Equilibrium Acid Fracturing

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  Equilibrium Acid Fracturing: A New Fracture Acidizing Technique for arbonate Formations S J Tinker SPE, Shell Western E P Inc. Summary The equilibrium-acid-fracturing technique was developed to stimulate wells in the Wasson San Andres Denver Production Unit. This new treatment technique maximizes acid contact time with the fracture faces while allowing control of the created fracture dimensions. Maximum acid contact time is essential to create highly conductive etched pathways on the fracture faces of cool dolomite formations that react slowly with acid. Control of fracture dimensions is important in the San Andres Denver Unit because fractures tend to grow uncontained in at least one vertical direction and the oil column is bounded by permeable gas-bearing intervals above and permeable water-bearing intervals below. With this technique, a fracture of desired dimensions is created by injection of acid at fracturing rates. The volume of acid required to create the desired fracture dimensions is determined by a 2D fracture-geometry program with design parameters determined from fracture field testing and laboratory testing. Injection is then continued at reduced rates that maintain equilibrium with the fluid leakoff rate from the created fracture faces. Maintaining equilibrium between injection and leakoff allows the created fracture to be held open without significant further fracture extension. Equilibrium is achieved in the field by maintaining the injection pressure below the fracture extension pressure but above the fracture closure pressure determined by fracture field testing. This paper presents the background and theory of this technique along with design procedures, field examples, results, and conclusions. Results of the equilibrium-acid-fracture treatments and other acid stimulations performed in the Denver Unit are also compared. Introduction The Denver Unit is one of several production units in the Wasson San Andres field in the west Texas counties of Gaines and Yoakum (Fig. 1). The target interval of the San Andres formation, a Permian dolomite, is at about 5,000 ft. The Wasson San Andres field was discovered in 1936. Waterflooding in the Denver Unit began in 1964, when the unit was formed; CO 2 flooding began in 1984, and expansion is ongoing today. The average permeability in the Denver Unit is about 5 md. 1,2 The pay-quality rock is split up into two major divisions. The firstporosity zones, in the upper part of the reservoir, are underlain by the main pay zones (see Fig. 2 for a type log). The reservoir temperature is about 105°F. An srcinal gas cap and an inactive aquifer exist. The oil column is bounded below by pay-quality waterbearing rock in all parts of the unit and bounded above by payquality gas-bearing rock in most of the unit. 1,2 An aggressive workover program made it possible to continue efforts to improve the effectiveness of well stimulations. The equilibrium-acid-fracturing technique was developed to improve the stimulation results achieved with other techniques. The typical acid formulation used for most acid stimulations in the Denver Unit, including equilibrium acid fractures, is 28 HCI. Many acid fracturing and acidizing techniques, ranging from matrix acid treatments to viscous fingering of acid through a gelled fluid, have been used throughout the industry. The equilibrium-acid-fracturing technique is significantly different from the other fracture acidizing techniques because it maximizes acid contact time with the fracture faces while allowing for control of the created fracture dimensions. A fracture of desired dimensions is created by injection of acid at fracturing rates. The volume of acid required to create the desired fracture dimensions is determined by the fracture-geometry program ENERFRAC3 with design parameters obtained from fracture field testing. After the fracture is created, the acid injection rate is reduced until it matches the fluid leakoff rate from the fracture. When these rates match, an equilibrium is established and the created fracture can be held open without significant further extension. In practice, equilibrium is obtained by adjusting the injection rate to maintain the injection pressure below the fracture extension pressure but above the fracture closure pressure (minimum in-situ stress) determined from fracture field testing. Equilibrium acid fracturing is used to obtain maximum oil stimulation without stimulating the adjacent water or gas zones outside the oil column. This is particularly important in carbonate formations where such properties as Copyright 1991 Society of Petroleum Engineers SPE Production Engineering February 1991 Young s modulus, Poisson s ratio, minimum in-situ stress, and propagation pressure are fairly uniform and few barriers to vertical fracture extension exist. The extended acid contact time is desirable in the relatively cool [105°F bottomhole temperature (BHT)] Wasson San Andres dolomite. The equilibrium-acid-fracturing technique was used successfully to stimulate wells in the Wasson San Andres Denver Production Unit. The significant aspect of this technique is the continued etching of the fracture faces for extended periods of time while the fracture is open without further fracture growth after the initial fracture dimensions are created. Other fracture acidizing techniques usually consist of high-rate continuous injection of either acid alone or alternating stages of acid and various gelled fluids. Often the total fluid volumes for these other methods are quite high and the stimulations m yor may not be designed with regard to the ultimate created fracture dimensions. When fracture growth is uncontained in at least one vertical direction, as in the Wasson San Andres field, the ultimate created fracture dimensions become important. The created fracture dimensions become extremely critical to the overall success of the stimulation when the oil column is bounded by productive gas zones above and water zones below. Stimulation of pay-quality zones outside the oil column usually result in excessive water and/or gas production, both of which negatively affect stimulation. Out-of-zone stimulation can also have detrimental effects if the field has secondary or tertiary recovery potential. Another fracture acidizing technique consists of creating a fracture with acid and/or other fluids, etching the fracture with acid, allowing the fracture to close, and finally injecting acid into the closed fracture at pressures below the closure pressure. 4 The concept of injecting acid into a closed fracture is almost opposite that of equilibrium acid fracturing. The equilibrium technique holds the fracture open while acid continues to etch its faces without significant further fracture extension and allows live acid to reach the fracture tip in cool dolomite formations. Injection into a closed fracture tends to concentrate the stimulation effects very near the wellbore because of the slow rates required to maintain a closed fracture. This paper does not present detailed data and discussion on the reactivity of the San Andres dolomite formation with acid because that topic is thoroughly covered elsewhere. 5 The effectiveness of the equilibrium-acid-fracturing technique was proved by field application in a heterogeneous, layered carbonate formation. Larger production increases at lower costs were obtained with the new technique than with the other stimulations in the same field. 25  Fig. 1 Locatlon of Denver Unit within the Wasson San Andres field. heory The equilibrium-fracture-acidizing technique maximizes acid contact time with slow-reacting hydrocarbon-bearing carbonate formations without fracturing into adjacent water-or gas-bearing zones. Because ofthe uncontained fracture growth and stiff rock (Young's modulus of 6,000,000 psi), very small volumes of fluid can create large fractures.6 These properties render impractical the use of large-volume, continuous, high-injection-rate acid fracture treatments. The alternative of a short, small-volume acid treatment is also unsatisfactory because of the San Andres dolomite's slow reaction rate. In the Denver unit, the acid flow-by or contact time is an important factor that affects stimulation response. The Wasson San Andres' low BHT causes the dolomite to react very slowly, even with 28 % HCI. The heterogeneous nature of the rock causes the differential etching required to create a conductive flow pathway, but the slow reaction rate requires extended acid contact time to create an effective fracture. f he fracture is not held open for extended contact time with live acid, the acid leaks off into the matrix, where its reaction has much less effect on the stimulation than it would if it spent on the fracture faces. The equilibrium technique provides a way to extend the acid contact time with the fracture faces without extending the fracture dimensions. The equilibrium technique takes advantage of the difference between the fracture propagation pressure (the pressure at which a fracture extends) and the minimum in-situ stress (the pressure at which a fracture opens or closes). To extend the acid contact time after the desired fracture dimensions are created by high-rate injection (at or above fracture propagation pressure), the injection rate is reduced to match the leakoff rate from the fracture faces. As the injection and leakoff rates come into equilibrium, the pressure drops below propagation pressure. f the pressure in a fracture is maintained above the minimum in-situ stress (or closure pressure) but below the fracture propagation pressure, the fracture will be held open without significant further extension. The pressure difference between fracture propagation and closure is called net fracture pressure. The fracture overpressure is the difference between the corrected instantaneous shut-in pressure (ISIP) and the minimum in-situ stress. The corrected ISIP is measured shortly after shut-in when the fracture remains open and nearly ceases to propagate.3 ,6,7 A typical overpressure in the Denver Unit is 500 psi. Keeping the fracture open throughout the low-rate injection portion of the equilibrium treatment allows the fracture faces to be stimulated vigorously. In cool dolomite formations, live acid pumped into the open fracture under equilibrium conditions reaches the fracture tip. Because the stimulation is kept in zone and live acid is allowed to reach the entire fracture area, the equilibrium technique optimizes acid use in both vertical and lateral placement. 26 G MM RAY SONIC LOG I--t-----Ir-::::B '; 4 700 I--+-r---H......-j 4800 FIRST POROSITY' MARKER ---4~;.-----IH-+~--:; iF---1 I-+--- f.iit--+--l 4900 ::~ . . MAIN PAY· M RKER ---4f-- :;,: -++~-~~--i SCALE IN FEET [ 50 o ~~ ;~ 1 ~ { 1--+------1 ,,:::+--1 5000 I- --- ~ --l 5100 1--+-1- ':,.-+--15200 Fig. 2 Denver Unit San Andres type log. Estimation of Leakoff Rate. After the fracture of the desired dimensions is created, the injection rate must be reduced to match the leakoff rate from the fracture, which can be estimated with a well-known equation.3.8 Eq. 1 can be used to estimate the total leakoff rate at any time t after the fracture has been created: qt t) = r o t da (1) o .Jt-TD a) where A = fracture area (ft2), Ao =A to), c t = total in-situ leakoff coefficient ft/minl/z), qt(t)=leakoff rate at time t (ft3 Imin), t=time (minutes), t = time to create the fracture (minutes), and TD a)=dimensionless time function. By knowing the leak-off rate at any time after the fracture has been created, one can calculate the volume of fluid that leaks off. This calculation enables the treatment designer to determine the equilibrium pump-rate schedule and the total acid volume required for the desired total acid contact time. Fracture Field Testing. Fracture field (minifracture) testing plays an important role in the design and execution of fracture treatments.3.6·8 Minifracture testing determines the in-situ fracturing parameters, such as the minimum in-situ stress, fracture propagation pressure, overpressure, and in-situ total leakoff coefficient. 3,6,7 Fig. 3 is an example of a fracture field test for a Denver Unit well. A predetermined volume of brine was pumped into the SPE Production Engineering, February 1991  40 5000 OJ 0 -i 35 -i 0 c - PUMP RATE : J: e ããã. BOTTOMHOLE PRES. -4500 0 30 r IT1 0 -0 QI ::n 25 IT1 If) (fI (fI ::> c 20 -4000 ::n ~ '. ''''''''' IT1 W : : - ISIP = 3910 PSI , , , « 15 ~,\ T a:: - . , ::Ii . ::> 10 ~ 3500 ::l a. N 5 '0 , ' 0 3000 0 10 20 30 40 50 60 TIME min) Fig. 3-Fracture field test on Denver Unit Well 4130. well at a rate sufficient to create a fracture. At shut-in, an ISIP of 3,910 psi (bottomhole) was observed. For a stationary fracture, the ISIP approximates the fracture propagation pressure. The well pressure was monitored until enough data were collected for an evaluation of the total in-situ leakoff coefficient. A fracture-reopening test at a low constant rate was also performed 6  7 to determine the upper-bound estimate ofthe minimum in-situ stress. Fig. 4 shows the results of the fracture-reopening test. The upper bound of the minimum in-situ stress, the point at which the pre~sure vs. time plot deviates from the compressibility-controlled straight line, was found to be 3,540 psi (bottomhole). A lower bound was estimated to be 3,400 psi from the flowback shown in Fig. 4. After the fracture was reopened, the well was flowed back at a relatively constant rate. The rate, however, was not recorded. A lower inflection in the pressure-vs.-time plot was identified at about 3,400 psi (bottomhole).7 The lower-bound estimate of the minimum in-situ stress was only slightly below the upper-bound estimate identified by the reopening test. This is consistent with the indication of closure seen from the extended pressure falloff in Fig. 3. The overpressure for this well was thus estimated to be 370 psi (3,910-3,540 psi). The total in-situ leakoff coefficient was then calculated to be 0.0005 ft/(min) h with the local-pressure-match technique described in Ref. 7. The in-situ measured fracture parameters are then used to design the fracture stimulation treatment for the well. esign rocess The general procedure for designing an equilibrium treatment should be adequate for most cases, but some minor modifications may be required in special situations. 1. Estimate the minimum injection rate required to create a fracture to ensure that a fracture is actually created. 2. Obtain fracture field test data and laboratory data to define the in-situ fracture design parameters and rock deformation properties, 3. Run an overpressure-calibrated fracture-geometry program to determine the volume required to create a fracture of the desired dimensions. 4. Establish treatment-pressure guidelines to prevent further extension of the fracture during equilibrium etching and to ensure that the fracture is open. 5. Using the leakoff equation, determine the expected pumping schedule, total treatment volume, and required pumping equipment. Fracture field test data should be obtained, preferably from a test on the subject well. If a fracture field test is not practical for that SPE Production Engineering, February 1991 25.-------------------------------~~ m o C 20 E 3800 o 3: :r o r   -  6 co n 15 2 3600 :0 v> v> C ::0 3200 N o 2 TIME (min) 3000 5 Fig. 4-Fracture reopening test on Denver Unit Well 4130. .0r------------------.------~~----__ l et .0005 lt/.r.iln I. R : 80 ft 80 70 ~ 60 o D ' :> 50 - .0 w ' 30 - . ::> -20 10 - TOTAL ACID VOL. : 6170 9 01 ã 2000 TOTAL ACID CONTACT HUE E 240 Min. FRAC EXTENSION PRES. 1500 <> , < ---------------------------- 1000 .0 - _. TeG.PRES. -PUMP RATE TIME (min.) Fig. 5- Typical equlllbrium-acid-fracture design. well, data may be applied from a test on a nearby well with the same rock and reservoir properties. The minimum in-situ stress, fracture propagation-pressure and overpressure, and in-situ leakoff coefficient are all required. Poisson s ratio and Young s modulus from laboratory tests on core material are also needed. The fracture field test and laboratory data are then used in ENERFRAC to determine the time and volume required (for a given injection rate) to create a fracture of the desired dimensions. Establishing the treatment-pressure guidelines is one of the most important steps in the treatment design. Once the fracture with the desired dimensions is created, the pressure must be controlled to prevent further fracture extension. Pressure is controlled by keeping the pressure below the fracture extension pressure. For continued fracture etching, however, the pressure should be kept above the fracture closure pressure (minimum in-situ stress). It is common to set the treatment pressure about halfway between the propagation and closure pressures. The pressure guidelines must be strictly followed once the fracture is created. The pumping schedule for the low-rate, equilibrium portion of the treatment is only a guide to help the pumper maintain the treatment pressure between the fracture extension and closure pressures. The pump-rate schedule, however, can be quite accurate if the fracturing parameters are accurately determined from the minifracture test. A program that uses the equation that describes the leakoff rate from a fracture can be used to generate a pumping schedule for the low-rate or equilibrium portion of the treatment. Intended fracture dimensions and a leak-off coefficient derived from a fracture field test are used to predict the leakoff rate as a function of time. The leakoff rate from a fracture of given dimensions may be estimated with Eq. 1, where the leakoff rate at a given time is equal to twice the fracture area multiplied by the fluid leakoff coeffi- 27  TEMPER TURE SCALE ( F) qq 103 107 III I I I I .':. 14---1---++ , --1--1-  ~T~~~~~ ã -;; TEM ERATURE e--I- ··c.:.:Jj---- ----14750' ::~ :~ ~- -- - ãã E ~~:.. TEMPERATURE f>--h ,,:::~---- ----1 4600 ,:.: ã ~~ _ NEUTRON LIXi ;~~~~~ ::: ' (POOOSITY) 4650 //////// GIIS On.. COHTflC.T Fig. 6-Postfracture gamma ray/temperature log for Denver Unit Well 6716. cient and a dimensionless time function divided by the square root of time since the start of injection. 8 The estimated leakoff-rate information is used three ways. The first use is for estimating the acid volume to be injected at low rates after the desired fracture dimensions are created. This volume is determined by integrating the rate-vs.-time curve for a specified total acid contact time. Second, the expected pumping schedule is determined from the required acid volume and the leakoff rate as a function of time. Finally, the leakoff-rate information is used to determine the type of pumping equipment required to perform the job. Because the leakoff rate declines with time, the pump rates at the end of the treatment can, for some wells, be as low as 5 gal/min at the end of the job. The equipment must be capable of pumping at the rates needed to match the leakoff rates throughout the entire equilibrium portion of the treatment. Example Design. Fig. 5 shows a typical example of an equilibriumacid-fracture treatment for the Denver Unit. A Young's modulus of 6,000,000 psi and a Poisson's ratio of 0.3 were determined from static-loading laboratory core tests. A totalleakoff coefficient e,) of 0.0005 ft/(min) 'h, an overpressure of 500 psi, an n of 1, and a viscosity of 1 cp were also used as input data. The constant n is the flow-behavior index. The desired pump rate of 2 bbl/min was used to create an uncontained radial fracture with an 80-ft radius (l60-ft total wellbore height) by injection of 2,940 gal of acid. This amounted to 35 minutes of acid contact time. The rate was then reduced so that the pressure dropped below the fracture extension pressure but stayed above the closure pressure. After the fracture was created, an additional 3,230 gal of acid was pumped into the open fracture at rates that matched the leakoff rate of the fracture. The pump rates for this portion of the treatment ranged from 40 to 10 gal/min (Fig. 5). Acid was pumped for an additional 205 minutes after the fracture was created for a total acid contact time of 240 minutes. 28 o .a .a 1000~---- ---- ----r----~ 10~---- ----4 ----4~----~ u C IL D U Oil W TER TOTAL FLUID 1~ ~ ~ ~ ~ 1981 1983 1985 1987 1989 YEAR Fig. 7-Productlon curve for Denver Unit Well 6716. Field-Application Considerations. Several items should be considered during the planning stages to ensure that the job is a technical and operational success. In cold dolomites like the Wasson San Andres, it is desirable to maximize the acid contact time. Total pumping times for typical jobs in the Denver Unit range from two to four hours. A maximum time of about 4 hours has been used for several reasons. First, it is operationally favorable if the total treatment time, including setup, production logging (if desired), the actual treatment, and any posttreatment logging, can be completed in daylight. Second, with the extended pump times, the required pump rates can be very low. Rates on the order of 4 to 5 gal/min have been experienced. Many service company pump trucks cannot pump at these rates. One way to solve this problem is to hook up a split-stream manifold, which allows part of the pump's output to be injected into the well while the remainder of the fluid is returned to the tank or transport. When a split-stream setup is used, all the monitoring equipment should be installed between the manifold and the wellhead so that accurate treatment pressure and rate information can be modified. Pressure guidelines play an important role in an equilibrium-acidfracture treatment. The treater and/or foreman supervising the job must know that the treatment pressure is actually within the prescribed guidelines. The treatment-pressure guidelines for the lowrate or equilibrium portion do not include the tubing friction or any other source of friction in the system. Although the friction is low or negligible for many jobs, in some situations the equilibrium rates are above 1 bbl/min and friction pressures can significantly affect the surface tubing pressure. The surface or tubing pressure usually is the only pressure that is monitored. One method of dealing with the frictional effects is to use friction charts for the tubing size used. Another method that was used successfully in the field (which also removes all the friction in the system) is to shut down periodically for 1 or 2 minutes. A brief shutdown to observe the real treatment pressure should not adversely affect the treatment because, in most cases, the fracture will not close in that short a time. In some situations, it is not practical to shut down the pumps, so friction charts must be used. Field Examples The first example discussed in this section shows what can occur when uncontained fracture growth exists and is not taken into account in the treatment design. The remaining three examples pertain to the equilibrium technique: equilibrium rates and pressures during pumping into a stationary open fracture, posttreatment temperature logs for a producer completed with an equilibrium acid fracture, and the injection-profile performance of a CO 2 injector completed with the equilibrium technique. The field examples are SPE Production Engineering, February 1991
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