laboratory tests for acidizing treatments · • spe 22393, "optimizing fracture acidizing...

Laboratory Tests for Acidizing Treatments 1

Laboratory Tests for Acidizing Treatments

Introduction

Laboratory support for stimulation treatment design is available at Halliburton's Duncan Technology Center

(DTC) in Oklahoma, USA, and Halliburton's Pune Technology Center in India.

Testing Capabilities

At Halliburton's testing facilities, technology staff members can perform a variety of analytical tests as

well as tests to optimize stimulation treatment designs. Table 1 lists Halliburton testing capabilities.

Table 1—Halliburton Laboratory Testing Capabilities Analytical Testing Capabilities Stimulation Treatment Tests X-ray diffraction (XRD) Mineralogical evaluations Acid solubility Mechanical rock properties Iron content Permeability and porosity ICP Acid-etched fracture-flow capacity IR and UV spectroscopy Rotating disk reactivity GC Linear flow test Scanning electron microscope (SEM) API fluid-loss and fluid-compatibility testing Bottom sediment, crude, and water analysis Corrosion inhibitor optimization studies

Supporting Documentation

The following papers illustrate the use of laboratory data in treatment design:

• SPE 27621, "The Application of Core and Well Testing to Fracture Acidizing Treatment Design" discusses how performing these tests can result in more effective fracture acidizing treatments.

• SPE 22393, "Optimizing Fracture Acidizing Treatment Design by Integrating Core Testing, Field Testing and Computer Simulations" describes available computer design programs.

• SPE 14654, "Stimulating Carbonate Formations Using a Closed Fracture Acidizing Technique," discusses the use of core testing for matrix acidizing design.


Laboratory Testing for Acid Treatments in Carbonate Reservoirs

When considering an acid treatment in a carbonate reservoir, personnel should perform the following

testing sequence:

• Basic mineralogical evaluations • Mechanical rock properties tests • Specialty core testing

Basic Mineralogical Evaluations

• Acid solubility • XRD • SEM

These basic mineralogical tests allow researchers to characterize the formation and determine if acidizing

is a viable option. This sequence of testing can be performed with a very small amount of formation core

(approximately 20 g).

X-ray fluorescence (XRF) is normally conducted in conjunction with the XRD analysis, which helps

identify trace minerals in the sample. If the core has low acid solubility, the acid-insoluble materials should

be analyzed. This analysis would require a larger sample.

SEM examinations provide useful information regarding the crystalline structure of the carbonate

matrix, the pore structure, and the cementing materials. In addition, SEM can indicate whether acid-

insoluble minerals are uniformly distributed throughout the core or if they are isolated in clasts or

microfractures.

Mechanical Rock Properties Tests

• Unconfined compressive strength (UCS) test • Penetrometer test

The information gained from these tests allows the hardness of the formation to be evaluated, which

indicates the likelihood of etched natural fractures or the possibility of collapse after acidizing.

Compressive strength tests require a core plug with a diameter of approximately 1 in. and a length of at

least 2.5 in. These tests yield Young's modulus and Poisson's ratio. This information is useful in fracture

simulation, and in anomalies on the stress-strain curves that can indicate shear failure along microfracture

planes that may not be visible or may appear to be cemented.


Penetrometer tests use a flat disc sample that is generally 3 in. in diameter and at least

0.5 in. thick. These tests indicate the Brinell hardness, which is used for estimating the conductivity of

an etched fracture under closure stress.

Specialty Core Testing for Fracture Acidizing Treatment Design

• Acid etching test • Rotating disc (RD) test

The flow regime and kinetic parameters (acid type and strength, reaction temperature, and reaction

time) affect the amount of rock removed during the acidizing process. The total amount removed must

be known, and an estimate must be made of the relative amounts removed in each element of the

fracture to optimize treatment designs and predict the magnitude of production improvement. The RD

test helps in predicting the distance that live acid will penetrate along a hydraulically induced fracture

and the amount of reaction that will occur in each segment of the fracture.

While kinetic parameters influence the depth of live acid penetration, formation characteristics dominate the

conductivity resulting from the acidizing process. The mineralogical composition of a formation is probably

the most important factor because the etching pattern is a direct result of the degree of heterogeneity in the

fracture face. Any rock characteristic that contributes to heterogeneity in the formation will enhance

differential etching. The physical and chemical composition of the formation rock will influence the reaction

rate of the acid; as a result, some areas will be dissolved more than others. For example, dolomites and

iron carbonates tend to react more slowly than calcite. In a formation with uniform chemical composition,

varied crystal sizes may also affect the rate of reaction, because the action of acid is a surface

phenomenon.

After a fracture acidizing treatment, the created fracture conductivity is a result of the action of acid on the

exposed formation face. The two primary factors influencing the fracture conductivity are the quantity of

rock removed and the pattern of rock removal. Once differential etching is achieved, formation hardness

and fracture closure stress influence the resultant fracture conductivity. As with propped fractures, the

conductivity of an etched fracture will decrease when closure stress increases. The extent of reduced

conductivity depends on the hardness of the formation and the ratio of supporting area to etched area. The

acid etching test allows modeling of the fracture conductivity results from the etching process to provide an

idea of true etching characteristics.


Differences in permeability, porosity, and small natural fractures can also affect the etching pattern

because of variable acid leakoff rates.

These tests are more time-consuming and require 8 to 10 in. of whole core. Generally three to five core

samples are tested. The cores selected for testing should represent high-/low-solubility intervals, high-/low-

permeability intervals, or variations from calcite to dolomite. In some cases, samples are selected from

areas of greater and lesser natural fracturing.

Acid Etching Tests

Etching tests are performed to measure the conductivity of created and acid-etched fractures. For this test,

a whole or slabbed formation sample approximately 3.5 in. (9 cm) long is required. Test samples are

circular discs, 2.25 to 3.75 in. (5.72 to 9.5 cm) in diameter and 0.5 in (1.27 cm) thick. These tests are

performed according to the following instructions.

Sample Preparation

1. Cut samples from the formation core so that the surface exposed to acid is in the same plane as a vertical fracture.

Note: Orientation of the samples can be varied, if necessary, depending on specific well conditions.

2. Mount the core disks in lead.

3. Using a lathe, turn the faces of the samples flat. Note: A flat surface will provide a better starting point of no flow, eliminating an experimental variable. In addition, initiating etching over a flat surface makes differential etching more difficult because of minimal turbulence at the fracture face during the first part of the test. Increased turbulence generally increases fracture face's rate of dissolution.

Test Procedure

1. Drill a hole through the axis of the sample and the lead mounting.

2. Place the sample in the etching test cell (Figure 1) with the turned face of the sample touching a Hastelloy plate, which acts as the opposite fracture face.


Figure 1: Etching Test Cell

Note: If enough core is available for all tests, a second core can be used as the opposite fracture face.

3. After placing the test sample in the etching cell, attach the fracture width micrometer.

4. Zero the micrometer. Figure 2 shows the original micrometer setting before testing.

5. Set the fracture width to 0.05 in. (0.13 cm).

6. Fill the system with water.

7. Bring the sample to test temperature.


Figure 2: Before Testing, Original Setting of 0.000 in.

8. Pressurize the system to 1,000 psi (6.89 MPa) with water.

9. Flow the acid upward through the center of the test sample and radially across the simulated

fracture face at 1,000 psi (6.89 MPa) backpressure and at test temperature (usually the bottomhole injection temperature should be calculated using the surface injection temperature, injection rates, and temperature gradients).

10. After the etching time (usually 15 minutes) has elapsed, displace the acid with water.

11. Cool to ambient temperature.

12. De-pressurize system to atmospheric pressure.

13. Obtain an acid-etched fracture flow capacity. The acid-etched fracture flow capacity reading (Figure 3) is used for the Supra CE calculations.

Figure 3: Supra CE Reading, Original Setting of 0.000 in.

14. Lower the Hastelloy plate against the core.

15. Apply the closure stress through the hydraulic ram placed under the core.


16. Obtain the acid-etched fracture flow capacity (Figure 4). This reading is used for the conventional calculations and in stimulation designs.

Figure 4: Conventional Reading, Amount Crushed = X.XXX in

17. While taking conventional acid readings, obtain a core "removed or crushed" value.

18. Repeat this complete series for a second etching period.

19. At the end of the second conventional reading, leave the closure stress on the core.

20. Complete the process a third time. This is the closed-fracture acidizing (CFA) period.

21. Maintain the closure stress throughout this stage of the testing. After this period, take only a conventional reading.

22. After completing the etching test, remove the core, examine the face of the test sample, and take pictures.

23. Describe anything unusual that occurs, such as a large amount of fines on the core face. Also note the etching pattern and any evidence of crushing. When combined with conductivity data, these descriptions allow a comparison of test results from the same formation and an evaluation of various acid systems.

The acid etching equipment construction allows one to test all strengths of acids. Acids may be

nonviscous, gelled, foamed with nitrogen, emulsified, or chemically retarded. The samples are mounted in

lead that tends to lose form around 8,000 psi (55.1 MPa) closure stress. Testing may be conducted at

temperatures up to 375°F (190°C).

Although the actual downhole fracture conductivity cannot be predicted with certainty, this test does help

optimize treatment design. In some cases, test results have indicated conditions that prompted

modification of treatment designs. These modifications sometimes allow successful stimulation of

formations that historically do not respond well to conventional fracture acidizing techniques.


When reviewing the data from such tests, researchers must note which numbers are meaningful and can

actually be considered as likely acidizing candidates. Supra CE data are very valid and significant during

the first acid stage of the test; however, if any significant crushing occurs, these numbers are misleading

during the second portion of the test. Generally, a formation with Supra CE numbers below 160,000 md-ft

should not be considered for a Supra CE job; other methods of fracture acidizing or other forms of

stimulation should be used. When conventional acidizing numbers are used, numbers should be greater

than 50,000 MD-ft to be considered.

Optimizing Treatment Designs with the Acid Etching Test

The acid etching test has not changed much since it was introduced almost 40 years ago. Most

modifications have been made to expand the range of operating temperatures and acid strengths. The

most significant modifications to the etching test pertain to the design of specific test procedures and the

interpretation of the data. This section provides specific examples of how etching tests are used to design

fracture acidizing treatments.

Low-Temperature Formations. Traditionally, retarded acids or retarding systems have been used for

obtaining deeper acid penetration. Some formations, however, require accelerated acid systems. The

most practical means to accelerate acid reaction is to heat the acid. Cold dolomites probably benefit from

heated acid more that any other type of formation. Generally, dolomites with bottomhole temperatures

(BHTs) less than 140°F (60°C) are considered cold. In addition to accelerating the reaction rate, heating

also helps remove organic materials such as oil, paraffins, or asphaltenes that can coat formation

surfaces and physically prevent or reduce acid/formation contact. Problems associated with organic

residues are most common in lower-temperature formations.

If an accelerated system is required, the acid system that will give optimum etched conductivity must be

determined. Generally, the maximum practical surface acid temperature is about 150°F (65.6°C). The

actual temperature for a specific field case is determined based on the ambient acid temperature and the

technique used to heat the acid.

If initial etching test results indicate that organic deposits were responsible for low conductivity, various

additives may be used during testing to evaluate how effectively the additives can strip away the organic

film coating the formation. In some cases, heat alone helps remove organics, and additives may not be

necessary.

Table 2 provides typical acid-etched fracture conductivities for cold formations treated with unheated and

heated acids. Fracture conductivity increases significantly as a result of the acid being heated. On the


basis of the etching test results, heated acid has been recommended and has successfully stimulated

many formations.

Chalk Formations. Because of their mineralogical and physical properties, chalk formations are difficult

to stimulate with standard fracture acidizing. Chalks tend to be uniform, and differential etching achieved

as a result of acidizing is generally poor. Even when the formation is etched unevenly, the formation

closure stress can crush newly formed channels. Generally, these conditions have been treated with

viscous preflushes.

By using the acid-etch test, analysts developed the closed-fracture acidizing (CFA) technique for treating

soft, uniform chalks. CFA can be simulated on chalks in the laboratory with the dynamic etching test. If the

etched fracture face is likely to be crushed, the test procedure is modified to make conductivity

measurements at a series of increasing closure stress levels. A plot of fracture conductivity vs. closure

stress may indicate the critical closure stress. Because the closure stress on a fracture depends on the


way a well is produced (and on the in-situ rock stresses and reservoir pressure), the results of the etching

test allow analysts to indicate a "recommended maximum drawdown." Table 3 shows the reduced etched

fracture conductivity as a result of increasing closure stress for several chalk cores.

If chalk cores do not develop etched conductivity, or the etched faces crush from the closure stress,

additional etching tests can be run to determine if CFA will produce adequate conductivities. As shown in

Table 2, CFA treatments provide higher initial conductivities and greater crush resistance than

conventional acidizing treatments. The etching test results and field results indicate that CFA has allowed

successful stimulation of many chalk formations. The greatest documented use has been in the Austin

Chalk.

Low-Solubility Carbonates. Fracture acidizing is not generally recommended for formations that are less

than 80% soluble in acid; the preferred solubility is greater than 90%. As formation solubility decreases, the


possibility of releasing acid-insoluble fines increases. These fines can cling to the fracture face and restrict

acid reaction. Low solubility with respect to fracture acidizing is different than low solubility from a

geological viewpoint; in this document, low-solubility formations are carbonates that are 75 to 85% soluble

in acid.

Acid etching tests allow analysts to determine how the presence of insolubles or slower-dissolving

materials will affect an acidizing treatment. If acid insolubles are concentrated in localized patches or at

bedding planes, they can enhance differential etching by acting as supporting areas for the fracture face.

Table 4 presents the results of etching with low-solubility cores containing patches of anhydrite, which act

as supporting material. Although the overall solubility of these cores was low, the differential etching was

excellent.

Problems with low-solubility formations generally occur when the acid insolubles are uniformly distributed

throughout the core. The more dispersed the fines are, the greater the chance of the fines either hindering

the acid reaction or migrating and plugging the created fracture. While the acid-etching test cannot

evaluate the potential of fracture damage from fines, it can indicate whether fines will hinder acid reaction.

If fines may be a problem, foamed acid may be helpful. Foamed acids tend to have an erosive effect, which

helps wash acid-insoluble fines from the fracture face. Etching tests allow evaluation of the benefits of

foamed acid when low-solubility formations are acidized. Table 5 compares etching results obtained with

conventional and foamed acids on several low-solubility cores. In some wells, foam helped return fines

during well cleanup. In nonfoamed treatments, no fines were noted in the recovery load.


Uniform Formations. Fracture conductivity after acidizing is least likely to occur in uniform formations. To

a geologist, a uniform formation has constant physical and mineralogical properties. In acid stimulation, a

uniform formation lacks the characteristics needed to achieve differential etching. Therefore, a large

interval may be geologically uniform, but sufficient heterogeneities exist that make a fracture acidizing

treatment feasible. In this document, a uniform formation is one that will not exhibit differential etching.

Acid-etching tests allow analysts to evaluate the feasibility of obtaining conductivity during a fracture-

acidizing treatment. If little or no differential etching occurs, the recommended stimulation treatment

could be hydraulic fracturing with proppants or a high-viscosity water preflush followed by acid.

With uniform formation cores, the first etching test determines whether differential etching will occur.

Normally, straight HCl is used for this first test. If poor differential etching results in low conductivities,

another acid system can be tested, such as a combination of organic and mineral acids. The same

formations will etch differently in different types of acid solutions. If differential etching cannot be achieved

with alternative acid systems, the formation should be hydraulically fractured with proppants.

For over 25 years, etching tests have been used to determine if treatments other than hydraulic

fracturing could be used on uniform carbonate formations. As in chalk formations, acid pumped at rates

below fracturing pressure following a conventional fracture acidizing treatment may result in enhanced

fracture conductivity (the CFA technique).


Closed-fracture acidizing has been commonly used in the San Andres formation in west Texas. Table 6

shows etching test results for several San Andres cores. Significant increases in conductivity occurred

when the last stage of acid was flowed under closure stress.

Rotating Disc Test

Two factors affect the spending rate of acid injected into a fracture: 1) the surface reaction rate and 2) the

mass transfer rate. The surface reaction rate is the rate at which the acid-rock reaction proceeds along the

fracture surface. The mass transfer rate is the rate at which acid is transported from the bulk acid solution

to the fracture face. Either or both of these factors may control the penetration distance of live acid.

In many instances, particularly when surface kinetics are being measured, observed reaction rates will not

be restricted totally by either the surface reaction rate or the mass transfer rate. For this reason,

experiments to determine reaction kinetics must be conducted in such a manner that the kinetic

expressions are the only unknowns. The experiment should be designed so that the rate of acid transfer to

the surface is known.


The Rotating Disc (RD) reaction-rate measuring device (Figure 5) is the industry-standard test apparatus

for kinetic studies with formation cores. It provides well-defined mass transport, the rate of which is a

function of the square root of rotational speed. The main components of the device are the reaction vessel

and the acid reservoir, which are both mounted in aluminum heating sleeves contained within insulated

jackets. The reaction vessel contains the RD and the acid reservoir serves as a pressurized, preheating

chamber; both are constructed of ampcoloy. The system is generally operated at a gauge pressure of

1,000 psi (6.89 MPa) and a maximum operating temperature of 200°F (93°C).

Figure 5: Rotating Disc Measuring Device


Five different acid strengths are run on each test core. The HCl systems used generally range from 0.5 N

to 2.5 N. These five acid concentrations are run at four different temperatures. Typically, these would be 100, 120, 140, and 160°F. This allows for the calculations of an Energy of Activation (E

a).

Gelled acids and emulsified acids cannot be run on the system because they will not be completely stirred

by the rotation of the core. While fluid-loss additives do affect reaction rates, they do not affect the test

because no pressure difference exists to make the additives adhere to the core face. Additives that

influence viscosity should not be used.

Test Procedure

1. Cut five formation core plugs, approximately 1/2 × 1 in. (1.27 × 2.54 cm) long from the formation

cores.

2. Place the core plugs inside a holding device that consists of two copper plumbing sweat caps. One

cap measures 1 × 7/8 in. ID (2.54 × 2.22 cm), and the other measures

9/16

in. OD × 1/2 in. ID (1.43 ×

1.27 cm). Use epoxy resin to hold the core plug in place. Note: The inset screw configuration ensures that the center of the core plug will be concentric with the axis of rotation.

Figure 6: Sample Placed in RD Reaction Vessel


3. After the epoxy has hardened, use a saw to cut the face of the core/epoxy/holding device.

4. Turn the core face flat with a lathe.

5. Blow fines off core face with compressed air.

6. Vacuum saturate cores in 5% NH4Cl.

7. Preheat acids in water bath to test temperature.

8. Place one sample (Figure 6) in the RD reaction vessel.

9. Bring the acid and reaction reservoirs to test temperature.

10. Place the required concentration of preheated acid in the acid reservoir.

11. Pressurize the entire system with nitrogen to 1,000 psi (6.89 MPa).

12. Rotate the disc at 120 rev/min. Use nitrogen pressure to move the acid quickly from the bottom of the acid reservoir into the reaction vessel. Continue rotating for 5 minutes.

Important: Maintain a system gauge pressure of 1,000 psi (6.89 MPa). Maintenance of pressure is

critical to the accuracy of the test. System gauge pressure less than approximately 730 psi (5.03

MPa) would allow carbon dioxide generated by the HCl reaction to come out of solution, affecting

both the reaction rate and mass transfer rate at the disc surface.

13. After 5 minutes, drain the acid, capturing the effluent.

14. Submit the effluent for analytical determination of calcium and magnesium concentrations by ICP. Note: Ion concentrations are generally in the range of 1 to 200 mg/L with an accuracy of ±1 mg/L.

15. Rinse the apparatus with deionized water.

16. Remove the core to be resurfaced on the lathe.

17. Using the next strongest acid, repeat Steps 5 through 17 using the next plug from this core.

After tests are completed from all four temperatures and five acid strengths and the data is returned from

analysis, the data points can be entered into an Excel spreadsheet that generates a reaction rate constant (RRC), reaction order (RO), and Energy of Activation (E

a).


When this data is entered, the actual test temperature must be entered; however, if a treating temperature

is entered, the RRC will be calculated at that temperature, automatically correcting for viscosity and density

change. The RRC is a misleading name since the RRC will vary with temperature. Generally, rate

constants will rise exponentially with temperature.

Determining Temperature Dependency

To determine the temperature dependency of the reaction rate expression, run the rotating disc test

several times at different temperatures. Determining the reaction rate expression involves a two-step

procedure.

1. Determine the concentration dependence (reaction order, n) at a fixed temperature.

2. Evaluate the temperature dependence of the RRC (activation energy, Ea) to give a complete

reaction rate expression. Note: The activation energy is a quantitative measure of the effect of temperature on a formation's surface RRC. Higher values of E

a indicate a higher temperature dependency. The effect of E

a is most important

when large variations exist along the fracture's length.

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03/12/2009

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