SPE 86542 UB in Carbonates1

Copyright 2004, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the SPE International Symposium and Exhibition on Formation Damage Control held in Lafayette, Louisiana, U.S.A., 1820 February 2004. This paper was selected for presentation by an SPE Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract Underbalance perforating has long been recognized as one of the best techniques for mitigating perforating damage. Earlier studies have attempted to establish the level of underbalance necessary to stimulate perforation clean-up in sandstone. These models are stated in terms of the underbalance pressure thought necessary to remove damage from perforations for a given matrix permeability (some models also give consideration to fluid properties and perforation geometry). The purpose of this study is to evaluate the extent to which these correlations for underbalance perforating may be applicable to carbonates.

Single shot laboratory flow tests were performed with limestone and dolomite cores to determine the extent of perforation damage as a function of underbalance pressure. The calculated perforation skins from the carbonate perforating experiments are not well described by the earlier sandstone correlations. In particular, the primary reliance on rock permeability as a prognosticator of perforation clean-up does not work for carbonates. Rather than relying upon the initial underbalance pressure and matrix permeability as has been done previously for sandstones, perforation skin for carbonates is best related to peak underbalance pressure and rock strength. The underbalance pressure condition is not surprising. Progressive perforation clean-up with increasing underbalance is intuitive and has been well noted by previous investigators. This study extends the concept by making reference to a peak dynamic underbalance pressure occurring during perforating. Relating rock strength instead of rock permeability to perforation clean-up is a relatively new idea that may also have merit for sandstones. The results of this study suggest that the initial mechnism of perforation clean-up is primarily related to failure of the perforation tunnel wall under stress as opposed to fluid surge flow.

Introduction For cased hole completions communication between the wellbore and producing formation is re-established through the perforations. It is accepted that underbalance perforating can facilitate perforation clean-up and thus permit more effective flow. Other researchers have investigated the level of pressure underbalance necessary to clean-up perforations in sandstone. The extent to which perforating underbalance correlations and their underlying assumptions may be applied to carbonates was unknown.

This report describes nine single shot laboratory flow tests performed at the Schlumberger Productivity Enhancement Research Facility (PERF) in Rosharon, Texas to evaluate underbalance pressure perforating strategies in carbonates. The perforating experiments were carried out under simulated downhole conditions. The perforation damage and subsequent clean-up were evaluated as functions of rock and simulated wellbore conditions. The perforating experiments were conducted with Bedford Limestone and Silurian Dolomite cores and 15 gram and 21.7 gram deep penetrating charges. The 15 gram and 21.7 gram charges are hereafter refered to as charge A and B respectively. The perforation tests were conducted at initial wellbore underbalance pressures ranging from 500 to 4000 psi. Five of the nine perforation tests were shot into limestone core targets using both charge A and B. Four of the nine perforation tests were shot into dolomite core targets with charge A.

The results of the perforating experiments with carbonate target cores are not well described by published sandstone correlations. In particular, the primary reliance on rock permeability as a prognosticator of perforation clean-up does not work for carbonates. Rather than relying upon the starting underbalance pressure and matrix permeability as has been done previously for sandstones, perforation skin for carbonates is best related to peak underbalance pressure and rock strength. Underbalance Correlations Several authors have proposed correlations to estimate the level of underbalance pressure necessary for perforation clean-up. This earlier work focused on sandstones and is not necessarily applicable to carbonates.

King, Anderson, and Bingham1 compared the level of total pressure underbalance and formation rock permeability to the success of acid treatment for 90 wells. Successful acidization was defined as improving the production rate by 10% or more.

SPE 86542

Underbalance Pressure Criteria For Perforating Carbonates Servio T. Subiaur, PEMEX; Craig A. Graham, Schlumberger; Ian C. Walton, Schlumberger; and David C. Atwood, Schlumberger

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They showed that the level of total underbalance necessary to achieve clean perforations (i.e. perforations which did not benefit from acidization) decreases with increasing formation permeability. They proposed that perforation clean-up was governed by the rock permeability and pressure differential. They envisaged perforation clean-up as resulting from flow through the formation matrix. Accordingly, high permeability rocks would more readily clean-up while very low permeability rocks may not clean-up regardless of the pressure differential. Their study is based on sandstone formations and does not consider limestone and limey sandstones.

Behrmann2 used a viscous drag force model to derive perforation skin as a function of underbalance pressure. His drag force equations depend on reservoir porosity, permeability, and diameter of the perforation tunnel. The work is based upon single shot laboratory experiments. Gold and Berea sandstone targets were perforated using 3.2 and 15 gram charges. Behrmanns correlations can also be used to estimate the perforation skin in those cases where the conditions are not right to fully remove the perforation damage. In contrast to Tariq3 turbulent flow perforation model, Behrmanns equations do not require oil compressibility and viscosity terms.

Walton4 offered an alternate explanation of the perforation clean-up mechanism. He proposed that underbalance perforating promotes productive perforations, not by increasing fluid surge, but by initiating mechanical failure of the damaged zone. Waltons theory predicts the optimum pressure underbalance in terms of the rock matrix strength, strength and extent of the perforation crushed zone, as well as the effective stress conditions. He illustrates his theory with data from several different sandstone lithologies.

Experimental Setup and Procedure The perforating test program simulated perforating an oil reservoir with brine across the perforations and the well open to surface. The cores used for this study are from Bedford (Indiana) Limestone and Silurian Dolomite outcrop rock. Core properties, a summary of the experimental conditions, and experimental results are shown in Tables 1 through 3. The detailed experimental facility and procedures are discussed further in the Appendix.

The cores were first dried and then vacuum saturated with 3% KCl brine. With the core under an effective confining stress of 3000 psi, K-1 kerosene was flowed through the cores until they reached irreducible brine saturation. The axial and cross-diameter permeabilities to kerosene at irreducible brine saturation were measured for each core. The initial permeability to kerosene serves as a benchmark by which the effective productivity of the perforated core can be measured. The flow outlet was to ambient pressure and inlet pressure was allowed to vary as governed by the flow rate and core permeability.

The design of the pressure vessel allows for the duplication of the downhole conditions including: confining stress, pore pressure and wellbore pressure. The core sample is enclosed in rubber sleeves to prevent communication with the confining fluid. An end attachment to the core sample is used for imparting the pore pressure. A simulated wellbore (SWB)

holds the wellbore fluid (3% KCl brine) and perforating gun. The shaped charge is placed inside a modified perforating gun in the wellbore at a standoff of 0.5 from the shooting plate (0.375 steel and 0.75 class H cement) to simulate the well casing. Pore and SWB accumulators simulate the far field reservoir and wellbore respectively.

For perforating, the pore pressure was set at 5000 psi for each of the tests and the wellbore pressure varied to obtain the desired underbalance or overbalance pressure in accordance to the test design. The confining pressure was 8000 psi for all tests for an effective stress (confining less pore) of 3000 psi for all the tests.

The tests are divided into three categories based on core and perforating charge type. The first two perforation tests were conducted to evaluate the perforation productivity of Bedford Limestone target cores perforated at initial wellbore underbalance pressures of 2000, and 3000 psi. Charge A was used for Tests 1 and 2. Tests 3 through 6 were conducted to evaluate the perforation productivity of Silurian Dolomite target cores perforated at initial wellbore underbalance pressures of 500, two at 1000, and 3000 psi. Charge A was also used for these tests. The target cores for tests 7 through 9 were Bedford Limestone. These tests were conducted at initial wellbore underbalance pressures of 1000, 3000, and 4000 psi with charge B and associated gun.

After perforating, the pore and wellbore pressures were allowed to equalize before readying the core for controlled pumped flow. Pressure in the SWB was bled down to atmospheric pressure. The confining pressure was reduced simultaneously to maintain constant effective overburden stress on the core. K-1 kerosene was then pumped through the core with an accurate low rate screw pump. Kerosene entered around the perimeter of the core, flowed radially through the core into the perforation and exited the core through the perforation tunnel. A constant flow-rate setup is used to flow the perforated core until a stable productivity is established. After all flow was completed the core was split open for inspection and measurement of the perforation tunnel.

Results The results are summarized in Tables 1 through 3. The tests are divided into three categories: Bedford (Indiana) Limestone perforated with charge A (Table 1), Silurian Dolomite perforated with charge A (Table 2), and Bedford (Indiana) Limestone perforated with charge B (Table 3). The results are in agreement with earlier findings of progressive clean-up of the perforation tunnel with increasing levels of initial underbalance (Figure 1). In Figure 1 perforation skin is plotted against the initial underbalance pressure. The progressive perforation clean-up with increasing underbalance pressure that has been well noted by previous investigators is also evident in Figure 1. However, the perforation skins for the higher permeability dolomite cores are consistently higher than the perforation skins for the lower permeability limestone cores (Figure 1). This finding is contrary to the conclusion of earlier studies that stipulate that for a given underbalance pressure higher permeability rocks should show more perforation clean-up than lower permeability rocks.

SPE 86542 3

Discussion and Interpretation Earlier findings of progressive perforation clean-up with increasing levels of initial pressure underbalance are corroborated by this study. The relationship between the level of perforation clean-up and pressure underbalance is intuitive and has been noted in previous studies of underbalance perforating in sandstone. In this respect the correlations developed for sandstones help define an important parameter for defining the clean-up of perforations in carbonates. However, the fact that the perforation skins for the higher permeability dolomite cores are consistently higher than the perforation skins for the lower permeability limestone is contrary to the earlier findings with sandstone. For this reason earlier correlations developed for sandstones, which rely primarily upon matrix permeability, are not applicable to carbonates.

In Figure 2, the carbonate test data are plotted along with Kings underbalance criteria necessary to eliminate acid stimulation in oil wells, but not specifically to achieve clean perforations with kc/k=1. The measured perforation skin is displayed next to each data point. All tests have positive perforation skins. Kings data is based on sandstones and therefore should not necessarily be expected to work for perforated carbonates. The higher permeability limestone and to a greater degree the dolomite perforation data do not fit Kings underbalance criteria. Moreover, the line delineating perforation clean-up cannot be simply shifted or readily modified to accommodate the carbonate data. There does not appear to be a direct relationship between underbalance pressure and permeability. According to Kings criteria it appears that carbonate rocks are more susceptible to perforation damage than sandstone rocks. That is, many of the carbonate core perforating experiments showed significant perforation skin despite possesing conditions which the King correlation suggests are sufficient for clean perforations.

The carbonate test data are plotted along with Behrmanns perforation skin criteria for Berea sandstone perforated with a 15.0 gram charge in Figure 2. Behrmann used Berea and Gold sandstone cores for his study. The discrepancy between Behrmanns correlation for Berea Sandstone and the carbonate test data shows the problem of applying an underbalance perforating correlations developed with sandstone to carbonates. The dolomite data points fall above the correlation (higher perforation skin) and with one exception the limestone data points fall below the correlation (lower perforation skin). From the large scatter in the data, this Berea Sandstone correlation is clearly not applicable to carbonates. One might be able to draw separate correlations for each of the dolomite and limestone series. Two charge sizes (15.0 and 21.7 grams) were used in this study in an effort to evaluate the effect (if any) of charge size on perforating clean-up in carbonates. In terms of perforating skin, a systematic difference between perforating was not noted in the limestone core targets.

Figure 4 shows the pressure response in the SWB at the time of perforating. Although there can be a casual relation between initial and dynamic underbalance pressure, initial underbalance pressure does not always reflect the near time (less than ~0.2 seconds) pressure responses which are thought

to be primarily responsible for perforation clean-up. The initial underbalance pressure does not correspond to the dynamic underbalance following charge detonation. From recent perforation flow research5, a dynamic underbalance pressure is chosen over the initial underbalance pressure as a measure of the underbalance pressure. It is the pressure conditions after and not prior to perforating that influence the level of perforation clean-up. Quantifying dynamic underbalance remains problematic. For this study peak dynamic underbalance is defined as the difference between the pore pressure, measured at the back of the core, and minimum SWB pressure. Two important assumptions are being made here: 1) the SWB pressure reflects the pressure inside the perforation tunnel and 2) similarly the pore pressure at the back of the core reflects the pore pressure immediately around the perforation tunnel.

Walton4 proposes that perforation skin is a function of the level of mechanical failure of the damage zone. This new idea is useful in reconciling the results of the limestone and dolomite tests. While conventional theory with sandstones proposes that perforation clean-up is enhanced in high permeability rocks, Waltons theory predicts enhanced perforation clean-up in lower strength rocks. For many sandstones there is a casual relation between permeability, porosity and rock strength. Thus high permeability sandstones tend to have low strength and vice-versa. Under these circumstances it is difficult to differentiate between the permeability and strength models for prediction of perforation clean-up. For the carbonate rocks tested here there is no consistent relationship between permeability and strength. This allows us to distinguish between the permeability and stress dependent models of underbalance perforating clean-up

Figure 5 shows the perforation skins for the flow tests. The perforation skin is plotted against the peak dynamic underbalance pressure divided by the nominal unconfined compressive strength (UCS) of the rock and an empirical correction for the intial rate of pressure drop following charge detonation. There seems to be a mathematical basis for this empirical rate correction, which we hope to more fully develop in the future. Reliance upon these parameters yields a better estimate of the perforation skin for Bedford Limestone and Silurian Dolomite. All testing was carried out at an initial effective stress of 3000 psi. Perforating tests at other stresses will be necessary to demonstrate whether effective stress, as opposed to the dynamic underbalance pressure, is a better measure of the stress for purposes of predicting perforation skin. According to Walton, this should in fact be the case. The prediction of perforation skin may be further refined with measurements of pore pressure nearer the perforation tunnel.

The discrepancy in perforation skin between earlier sandstone correlations and these carbonate tests is probably related to rock strength, lithology, and the definition of the effective underbalance pressure. Perforation clean-up in sandstone may also be a function of rock strength. Unfortunately, earlier perforation tests lack dynamic pressures and rock strength data to test this hypothesis. All the tests in this study were conducted with 5000 psi pore pressure and 8000 psi confining pressure for an effective stress of 3000 psi. Under these test conditions the dynamic underbalance pressure

4 SPE 86542

and rock strength are sufficient to predict perforation skin. Testing at other starting effective stresses may verify Waltons hypothesis that the optimum underbalance for perforation clean-up is a function of the effective stress, the rock strength, and the strength and extent of the perforation damage zone.

Conclusions These tests verify earlier findings of a general progressive clean-up of the perforation tunnel with increasing levels of initial underbalance pressure. However, the measured perforation skins for the carbonate target cores are not well characterized by earlier sandstone correlations. In particular the primary reliance on rock permeability as a prognosticator of perforation clean-up does not work for carbonates. Rather than relying upon the initial underbalance pressure and matrix permeability as has been done for sandstones, perforation skins for carbonates are best related to peak underbalance pressure and rock strength. The underbalance pressure condition is not surprising. Progressive perforation clean-up with increasing underbalance is intuitive and has been well noted by previous investigators. This study makes reference to the peak dynamic underbalance pressure occurring during perforating. Relating rock strength instead of rock permeability to perforation clean-up is a relatively new idea that may also be applicable to sandstones.

Acknowledgements The authors wish to thank Schlumberger and PEMEX for permission to publish this paper, Larry Behrmann for constructive comments during its preparation. Assistance by Brian Taylor and Solomon Zepeda in conducting the experiments is much appreciated.

References 1. King, G.E., Anderson, A., and Bingham, M.: A Field Study of

Underbalance Pressures Necessary to Obtain Clean Perforation Using Tubing Conveyed Perforating, JPT (June 1986) 662-664; Trans, AIME, 281

2. Behrmann, L.A.: Underbalance Criteria for Minimum Perforation Damage, paper SPE 30081 presented at the 1995 SPE European Formation Damage Conference, The Hague, The Netherlands, May 15-16

3. Tariq, S.M.: New Generalized Criteria for Determining the Level of Underbalance for Obtaining Clean Perforations, paper SPE 20636 presented at the 65th Annual Technical Conference and Exhibition, New Orleans, Sept 23-26, 1990

4. Walton, I.C.: Optimum Underbalance for the Removal of Perforation Damage, paper 63108 presented at the 2000 SPE Annual Technical Conference and Exhibition, Dallas, Oct 1-4, 2000

5. Walton, I.C., Johnson, A.B., Behrmann, L.A., and Atwood, D.C.: Laboratory Experiments Provide New Insights into Underbalance Perforating, paper 71642, presented at the 2001 SPE Annual Technical Conference and Exhibition, New Orleans, Sept 30 Oct 3, 2001

6. API RP 43: Recommended Practices for Evaluating of Well Perforators, American Petroleum Institute, fifth edition, Washington, DC (Jan. 1991).

7. Halleck, P.M and Dogulu Y.S.: The Basis and Use of the API RP43 Flow Test for Shaped-charge Oil Well Perforators, JCPT May 1997

SI Metric Conversion Factors psi x 6.895 E+00 = kPa in x 2.54 E+02 = m

SPE 86542 5

Core Test 1 Test 2 Setup Pressures Confining Pressure (psi)

8000 8000

Pore Pressure (psi) 5000 5000 Simulated Well Pressure (psi)

3000 2000

Effective Stress (psi)

3000 3000

Initial Under-balance (psi)

2000 3000

Peak Dynamic Under-balance (psi)

2640 3350

Core Properties Rock Type Bedford LS Bedford LS UCS Plug (psi) 7478 7219 Core Length (in) 15.875 16.75 Core Diameter (in) 7 7 Porosity (%) 12.5 12.4 Axial Permeability (md)

8.5 7.8

0-180 Diametral Perm (md)

6.9 8.0


7.2 7.8

Average Perm (md) 7.5 7.9 Axial PI 0.222 0.19 0-180 Diametral PI 0.95 1.11 90-270 Diametral PI

1.00 1.08

Perforation Charge A A Gun Standoff (in) 0.5 0.5 Total Length in rock (mm)

165 117

Clear Length in rock (mm)

135 112

Average Diameter (mm)

4 4

Post Shot PI (radial)

0.409 0.421

Post Perforation Flow

PI(post)/PI(pre) 0.470 0.384 CFE 0.459 0.569 Skin 4.6 2.9

Table 1: Experiment setup conditions and a summary of the experimental results (Bedford Limestone, Charge A). PI is defined as flow rate/differential pressure in units of cc/sec/100 psi.

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Core Test 3 Test 4 Test 5 Test 6 Setup Pressures Confining Pressure (psi)

8000 8000 8000 8000

Pore Pressure (psi) 5000 5000 5000 5000 Simulated Well Pressure (psi)

4500 4000 4000 2000


3000 3000 3000 3000


500 1000 1000 3000


1750 2160 2150 3160

Core Properties Rock Type Silurian Dolomite Silurian Dolomite Silurian Dolomite Silurian Dolomite UCS Plug (psi) 4971 4614 6699 5721 Core Length (in) 17.625 17.5 17 17.5 Core Diameter (in) 7 7 7 7 Porosity (%) 17.9 17.3 16.3 18.6 Axial Permeability (md)

142.2 68.8 108.6 83.1


173.4 88.9 92.2 86.5


174.4 80.1 106.4 80.9

Average Perm (md) 162.6 78.8 102.1 83.5 Axial PI 3.5 2.03 2.83 2.03 0-180 Diametral PI 24.0 9.51 12.8 11.5 90-270 Diametral PI

24.1 12.3 14.7 12.0

Perforation Charge A A A A Gun Standoff (in) 0.5 0.5 0.5 0.5 Total Length in rock (mm)

225 285 165 185


115 280 152 178


7 10 12 14


8.07 6 5.6 6.3


PI(post)/PI(pre) 0.336 0.550 0.408 0.536 CFE 0.224 0.259 0.294 0.327 Skin 11.2 8.2 6.5 5.2

Table 2: Experiment setup conditions and a summary of the experimental results (Silurian Dolomite, Charge A). PI is defined as flow rate/differential pressure in units of cc/sec/100 psi.

SPE 86542 7

Core Test 7 Test 8 Test 9 Setup Pressures Confining Pressure (psi)

8000 8000 8000

Pore Pressure (psi) 5000 5000 5000 Simulated Well Pressure (psi)

4000 2000 1000


3000 3000 3000


1000 3000 4000


2480 3580 4000

Core Properties Rock Type Bedford LS Bedford LS Bedford LS UCS Plug (psi) 4005 4055 4200 Core Length (in) 18 18 18 Core Diameter (in) 7 7 7 Porosity (%) 16.7 17.1 16.7 Axial Permeability (md)

3.0 3.4 3.6


3.4 3.7 4.5


3.1 4.1 4.7

Average Perm (md) 3.2 3.7 4.2 Axial PI 0.07 0.08 0.09 0-180 Diametral PI 0.47 0.51 0.63 90-270 Diametral PI

0.43 0.56 0.65

Perforation Charge B B B Gun Standoff (in) 0.5 0.5 0.5 Total Length in rock (mm)

230 165 260


120 130 200


6 9 8


0.206 0.48 0.935


PI(post)/PI(pre) 0.459 0.897 1.464 CFE 0.316 0.731 0.814 Skin 7.3 1.1 0.7

Table 3: Experiment setup conditions and a summary of the experimental results (Bedford Limestone, Charge B). PI is defined as flow rate/differential pressure in units of cc/sec/100 psi.

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0 500 1000 1500 2000 2500 3000 3500 4000 4500 50000

2

4

6

8

10

12Perforation Skin versus Initial Underbalance Pressure

Initial Underbalance Pressure (psi)

Per

fora

tion

Ski

n

Limestone, Charge ADolomite, Charge ALimestone, Charge B

7.5 md

7.9 md

162.6 md

78.8 md

102.1 md

83.5 md

3.2 md

3.7 md 4.2 md

Figure 1: Perforation Skin versus Initial Underbalance Pressure

102 103 104100

101

102

103

Initial Underbalance Pressure (psi)

Per

mea

bilit

y (m

d)

Permeability versus Initial Underbalance Pressure


5.3

7.3

2.9

11.2

8.25.2

4.6

6.5

1.10.7

Acid improves production

Acid does not improve production

Figure 2: Carbonate Core Skin compared to King Sandstone Correlation

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0 200 400 600 800 1000 1200 1400 1600 1800 20000

2

4

6

8

10

12

dP*k**0.5/phi*CD**0.3

Per

fora

tion

Ski

n

Perforation Skin versus Drag Force


Berea sandstone correlation

Figure 3: Carbonate Core Skin compared to Behrmann Berea Sandstone Correlation

-0.1 0 0.1 0.2 0.3 0.4 0.50

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000Well Pressure versus Time

Time (seconds)

Wel

l Pre

ssur

e (p

si)

Test 1Test 2Test 3Test 4Test 5Test 6Test 7Test 8Test 9

Initial Pore Pressure = 5000 psi

1

2

3

4

5

6

78

9

Figure 4: Wellbore Pressures during Perforating

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20 25 30 35 40 45 50 55 600

2

4

6

8

10

12

Peak Dynamic UB/UCS * Initial Pressure Rate**0.4

Per

fora

tion

Ski

n

Perforation Skin versus 'Dynamic' Underbalance


Figure 5: Carbonates Skin Correlation compared to Dynamic Underbalance and UCS

SPE 86542 11

Appendix A: Perforating Procedure The perforating test procedures are based upon the recommended practices for evaluation of well perforations as set forth in the fifth edition of the API RP 436. Core Preparation and Initial Permeability Each core is dried for 24 hours at a temperature of 200o F. Next a vacuum is pulled on the core for a minimum of 8 hours. The weight of the dry core is recorded. The dried and evacuated core is flooded in the evacuation chamber with 3% KCl brine while the rate at which the saturating fluid is admitted is carefully monitored so that it does rise faster than the capillary rise of the fluid in the core. After saturation the core is lightly wiped to remove free brine from the surface and weighed again. A porosity determination is made from the dry and saturated core weights.

Determination of the axial permeability involves flowing kerosene along the axis through the full face of the core. A rubber sleeve around the core diameter isolates the core from the confining fluid. Flow is continued along the core axis until the core reaches irreducible brine saturation and its permeability to kerosene measured.

Following the determination of axial permeability to kerosene, the core is prepared for cross-diameter flow. Determination of cross-diameter permeability involves using two sets of opposing rods. Each set of rods extends a quarter of the way around the core diameter. The core and rods are placed in a rubber sleeve to isolate the core from the confining fluid. The top faceplate seals that end of the core. The bottom faceplate has openings for pore fluid inlet and outlet while sealing the bottom of the core face from confining fluids. Kerosene flows through the inlet in the bottom faceplate, around the inlet rods and across the core diameter. Kerosene gathers at the outlet rods and flows through an outlet in the bottom faceplate. A second cross-diameter permeability is then made perpendicular to the first cross diameter permeability. Perforation Experimental Setup and Procedure After the initial permeability of the test core to kerosene at irreducible brine saturation has been determined, the core is mounted in the pressure vessel as shown in Figure A1. The design of the pressure vessel allows for the duplication of the pressure conditions downhole: confining pressure, pore pressure, and wellbore pressure. The simulated wellbore holds the wellbore fluid (3% KCl brine in this case). The core is enclosed in rubber sleeves to prevent communication with the confining fluid. Pore pressure is maintained through an end attachment. The shaped charge is placed inside a modified perforating gun in the wellbore at a known standoff from the shooting. The shooting plate is 1.125 thick (0.375 steel and 0.75 class H cement) and is used to simulate the downhole casing and cement. Shooting leads are routed from the gun through fluid-to-air connectors to the shooting box. Perforating conditions for the tests are summarized in Tables 1 through 3. After perforating, the pore and wellbore pressures are allowed to equalize before readying the core for flow.

A constant flow setup is used to flow the core sample. Kerosene is pumped through the core with an accurate low rate screw pump through the perforated core and the productivity of the perforated core sample is calculated from pressure and flow data. An effective stress of 3000 psi is maintained on the core throughout the flow. Upon completion of all flow, the core is removed from the test vessel and the perforation entrance hole is measured and inspected. Lastly, the core is split open for inspection of the perforation tunnel. Appendix B: Calculation Notes Definition of PI, CFE, and skin Core Flow Efficiency (CFE) is defined as the measured radial flow rate divided by the calculated radial flow rate into a theoretical perforation of the same depth and diameter. CFE can be viewed as the ratio of the productivity of the actual perforation relative to an ideal undamaged perforation.

calculated

measured

QQ

CFE = where:

+=

)(ln

21

rRrRk

rRDkPQcalculated

and: D = perforation depth r = perforation radius R = core radius k = permeability, bedding perpendicular to the axis:

hkk =1 3

122 )( hvkkk =

Productivity Index (PI) is defined as the flow rate divided by the total pressure drop. In this paper the PI is measured in units of cc/sec/100 psi. The geometry and state of the perforation affect the PI, however, the determination of PI does not directly take the perforation depth and diameter into consideration.

Skin is a composite variable. It represents the additional (or reduced) pressure drop associated with a given completion. A positive skin denotes that the pressure drop in the near-wellbore zone is greater than would have been expected from the normal, undisturbed, reservoir flow mechanisms. Conversely, a negative skin denotes that the pressure drop in the near-wellbore zone is less than would have been expected from the normal, undisturbed, reservoir flow mechanisms.

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p

e

p

cp

c

o

rr

CFErtr

kk

S ln11ln1

=+

=

where: S = skin ko = initial permeability kc = perforation damage zone permeability re = core radius

rp = perforation radius tc = damage zone thickness This calculation assumes Darcy flow, which is a good assumption under these test conditions. A more detailed derivation of the equations for perforation CFE and skin can be found in a paper by Halleck and Dogula7.

P

1

2

3

6

4

9

7

8

10

5

11

12

Figure A1: Schematic view of experimental facility. Showing 1. Confining chamber with confining fluid (kerosene) 2. Simulated wellbore with wellbore fluid (KCl Brine) 3. Core sample with pore pressure and pore fluid 4. Gun with the shaped charge 5. Shooting leads 6. Shooting Plate 7. Micrometer valve 8. SWB pressure gauges 9. Confining pressure gauge 10. Pore pressure gauge 11. Pore fluid accumulators 12. Wellbore fluid accumulators

SPE 86542 UB in Carbonates1

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