grebe and S. M. Stoesser: U. S. Patent (1934). 3. P. E. Fitzgerald and F. R. Holland: Use of Gel Compounds Aids in Acidizing Wells. Oil and Gas Jnl. (Feb. A. 1937) R. J. Sullivan: Gas-oil RaiG-Cbntrol in Flowing Wells. Amer. Petr. Inst. Drill. and Prod. Practice (1937) G. L. Leach: Improved Gas Well Acidizing. The Oil Weekly (March ) 17.