acid re frac

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SPE 124874

Acid Re-fracturing: Is it a Good Practice?M. Pournik, SPE, M. Mahmoud, SPE, and H.A. Nasr-El-Din, SPE, Texas A&M University

Copyright 2009, Society of Petroleum Engineers

This paper was prepared for presentation at the 2009 SPE ATCE held in New Orleans, Louisiana, 47 October 2009.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

AbstractThe success of acid fracturing depends on the conductivity created and retained under closure stress in addition to the length ofconductive fracture. Majority of acid fracturing treatments show a sharp decline in conductivity with increasing closure stresswith almost no significant conductivity after a short production time. As a result, many wells are re-fractured in order torestore back to the original productivity after the initial fracture. However, the success of these re-fracture treatments has beendiverse with respect to extent of stimulation. The effect of re-fracturing on acid fractured wells has not been studied and theconditions that result in success or failure of re-fracturing operations are not understood.

An experimental study was conducted to investigate the effect of re-fracturing on already acid fractured cores exposed to

closure stress. Indiana limestone cores were acidized with a typical acid system of 15 wt% HCl acid viscosified with a polymerunder typical field conditions. After the first acidizing process, conductivity measurements were conducted on acid-etchedcore faces up to a certain closure stress. While the fracture was kept under the closure stress, a re-fracturing treatment wasconducted under the same conditions as the initial acidizing. The re-etched fractures were once again placed under differentlevels of closure stress and conductivity measurements taken at each stress. Experiments were conducted under differentconditions of leak-off, polymer concentration and closure stress after the first acidizing in order to determine influence of theseparameters on the re-fracturing conductivity.

Acid re-fracturing enhanced fracture face etching, while significantly increased fracture conductivity under closure stress.

However, leak-off, polymer concentration, and closure stress did influence the degree of success of acid re-fracturing. The re-fractured sample with leak-off and lower polymer concentration resulted in the most enhanced fracture conductivity incomparison to the initial acid fracturing process.

IntroductionAcid fracturing is a well stimulation process in which acid dissolution along the face of the hydraulically induced fracture isexpected to create lasting conductivity after fracture closure. However, conductivity after fracture closure requires that thefracture face is non-uniformly etched by the acid while the strength of the rock is still maintained at high levels to withstand

the closure stress. While the etched pattern has a dominant influence on the resulting fracture conductivity at low closurestresses, conductivity is more dependent upon the strength of fracture face asperities as the surface features along the fracturefaces are crushed under higher closure stresses.

There are many conditions that result in failure of acid fracturing treatments either due to lack of conductive path orclosure under stress. The primary failure conditions are uniform face dissolution, soft formations, excessive softening offormation, low acid solubility of fracture face, and insufficient acid reaction time with fracture face (Fredrickson 1986). Inorder to overcome some of the shortcomings of standard acid fracturing operation, closed fracture acidizing technique was

designed to allow acid flow through existing fractures below fracturing pressure in a channeling manner. As the acid tends tofollow areas of higher solubility and least flow resistance developed from the initial fracture, channels with wider width and

better conductivity which can remain open under even high closure stresses are developed. The process creates additionaluneven etching or pillars that keep fracture from premature collapse. While the small portion acidized provides the conductiveflow path, the remaining large unetched area holds the fracture open under closure stress without completely collapsing theetched channel. In addition, acid flows more rapidly and dissolves more formation face due to the closed nature of fracture.The process should also be considered for formations that greatly soften with acid as an initial fracture created by non-acidsystem can be acidized under closure to develop channels which are supported by large unsoftened and undamaged areas

(Fredrickson 1986). The use of closed fracture acidizing is usually for situations where a conductive fracture is not created by

the initial fracture acidizing. The conditions that do not usually produce sufficient productivity enhancement following a


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fracture acidizing are: formations that are readily soluble in acid resulting in uniform dissolution of fracture face, formationsthat are etched in an uneven manner, however the etched flow channels are crushed under closure due to either soft formationas in chalk formations or excessive acid leak-off softening the fracture faces, and also for formations that have low solubilityin acid, resulting in acid-insoluble fines remaining on the fracture face (Taylor et al. 2006) and restricting additional acidreaction needed to create permanent conductivity. In such cases, CFA is a good practice as it can create wide grooves orchannels along the fracture face that remains open under severe closure conditions. The technique is also applicable for

naturally fractured formations and in previously created fractured including propped fractures. However the primary

application is as a final matrix acidizing stage following an acid fracturing treatment (Kalfayan 2000).Several chalk formations in the Middle East where re-treated with acid after four to five years of production with excellent

production increases (Fredrickson 1986). The initial acid fracturing treatment had resulted in production rate of 500 BOPDwhich decreased to 300 BOPD after five years. The acid re-fracturing treatment resulted in initial production rate of 3,000BOPD which decreased to 1,800 BOPD after one year. While many fields produced excellent results to closed acid fracturing

treatments, laboratory tests indicate that the process will not work in all formations (Fredrickson 1986). The greatestdocumented use of closed fracture acidizing has been in San Andres formation of West Texas which is a relatively uniform

formation. Both experimental and field applications have shown significant increases in conductivity with over one order ofmagnitude increase. In addition, uniformly etched, soft chalk formations, especially Austin chalk, have been successfullytreated with closed fracture acidizing. The treatments have resulted in enhanced etched fracture conductivity and renderedfracture face less susceptible to crushing. Results clearly indicate that closed fracture acidizing is a viable option forformations that etch uniformly or ones with etched fracture faces that crush under closure stress (Anderson and Fredrickson1989). Another successful closed fracture acidizing treatment was performed in a tight limestone formation which was also

moderately soft and highly acid soluble (Sizer et al. 1991). After failure of conventional acid fracturing treatments in thepredominately limestone with dolomite streaks formation of Lisburne field in Prudhoe Bay, laboratory and field applications

of closed fracture acidizing using emulsified acid showed increased improvements with sustained high production rates(Bartko et al. 1992). Following a closed fracture acidizing treatment, flow regime could be controlled by flow throughchannels or wormholes or flow through both fracture and wormholes, which are both very different from conventional openfracture acidizing with flow through fractures only. Models of such flow regimes have been developed and have shown theimprovements in production rates as a result of the new flow paths resulting in much larger flow conductivities in excess of100,000 md-ft (Soliman et al. 1990).

Another method to enhance production improvements in slow reacting formations has been the equilibrium acid fracturing

where after a fracture of desired dimensions is created, acid injection is continued at a lower rate below fracture extensionpressure until it matches fluid leak-off rate which allows for increased acid contact time with fracture faces and improvedetching and fracture conductivity. The method is different from closed fracture acidizing which injects acid into a closedfracture, tending to concentrate on stimulating near wellbore due to much slower rates required to maintain a closed fracture.

The effectiveness of equilibrium acid fracturing treatment was proven in field applications in a layered San Andres dolomiticformation in Denver unit which showed larger production rates as compared to traditional acid fracture treatments. The

average increase in production rate was about 29 compared to 15 BPD for other stimulation techniques for over 30 differenttreatments. The increased production rate was sustained for the 9 months of available data (Tinker 1991).

Many times conductivity of acid fractured reservoirs declines rapidly to the initial pre-stimulated condition and re-fracturing treatment is considered. However success of re-fracturing treatments is not well known. The decision to re-fracturedepends on permeability, initial fracture length, and conductivity. Usually presence of existing permeability and effectivefracture length do not warrant re-fracturing treatment (Aud et al. 1992). A successful re-fracturing treatment should createhigher fracture conductivity and/or penetrate an area of higher pore pressure than the previous fracture. Generally, shorteffective fracture lengths are good candidates for re-fracturing, while fractures with high infinite acting conductivity andpenetrating deep into reservoir are considered poor candidates for re-fracturing (Elbet and Mack 1993). There are threedifferent categories of previously fractured wells that might benefit from a re-fracturing treatment: wells with insufficient

production increase from first fracture, wells with production fallen below projected decline level after the first fracture, andwells producing at projected decline rates. While the reason for failure of first fracture treatment for the first two categoriesshould be determined before decision on re-fracturing, possible improvements in production from a re-fracture should bedecided on for the third category of fractured wells. A good understanding of the reservoir response to the previous fracturingtreatment is necessary to design a successful re-fracturing job. The time frame and severity of reduced production rate willhelp in determining the degree of modification to the original fracture for wells with faster than predicted production declinerate (Conway et al. 1985).

Literature survey of re-fracture stimulation shows mixed success with the findings that a successful re-fracturing treatmentfor tight formations requires increased fracture length while enhanced fracture conductivity is needed for permeable reservoirs.Also good wells with high initial productivity make better candidates for re-fracturing with higher success rate (Reese et al.1994). This result may indicate that re-fracturing to increase fracture conductivity are more likely to succeed as conductivitycan be achieved more cost effectively and more importantly the permeability of these wells are greater in the first place.However re-fracturing of depleted reservoirs is not a commercially viable option (Reese et al. 1994). Parrot and Long (1979)

observed both successful and unsuccessful re-fracturing treatments in tight gas formation of Wattenburg field and concludedthat wells with greater initial production make better candidates for re-fracturing. Hunter (1986) showed that re-fracturing


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success was controlled by initial treatment size and reservoir flow capacity in the tight gas field of Oak Hill Cotton Valley. Re-fracturing with increased proppant volume did not dramatically improve productivity. Pearson et al. (1990) observedsignificant production enhancement by the re-fracturing treatments in the Kuparuk River Unit with 20-80 md permeability. Re-fracturing of 88 wells showed an average increased productivity of over 400 BOPD per well which was achieved withincreasing amount, size, and strength of proppants, enhancing the fracture conductivity. Re-fracturing of several permeablereservoirs in different fields that were initially fractured with low proppant concentrations have proven successful in

improving their waterfloods (Bagzis 1989; Fleming 1992; Griffith and Madison 1988; Olson 1991). Branch and Drennan

(1991) also observed successful re-fracturing treatments of damaged wells in the Norge Marchand Waterflood Unit. Theimprovement was from use of higher strength proppants that minimized proppant crushing which occurred after the initialfracturing. Another success was in the re-fracturing of AWP Olmos Field which overcomes the effects of poor initial treatmentdesigns and mechanical failures through use of larger size treatments and higher sand concentrations (Venditto et al. 1986).

Experimental StudiesThe experimental apparatus and procedure were designed to enable experiments to be conducted at conditions representing

field treatments more closely and accurately. Detailed information about the experimental apparatus and procedure can befound in Pournik et al. (2007). The experimental apparatus was designed to accommodate larger rock samples, higher injectionrates, and higher temperatures, while minimizing corrosion and mechanical failure. The core samples are placed inside amodified API conductivity cell that has a body dimension of 10 in. 3 in. 8 in. with a 7 in. 1 in. opening, allowingthe use of rock samples with up to 3 in. thickness. A pump with a maximum operating pressure of 2,200 psig and a maximumflow rate of 1.04 L/min was used to allow injection at rates that scale appropriately to field conditions. Cylindrical ceramic

radiant heaters were wrapped around the flow line to heat the fluid before entering the cell so experiments could be conductedat temperatures similar to field conditions. The heaters allow the fluid to be heated to approximately 300 oF. A back pressure

regulator is installed on the leak-off line to control the pressure drop across the core. This allows the leak-off flow rate to becontrolled at the desired rate to represent field conditions more accurately. Another back pressure regulator is installed on thecell effluent line to maintain the cell pressure above 1,000 psi to prevent CO 2gas from coming out of the solution during theacidizing process.

Experimental Procedure. An experiment consisted of eight consecutive steps: acid etching, rock embedment strengthmeasurement before and after acidizing, etched surface characterization, fracture conductivity measurement, re-acidizing

etching, rock strength measurement and etched surface characterization after re-acidizing, and final fracture conductivitymeasurement. After the core samples were cut, coated, dried in oven at 120

oC for 2 hours, and weighed, rock strength

measurements were taken on the back side of both fracture faces. Then the acid etching process was conducted on the samplesunder the specified condition with a specified fracture width and no closure stress on the fracture. After the acidizing, the

samples were removed from the API cell and surface profile characterization was conducted using the profilometer. Thesamples were placed in an oven at 120 oC for 2 hours and then weighed. Afterwards, rock strength measurement wasconducted on both fracture faces. The etched samples were placed back into the API cell and fracture conductivity

measurements were taken. Once conductivity at the highest closure stress was measured, a cell holder was used to lock thecores in the API cell in place under the closure stress.

The second stage of acid etching was conducted on the samples while the closure stress was maintained on the fracture,hence the fracture faces were touching each other and fracture width was much smaller than the initial acidizing step. After thesecond acidizing process, core samples were removed and the same steps as done after the first acidizing were performed onthe core samples. The final step was the measurement of fracture conductivity on the re-acidized samples.

Materials. Indiana limestone samples with average permeability of 4 md were used in all experiments. The core sampleswere cut into parallelepiped shape with the ends curved into half-circles to fit the API cell. The overall length of the cores wasapproximately 7.11 in., with a width of about 1.61 in. and a thickness of about 3 in.. The cores were covered on the sides witha silicone rubber compound inside a mold of the API cell in order to secure a perfect fit of the core in the cell and to prevent

any bypass of the acid around the sides of the core samples. The fracture face was kept as smooth and flat as possible byensuring straight and smooth cut of the fracture face.

Gelled acid was used in all experiments and it was prepared with HCl acid (nearly 31.45 wt%), a corrosion inhibitor, andan acid-soluble polymer. Tap water (TDS = 500 mg/l) was used to dilute the acid to desired concentration. Acid concentration

was kept constant at 15 wt% HCl. The corrosion inhibitor was used at 0.2 wt% and polymer concentrations of 1.5 wt% and 2.5wt% were used. The corrosion inhibitor and polymer were added to the acid such that the final acid concentration was 15 wt%.

The polymer was added to the solution slowly with the addition taking about two hours. The solution was mixed using both amagnetic stirrer at the base of the fluid tank and an impeller mixer which was situated in the middle of the fluid level. Themixing continued for about one hour after the addition of polymer to ensure proper mixing and hydration of polymer in acid.About 150 ml sample of prepared acid solution was taken for viscosity and fluid analysis. The fluid was mixed all the timeuntil the acid injection process was finished. The total amount of acid prepared was 18 liters as the capacity of the tank wasonly about 20 liters and about 15 liters of acid injected during the acidizing process.


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Analysis Techniques. The apparent viscosity of acid solutions was measured using a M3600 viscometer. All themeasurements were conducted at temperature of 28C and atmospheric pressure as a function of shear rate in the range of 0.1-

300 s-1

. The effluent samples from both the fracture flow and leak-off direction were analyzed for concentration of calciumions, HCl concentration, pH, and density. Calcium ion concentration was measured by the Atomic Absorbance Spectrometerflame type (A Analyst 700). The concentration of HCl acid in effluent was measured by titration with a known concentrationof sodium hydroxide using the autotitrator, while pH was measured using Ross pH electrode. The density was measured with aDMA-35N densimeter at 25

oC.

Acid Etching Apparatus. The acid etching apparatus is shown in Fig. 1. Cores were placed in the cell and a fracture gap ofabout 0.12 in. was kept between the two fracture faces using metallic shims. The cell was placed in a vertical direction suchthat fluid flows upwards through the fracture to prevent gravity effects on the etching of the core faces. After ensuring that thepistons were in place and the fracture width maintained at the desired value, a pre-flush of tap water was injected. Afterachieving the desired cell pressure, leak-off rate, and temperature, acid injection was started. During acid injection,

temperature recordings were taken several times and leak-off volume and leak-off differential pressures were monitored toensure appropriate leak-off rates. Finally, tap water was injected.

Fig. 1 A schematic diagram of the acid etching process.

Surface Characterization.A profilometer was used to characterize the etched fracture faces after the acidizing process. A

profilometer is a precise vertical distance measurement device that can measure small surface variations in vertical surfacetopography as a function of position on the surface. The vertical measurement is made using a laser displacement sensor whilethe sample is moved along its length and width on a moving table. All the experiments used a 0.05 in. measurement interval inthe x and y directions. The resolution on the vertical measurement is 0.002 in., while in the horizontal directions (both x and ydirection), the transducer resolutions are 0.00008 in.

Rock Strength. The hardness or strength of core samples was measured both before and after acidizing, using the rockembedment strength method (Howard and Fast 1970). Rock embedment strength was measured using a ball-point of 0.0625 in.

diameter which is mounted on the upper platen of a hydraulic testing machine. During the test, the ball-point was brought intocontact with the core and, by applying hydraulic load, was embedded to a depth of a quarter of the ball diameter (0.0156 in.).The load required to embed the ball to that distance was recorded from the pressure gauge on the hydraulic load system. Rockembedment measurements were taken at 3 different locations across the fracture both before and after each acidizing process.The initial rock embedment measurements before the first acidizing was taken on the back of the fracture face in order not toaffect the acidizing process, however the strength measurements after each acidizing process were performed on the fractureface in order to determine the effect of acid on the rock strength.

Fracture Conductivity.The fracture conductivity was measured by flowing nitrogen gas between the two acid etched fracturefaces and recording the absolute pressure at the mid-point of the fracture and the pressure drop across the fracture. The cell

was placed in a horizontal direction with horizontal flow of nitrogen gas through the fracture as shown in Fig. 2. Theconductivity cell was placed in a load frame which provided the application of different closure stresses. The closure stress

was applied in increments of about 500 psi, starting from 500 to 3,500 psi, changing the load after approximately 60 minutes.

Heater

Brine Acid

High pressure

pump

Flowmeter

Data

Acquisition

APIConductivity Cell

Back PressureRegulators

Leak-offAcid

SpentAcid


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Nitrogen flow was resumed after placing the first desired load on the cell. The pressure drop across the length of the fractureface (one inch from each end of core, therefore 5.25 inches in total length) and absolute pressure at the center of the fractureface were measured during the experiment. After around 60 minutes, these pressure readings were recorded at five differentflow rates after the pressure stabilized at each flow rate. The flow rates ranged from about 5 to 25 liters/min with flowmeasured using a mass flow meter. Forcheimers equation for turbulent gas flow through a porous medium was used tocalculate the fracture conductivity from flow rate and differential pressure measurements (Pursell et al. 1988).

Fig. 2 A schematic diagram of the fracture conductivity setup.

Results and DiscussionA set of four different experiments were conducted using different acid composition, leak-off condition and closure stressloading in order to determine the effect of these parameters on acid re-fracturing process which includes effect on dissolvedvolume, fracture etching pattern, rock strength, and fracture conductivity. Indiana limestone samples were used with the firstacid fracturing process under a fixed 0.12 in. fracture width. All the acid etching steps were conducted at an injection rate of 1L/min with injection time of 15 minutes, fluid temperature of around 130 oF, and backpressure of 1,000 psi. The leak-off fluxwas fixed at nearly 0.0035 ft/min except for one experiment which had no leak-off. The acid used was 15 wt% HCl gelledacid with 1.5 wt% polymer except one experiment which used 2.5 wt% polymer. All the other experimental conditions werethe same except one experiment with the closure stress taken up to 3,500 psi rather than 2,500 psi after the first acid fracturing

process. The acidizing conditions and conductivity load for all the experiments are summarized in Table 1.

TABLE 1SUMMARY OF EXPERIMENTAL CONDITIONS.

Case Polymer Concentration, wt% Leak-off Flux, ft/min Closure Stress,

a

psi

Base 1.5 0.0035 2,500

No Leak-off 1.5 0 2,500

Higher Polymer 2.5 0.0035 2,500

Higher Stress 1.5 0.0035 3,500

a. during acid re-fracturing.


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Acid Etching Process. During the acid etching process, the pressure profiles in the fracture flow direction and leak-off

direction were monitored using LabVIEW program. The pressure drops across the fracture flow and leak-off line for onesample experiment are shown in Figs. 3 and 4, respectively, for the first acid etching and the re-acid etching process. Thepressure profiles were similar among all the different experiments, indicating leak-off, polymer concentration and closurestress do not significantly alter the pressure profiles during acid etching. Pressure drop across the fracture flow line during there-acid etching process clearly shows a substantial decrease in pressure drop during the acid etching as the fracture width

grows significantly. However, due to the much smaller fracture width during the re-acid etching process (fracture width five to

ten times smaller than first acidizing), the pressure drop across the fracture is much higher during the re-acidizing process. It isinteresting to note that by the end of both acidizing processes, the pressure drop across the fracture are almost the same, whichsuggests that the fracture widths are almost the same at the end (there will be more discussion of this on the etched widthsection). On the contrary, there is much greater pressure drop across the leak-off line initially during the first acidizing process.The reduced leak-off pressure drop during re-acidizing process is due to enhanced permeability after the first acidizing and

also the increased pressure drop in the fracture flow direction, resulting in more tendency for fluid to leak-off.

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0

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10

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20

25

30

35

00:00 02:53 05:46 08:38 11:31 14:24

Time, min:sec

PressureDropAcrossFracture,psi

First Acidizing

Re-Acidizing

w = 0.12 in.

w = 0.03 in.

q = 1 L/min

Fig. 3 Pressure drop across fracture flow line for the first and re-acidizing process for base case.

0

5

10

15

20

25

30

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00:00 02:53 05:46 08:38 11:31 14:24

Time, min:sec

PressureDropAcrossLeakoffFlow,psi

First Acidizing

Re-Acidizing

Flux = 0.0035 ft/min

Fig. 4 Pressure drop across leak-off line for the first and re-acidizing process for base case.


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During the pre-flush with tap water to heat the system to required temperature before acid injection, the pressure dropacross the fracture was also measured. This pressure drop was used to calculate the fracture width using the Hagen-Poiseuillelaw for slit flow:

L

phwq

=

3

12

1 (1)

where wis the fracture width, his the thickness of fracture face (1.61 in.), andLis the fracture length across which pressure

drop is measured (5.25 in.). The calculated fracture width before the first and the re-acid injection are shown in Fig. 5. Thefracture width before first acidizing process are very similar for all tests with width of nearly 0.13 in. which is close to thefixed width of 0.12 in., however the fracture width before the re-acidizing process is much smaller at about 0.015 in. except forthe test with no leak-off which has much higher fracture width of 0.03 in.. The much smaller fracture widths during the re-acidizing process explain the higher pressure drop across the fracture as compared to the first acidizing. The higher etchedwidth of no leak-off experiment before the re-acidizing process is consistent with the conductivity measurements after the firstacidizing which shows higher final conductivity for the no leak-off experiment.

Base CaseNo Leak-off

2.5 wt% Polymer3,500 psi Stress

First Acidizing

Re-Acidizing

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

FractureWidthbeforeAcidInjection,

in.

Initial width = 0.12 in.

Fig. 5 Fracture width before acid injection of the first and the re-acid for all experiments.

Etching Pattern.Scanning of etched fracture faces with the profilometer provides 3D profile images of the etching pattern. A

sample of etched profiles obtained from the profilometer are shown for both the first acidizing and the re-acidizing process inFigs. 6and 7, respectively. After the first acidizing, the etched profiles show little amount of etching with almost the sameamount of etching across the fracture face (uniform etching). There seems to be more etching on the fracture face A with a

rather deep valley at the inlet of the fracture with also rougher etching compared to face B. However, after the re-acidizingprocess, much more etching has occurred with rough etching pattern. The etched profile shows many locations with very deepetching, representatives of wormholes visible on the fracture face. It is interesting to note that more etching is shown now on

fracture face B which is opposite to the results from the first acidizing. All the experiments indicate the re-acidizing processresults in much more etching with a rougher etched profile and greater wormhole density in the leak-off direction as compared

to the first acidizing.


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Depth, in. Depth, in. Depth, in.

Fig. 9 3-D etched profiles after re-acidizing process for base case, no leak-off, and 2.5 wt% polymer samples, respectively.

Etched Width.Based on the etching pattern analysis, the etched fracture surface volume was calculated from the difference in

surface volume between the before and after acidizing samples. From the etched surface volume, the etched surface width wascalculated using the cross sectional area of the fracture. While the actual etched surface width values after both the first

acidizing and the re-acidizing process are shown in Fig. 10, Fig. 11 shows the ratio of etched surface widths after the re-acidizing process to the first acidizing process. Clearly there is much more etched surface width after the re-acidizing ascompared to the first acidizing with ratios ranging from two to seven. This is consistent with the etched profiles shown in Figs.6 and 7. Also the results are consistent with etched patterns showing more etching for face A after the first acidizing while faceB was etched more during the re-acidizing process, resulting in larger ratios of etched surface width between re-acidized andfirst acidized process for face B. The experiment with higher load applied after the first acidizing shows similar extent of

etching between the two faces as the fracture face was completely crushed after the first acidizing and the etching during there-acidizing process were very similar on the two faces. Based on initial fracture width of 0.12 in. kept open during the first

acidizing, the average fracture width at the end of first acidizing was around 0.15 in. compared to average fracture width ofaround 0.12 in. after the re-acidizing process. The close match between final etched surface widths after both acidizingprocesses explains the almost same pressure drops across the fracture at the end of each process as was observed in Fig. 3.

There are significant effects of leak-off and polymer concentration on etched surface widths as shown in Figs. 10 and 11.There is significantly greater enhancement in etched surface width during the re-acidizing process with leak-off allowed as theleak-off enables more acid to be transported to the fracture face and dissolve fracture face. There was less improvement inetched surface width during re-acidizing process with increasing polymer concentration as the higher polymer concentration

reduces acid transport to fracture face and reduces etching. The effect of higher closure stress applied after the first acidizingwas to reduce the amount of etching during the re-acidizing process as the fracture was almost completely closed and therewere insufficient fracture flow areas open for acid dissolution. There was no significant change in etched width during the firstacidizing with changing leak-off condition or polymer concentration.

Base CaseNo leak-off

2.5 wt%

Polymer 3,500 psi

Stress

First Acidizing

Re-Acidizing0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

EtchedFractureWidth,in.

Fig. 10 Etched fracture width from surface profile after the first acidizing and re-acidizing process for all experiments.

Base Case No Leak-off 2.5 wt% Polymer


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Base CaseNo leak-off

2.5 wt%

Polymer 3,500 psi

Stress

Face A

Face B0

1

2

3

4

5

6

7

RatioofRe-Acidized

EtchedWidthto

FirstAcidizedEtchedWidth

Fig. 11 Ratio of etched fracture widths after the re-acidizing process to the first acidizing process for all experiments.

Acid Viscosity. Viscosity measurements were conducted on prepared acid solutions in addition to acid concentration anddensity measurement. Acid concentration and density for most of the experiments for the first acidizing and re-acidizingprocess are shown in Table 2while viscosity of gelled acid with 1.5 wt% and 2.5 wt% polymer are shown in Fig. 12. All

measured properties of live gelled acid are similar to each other among different experiments which indicate that the gelledacid was prepared consistently. All gelled acids behaved as non-Newtonian shear-thinning fluids over the range of shear ratestested. The apparent viscosity can be described using the power-law model, and the constants nandKare given for the twodifferent polymer concentrations in Table 3.

TABLE 2PROPERTIES OF INJECTED ACIDS.

Base Case No Leak-off 2.5 wt% Polymer

First Acid Re-Acid First Acid Re-Acid First Acid Re-Acid

Density,ag/cm3 1.066 1.069 1.065 1.067 1.069 1.068

HCl Concentration, wt% 14.6 14.5 14.4 14.7 14.8 14.4

a. measured at 25C.

y = 361.19x-0.4063

R2= 0.9976

y = 101.17x-0.3433

R2= 0.9739

1

10

100

1000

1 10 100 1000

Shear Rate, s-1

Viscosity,mPa.s

1.5 wt% Polymer

2.5 wt% Polymer

Fig. 12 Viscosity of original acids with 1.5 and 2.5 wt% polymer concentrations.


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TABLE 3POWER-LAW CONSTANTS AS A FUNCTION OF POLYMER CONCENTRATION.

Polymer Concentration, wt% n K, g/(cm s2-n

)

1.5 0.66 101.2

2.5 0.59 361.2

Effluent Analysis. The calcium concentration after the first acidizing and the re-acidizing process for one experiment isshown for the both the fracture flow and leak-off direction in Fig. 13while HCl concentration in both directions is shown inFig. 14. Majority of experiments showed similar trends of concentration change. Both fracture and leak-off effluentconcentrations of calcium and HCl indicated more etching during the re-acid etching as the calcium concentration was higherwhile HCl concentration was lower during re-acidizing. However the concentrations in the fracture flow direction of first acidand the re-acid were much more similar than concentrations in the leak-off directions. The similarity of concentrations in the

fracture flow direction suggests that most of the increased etching during the re-acidizing process is due to the leak-offprocess. The increased dissolution of calcium and spending of HCl acid is as a result of smaller fracture width during the re-acidizing process which allows higher acid diffusion rate to the fracture face.

0

5,000

10,000

15,000

20,000

25,000

30,000

0 2 4 6 8 10 12 14 16 18 20

Time, min

CalciumConcentration,mg/l

.

First Acidizing - Fracture FlowFirst Acidizing - Leak-off Flow

Re-Acidizing - Fracture Flow

Re-Acidizing - Leak-off Flow

Fig. 13 Calcium concentration in the fracture and leak-off flow during the first and re-acidizing process for one experiment.


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14

16

0 2 4 6 8 10 12 14 16 18 20

Time, min

HClConcentration,wt%

.

.

First Acidizing - Fracture Flow

First Acidizing - Leak-off Flow

Re-Acidizing - Fracture Flow

Re-Acidizing - Leak-off Flow

Fig. 14 HCl concentration in the fracture and leak-off flow during the first and re-acidizing process for one experiment.

Rock Strength.The measured rock embedment strengths before and after each acidizing process are shown in Fig. 15. All therock strengths after the first acidizing process except for face B of no leak-off experiment, are much lower that the initial rockstrengths with an average decline of about 25% in rock strength. The reduction in rock strength following the first acidizing

process are consistent with results from other studies on acid fracturing (Nasr-El-Din et al. 2008; Abass et al. 2006). However,the majority of experiments indicate stronger fracture faces after the re-acidizing process with an average increase in rock

embedment strength of also 25% compared to the first acidizing values. Face B of the no leak-off experiment and theexperiment with higher load show an opposite trend with decreasing rock embedment strength following the re-acidizingprocess. In general, rock strength measurements show that while the first acidizing weakens the fracture face, the following theclosure stress applied on the fracture face after the first acidizing, the weaker sections of fracture face are crushed, leaving thestronger points, which are less weakened during the re-acidizing process, resulting in a stronger fracture face after the re-acidizing process as compared to the first acidizing. While the re-acidizing process might have reduced the rock strength to thesame extent or even greater than the first acidizing, however due to presence of stronger areas of fracture face for acid attack,

rock strength measurements do not indicate weakening as compared to the fracture face strength measurements after the firstacidizing before any closure stress application. The rock strength variations between faces A and B are consistent with theextent of etching. In general, fracture face A shows a greater degree of rock weakening after the first acidizing process asfracture face A was more etched as was shown in etched width values. However, fracture face B in general shows greaterdegree of change in rock embedment strength during the re-acidizing process as face B was etched more than face A in the re-acidizing process.

The effect of leak-off on rock strength was to increase the weakening of fracture face after the first acidizing process,however rock strengths are similar after the re-acidizing process for both cases of leak-off and no leak-off. Increase in polymer

concentration clearly reduced the amount of rock weakening and resulted in stronger fracture face after both the first and re-acidizing process. The effect of applying greater closure stress after the first acidizing was to crush the fracture face more,resulting in much weaker fracture face after the re-acidizing process. As a result, the only experiment with a decrease in rockstrength following the re-acidizing process was the experiment with higher closure stress applied following the first acidizing.The higher closure stress crushed even the stronger fracture face asperities, resulting in a very weak fracture face remainingduring the re-acidizing process.


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SPE 124874 13

Face

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

RockEmbedmentStrength,SRE(psi)

Base Case

A

Base Case

B

No leak-off

A

No leak-off

B

2.5 Polymer

A

2.5 Polymer

B

3500 Stress

A

3500 Stress

B

Initial

First Acidizing

Re-Acidizing

Fig. 15 Rock embedment strength measurement before, after first acidizing, and after re-acidizing process.

Fracture Conductivity. Conductivity measurements after the first acidizing and the re-acidizing process are shown in Fig. 16.

Fracture conductivity under all different closure stresses was increased by about one order of magnitude after the re-acidizingcompared to the first acidizing for all the experiments. The increase in conductivity with the re-acidizing process is consistentwith increased etched fracture width and rougher etching pattern as was shown in Fig. 10 and Fig, 7, respectively. While therewas a clear trend of increasing conductivity with re-acidizing, conductivity decline with closure stress did not show anygeneral pattern. The base case experiment shows lower rate of conductivity decline after the re-acidizing process while the

experiment with no leak-off shows an opposite trend with an increased conductivity decline rate, and almost no difference inthe conductivity pattern with closure stress following the re-acidizing process for the test with higher polymer concentration.

In general, values of conductivity after each acidizing process are similar regardless of experimental condition of leak-off orpolymer concentration.

The effect of leak-off on fracture conductivity indicates different trend following the first acidizing and the re-acidizingprocess. While conductivity is higher for the sample with no leak-off following the first acidizing especially at higher closurestresses, both tests with and without leak-off have similar conductivity values after the re-acidizing process. The experimentwith no leak-off had higher conductivity after the first acidizing as more fracture face was etched (Fig. 10) while fracture facerock strength was maintained at a higher level than the test with leak-off (Fig. 15). However, after the re-acidizing process, theleak-off experiment had much higher etched fracture width compared to the test with no leak-off (Fig. 10), while it had similar

fracture face strength (Fig. 15), which resulted in similar conductivities between the two experiments after the re-acidizingprocess. There was a lesser degree of fracture conductivity change with polymer concentration. Fracture conductivity were

very similar between the two experiments with different polymer concentration, however the decline of conductivity withclosure stress were different. While the higher polymer concentration had lower conductivity decline rate after the firstacidizing, it actually had a higher reduction in conductivity with closure stress following the re-acidizing process. As a result,conductivity values are very similar at lower closure stresses and then the differences in conductivity increase with closurestress. Conductivity pattern is consistent with etching as there were similar etching patterns and etched fracture widths for

different polymer concentrations (Figs. 8-10). There is no clear correlation between conductivity decline with closure stressand rock strength measurements as the sample acidized with higher polymer concentration shows stronger fracture strength

following both the first and the re-acidizing process (Fig. 15). Conductivities after the first acid are very similar between theexperiment at higher load and the one at lower load as there was no difference in the two experiments up to these firstconductivity measurements. The consistency between the two experiments shows the validity of results. There was a largecrack on the fracture face of the higher load experiment after the re-acidizing process which resulted in the very highconductivity that could not be measured due to the pressure transducer limitation. The value of conductivity was over 100,000md-ft and continued to be above 100,000 md-ft up to 2,000 psi of load.


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14 SPE 124874

10

100

1,000

10,000

100,000

0 500 1000 1500 2000 2500 3000 3500 4000

Closure Stress, psi

Conductivity,md-ft

Base Case First Acid

Base Case Re-Acid

No Leak-off First Acid

No Leak-off Re-Acid

2.5 wt% Polymer First Acid

2.5 wt% Polymer Re-Acid

3,500 psi Stress First Acid

Re-Acid

First Acid

Fig. 16 Fracture conductivity after the first acidizing and the re-acidizing process for all the experiments.

ConclusionsThe effects of polymer concentration, leak-off flux, and closure stress on acid fracture conductivity were studied throughexamination of etching pattern, etched width, rock strength, and most importantly final fracture conductivity. Based on theresults obtained, the following conclusions can be drawn:1. Acid re-fracturing resulted in much larger created etched fracture width with rougher etching pattern regardless of leak-off

flux, polymer concentration, or closure stress. However, there was greater enhancement in etching with the acid re-fracturing for acids with lower polymer concentration of 1.5 wt% and also when leak-off was allowed.

2. Acid re-fracturing process also generated larger and denser concentration of wormholes in the leak-off direction, resulting inmore enhanced permeability around the fracture face. However, the acid with polymer concentration of 2.5 wt% did not

show increased leak-off etching due to polymer entrapment in the pore space following the first acid fracturing with smallpore space for acid flow.

3. While the fracture faces showed weaker rock embedment strength following the first acid fracturing, the acid re-fracturingprocess did not result in further weakening of the fracture face. The reason might be due to crushing of the weakersections of fracture face during the first acid fracturing conductivity measurement, leaving the stronger points, which areless weakened during the re-acidizing process.

4. Most importantly, all the acid re-fracturing experiments had an average of one order of magnitude larger created fracture

conductivities at all the different closure stresses tested up to 2,500 psi. While there were not significant differences inconductivities among the experiments, lower polymer concentration and allowing leak-off tended to generate moreenhancements in fracture conductivity during the acid re-fracturing.

AcknowledgementsThe authors would like to thank Ahmed M. Gomaa for performing the acid viscosity measurements. We would also like tothank Dr. A.D. Hill for lending fracture conductivity setup that was used in this study. M. Mahmoud acknowledges a

scholarship from the Egyptian government. Dr. M. Pournik also acknowledges partial funding from Saudi Aramco.


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SPE 124874 15

Nomenclatureh = Fracture height, L, mK = Power-law constant, m/Lt2-n, g/cm s2-nkfw = Fracture conductivity, L

3, m3[md ft]L = Fracture length over which pressure is measured, L, mn = Power-law exponent, dimensionless

p = Pressure, m/Lt2, Pa

q = Flow rate, L3/t, m3/sSRE = Rock embedment strength, m/Lt

2, Pa [psi]

w = Fracture width, L, m = Viscosity, m/Lt, Pa s

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