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Cathodic Protection of Oil and Gas Well Casings
Conference Paper · January 2008
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1st National Iranian Drilling Industry Congress.
Cathodic Protection of Oil and Gas Well Casings
Pejman Malekinejad
Technical Inspection Engineer, Iranian Central Oil Fields Company
East Oil and Gas Production Company.
Saeed Alavi
QC Manager, Moshanir Company, Iran.
Abstract :
This article discusses the use of CP (cathodic protection) for the cost effective
control of external well casing corrosion. CP is an important tool because
maintaining casing integrity is essential to oil and gas production, water and gas
injection, and gas storage fields. When a leak develops, production (or injection)
usually ceases until the leak is repaired or a liner is installed. When corrosion is
severe, the casing can collapse and the well may have to be abandoned, which can
result in lost reserves. CP can be utilized in maintaining casing integrity caused by
external corrosion, thereby reducing operating costs and maximizing total
production and profits.
Key Words:
CP (Cathodic Protection), Casing, Ground Bed, Current Density.
1. Background :
Cathodic protection has been employed in the oil and gas industry for use on well
casings since the late forties. The use of CP on well casings is preceded by its
application to pipelines. Because of its success and because it is the only technique
that can be used to mitigate corrosion after the well is in place, CP is now an
accepted procedure in the oil field.
2. CP of Well Casings versus Pipelines :
CP of well casings differs from pipelines in the following ways [1]:
1. The pipe is vertical to the surface rather than parallel.
3. Pipe-to-soil potential measurements can only be made at the end from which the
current is drained. Potentials cannot be directly measured along the outside surface
of the casing.
4. Well casings are connected by threaded collars rather than welded connections,
which may increase the resistance of the metallic path.
5. Soil/formation changes with length/depth.
6. Well casings are typically installed without an organic coating on the OD (outside
diameter), although most have a partial cement coating.
7. Long lengths of the production casing are shielded from CP by surface and
intermediate casing strings.
3. Up to How much CP of Well Casings is Applied? :
1st National Iranian Drilling Industry Congress.
Oil and gas well casing cathodic protection is widely used all over the world. For
example table 1 shows earliest well casing CP installations (which date from the
1950's) and some of the largest field wide CP systems in the United States done by
Chevron company. The list does not include more than 800 onshore wells with CP
located in Canada.
Table 1: Chevron Operated Oil Fields with Well Casing Cathodic Protection
Location
Date
Installed
# Anode
Beds
# Wells
SACROC
TX
1979
750
1560
Elk Hills
CA
1963
650
2000
East Texas
TX
1970
132
225
Baxterville
MS
1960
125
270
Kettleman
Hills
CA
1958
75
200
Pittsburg
TX
1988
36
45
Taft
CA
1965
30
30
Raleigh
MS
1961
26
26
Coalinga
CA
1965
25
60
Heidelberg
MS
1984
18
24
Field Name
The number of wells that have been placed under CP is impressive, but includes
only a small fraction of the total number of wells operated by the company. The
majority of these systems were installed after well casing leaks became a significant
problem.
4. Limitations :
While CP is a great tool for corrosion control of well casings, there are limitations to
its effectiveness. In some cases it simply may not be practical to get protection to the
bottom of deep well casings. [2]
5. Exposed External Surfaces :
CP can mitigate corrosion only on the exposed external surfaces of a well casing. A
conventional CP system has no effect, positive or negative, on the internal surfaces
of the well casing or any production equipment inside the well casing. Because of
space restrictions, it is not practical to install a long anode (or series of anodes) that
is capable of protecting the inside of the well casing. CP is only effective on those
external surfaces of the casing strings which are in direct contact with the
environment. Conductor pipe, surface casing, and intermediate casing will shield the
areas of the production casing string contained within it. Figure 1 illustrates a typical
well completion diagram and the areas on the casing that receive protection [1].
6. Well Densitym :
1st National Iranian Drilling Industry Congress.
Well density (or spacing) limits how far the anode bed can be placed from the
subject well without interfering with neighboring wells. Ideal anode bed placement
would be the half way point between equally spaced wells. The farther the anode
bed is placed from the subject well, the deeper the well can be protected and the
more even its current distribution. Figure 2 is a plot of the equal potential lines
radiating out from a anode bed. It demonstrates that a well placed farther from an
anode bed will receive more uniform potentials and therefore more uniform
protection.
Figure1: Typical Well Casing Completion Diagram
Figure2: Equal Potential Lines
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Figure3: Current Density versus Anode Bed Distance
7. Varying Earth Resistances :
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Current will always take every available path in proportion to the resistance of the
path. Variation in earth resistances can complicate the current flow and the
distribution of current to the well casing. At deeper depths, high resistance
formations can limit the amount of ionic current flow to the casing.
8. Temperature :
The current requirement of steel increases with increasing temperature. To make
matters worse, attenuation in the well casing reduces the current density with depth
(i.e. the areas of the casing which receive the least protection need it the most).
9. Well Logs :
Well logs [3,4] are essential to a complete CP study because:
1. Casing leaks must be identified as to the cause and location.
2. Effectiveness of the CP system must be evaluated.
3. Downhole resistivities are needed for modeling.
Casing inspection logs are a vital part of a CP study because it would do little to
install a CP system on a field that had experienced only internal corrosion or
corrosion behind a surface casing string. Significant space is devoted to well logs
because they are the only tools which reveals the present condition of the well. All
casing inspection logs require that the rods and tubing be removed from the well. It
is also good practice to make a bit and scraper run before any logs are run in a well.
The extra tubing trip will remove scale, paraffin, corrosion by-product, and allow
any accumulated fill to be washed out. It is important that the scraper tool have
sharp blades and strong springs.
Because of the cost of well preparation, the best opportunity to run a casing
inspection log is during casing repair workovers. This approach not only provides
data for the CP project, but gives the production engineer an idea of where to look
for the leak with his squeeze packer. The inspection log may also indicate that the
casing is in such bad condition that repair by cement squeeze is futile and a liner is
required. Casing inspection tools utilize one of four technologies:
1. Electromagnetic
2. Mechanical
3. Acoustic
4. Optical
In addition, two other types of logs are important in well casing CP:
1. Resistivity
2. Casing Potential Profile [5]
A well logging company representative will be quick to point out that each has its
advantages and to perform a thorough job more than one tool will be needed. While
this is true, it can be argued that a single log will provide enough information for the
CP project. If only one log is run, a combination DC induction (flux leakage/eddy
current) log should be selected. The flux leakage/eddy current log is most adept at
identifying pitting corrosion and can discriminate between internal and external
defects [1].
10. Current Requirements :
1st National Iranian Drilling Industry Congress.
During CP system design and after its installation, one of the most important
questions is how much current to apply. Until a field is energized and wells are
logged, there are no easy answers to this question. This section describes how to
establish a current requirement value [1].
11. Rules-of-Thumb :
There are two rules-of-thumb that circulate in the well casing CP community. The
first is a current density based on the surface area of the well casing. A value of 2
mA/ft2 is used to calculate the current required to protect all steel not covered by a
cement coating. A value of 0.1 mA/ft2 is used to estimate the amount of current to
protect steel covered by cement. A second rule-of-thumb for estimating a current
requirement assumes that the sizes of well casings are roughly the same and that 1
amp of current should be adequate for each 1000 feet of well depth. Both of these
estimating tools break down in actual use because current is not evenly applied to
the well casing. In some cases current may never reach the deeper sections of the
casing. Use these rules-of-thumb for quick ballpark estimates, but do not consider
the answer to be final [6].
12. Mathematical Modeling :
There are three mathematical models currently in use for calculating well casing
current requirements. The first is a model by Schremp-Newton. The technique is a
modification of the Pope attenuation model used for pipelines. The SchrempNewton model requires only some wellhead potential measurements collected from
a specific well already under CP. This is a useful technique for single wells that have
minimal interference effects. It is the preferred procedure for calculating downhole
potentials for various currents on an actual well casing. A second mathematical
modeling technique is an electrical transmission model developed by Dabkowski.
Resistivities at various depths and well patterns are required for input. Dabkowski's
model allows the effects of many different resistivity layers to be evaluated. It also
can evaluate interference effects of multiple anode beds and wells. The third
technique is the BEM (boundary element model). This model is currently in use to
evaluate current requirements and potentials on complex structures, such as offshore
platforms. Because of difficulty in setting up input files with layered resistivities,
convergence problems, and their CPU intensive nature, BEMs are not commonly
used to evaluate well casing CP [7].
Some computer models capable of running on an IBM or compatible are available
for specific applications.
13. Suggested Well Casing Current Requirements :
Table 2 has been prepared in part from Chevron company experience and in part
from calculations made from Dabkowski's model. It is suggested that this table be
utilized as the initial starting point for estimating well casing current requirements.
To find a suggested current requirement simply look up the value given at the
intersection of the well depth and well spacing. Table 2 assumes seven inch casing
and partial primary cement coverage. Numbers were calculated from Dabkowski's
1st National Iranian Drilling Industry Congress.
model using hypothetical field data. For casings deeper than 10,000 ft. or those that
fall into the NR category do not
expect to get protection all the way to the bottom. To facilitate complete CP
coverage, an organic coating should be strongly considered for use on new well
casings that are to be drilled to a depth over 10,000 ft. or on wells that are tightly
spaced. The greatest cost benefit can be achieved by coating the OD of the surface
casing, because the highest current density is near the surface.
Table2: Suggested Well Casing Current Requirements
Well
Spacing
Single
Well
160 Acre
80 Acre
40 Acre
20 Acre
Current
(amps)
Current
(amps)
Current
(amps)
Current
(amps)
Current
(amps)
2,000 ft
1.0
1.5
2.0
2.5
3.0
4,000 ft
3.0
3.5
4.0
5.0
5.5
6,000 ft
5.5
6.0
7.0
10.0
13.0
8,000 ft
7.0
8.0
9.0
11.0
NR
10,000 ft
10.0
12.0
13.0
NR
NR
12,000 ft
12.0
13.0
NR
NR
NR
Well
Depth
To facilitate complete CP coverage, an organic coating should be strongly
considered for use on new well casings that are to be drilled to a depth over 10,000
ft. or on wells that are tightly spaced. The greatest cost benefit can be achieved by
coating the OD of the surface casing, because the highest current density is near the
surface.
14. Anode Bed Design :
Anode bed design is the same as that for cathodic protection of pipelines. Only the
unique considerations for well casing systems will be covered in this article.
15. Type of System :
Well casings will require an impressed current system to provide enough current
unless they are very shallow (1,000 feet or less) or externally coated. Areas with low
surface soil resistivity and ample well spacing are suitable for shallow surface anode
beds. Fields with high surface soil resistivity, close well spacing, and-or dry climates
make the use of deep anode beds more attractive. Despite a high installation cost, a
deep anode bed is the most desirable design because it:
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1. Reduces interference on the well casings by placing anodes in lower resistivity
soil.
2. Reduces interference on pipelines and other surface facilities by placing the
anodes farther away.
3. Has minimal space requirements.
4. Is not subject to wet/dry conditions.
16. Rectifiers :
Of the two conventional types of rectifier stacks (silicon diode and selenium) the
silicon diode or bridge provides the best power conversion efficiency and least
expensive replacement. New high efficiency solid state switching type rectifiers are
now available. Maintenance history should be examined to confirm unit reliability.
The technology that offers the most promise is the pulsed rectifier. Pulsed rectifiers
generate spikes of high current output many times per second. Chevron data shows
that polarization is actually slower, but the final polarized potential is much more
negative. Manufacturers claim that pulse technology uses four times less total
current output and allows the anode bed to be downsized by a comparable amount.
Pulsed rectifiers have been around for a number of years, but were not very cost
effective. A pulsed rectifier should be considered for deep or closely spaced well
casings or where reducing the anode bed size will provide a significant cost savings.
17. Well Completion Enhancements :
There are two major completion related items that can enhance the performance of a
well casing CP system. The first is to make the negative lead connection at the
bottom of the well and run a lead wire to the surface (usually not practical). The
second is to externally coat as much of the casing as possible. Cathodic protection
has a very strong symbiotic relationship with coatings, both organic and inorganic.
This is because coatings reduce the effective surface area of steel exposed to the
environment. There are three compelling reasons to coat a well casing:
1. To ensure adequate protection to the total casing depth
2. To reduce the current requirement
3. To minimize interference
Coating well casings that are to be drilled in closely spaced clusters, onshore or
offshore, may be the only method that will allow protection to depth [1].
18. Improved Primary Cement Jobs :
The simplest thing that can be done to improve current distribution is to insist on a
complete coverage cement job. Cement will decrease the current requirement by a
factor of 20 over bare steel. Figure 4 is a CPP log of a well that contains a DV tool
installed to improve cement coverage. Notice how flat the slope of the curve is in the
areas covered by cement and how steep the slope is in the areas not cemented.
Figure 4: CPP Log of a Well with Improved Cement Coverage
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19. Factory Applied Organic Coatings :
A factory applied organic coating is the most untraditional, yet effective method
available to improve the effectiveness of well casing CP. Current density and
interference are greatly reduced thereby allowing protection to be extended to the
bottom of the casing. A FBE (fusion bonded epoxy coating) can reduce current
densities on the order of ten times lower than a cement coating and 200 times lower
than required for bare steel. There are four documented installations of well casings
that were externally coated before installation. In 1960, United Fuel Gas of West
Virginia drilled 10 wells to 2500 feet. The lower 900 feet of casing were externally
coated. The wells were pulled after 18 months and the coating examined. All of the
coatings were field applied by hand, yet two of the epoxy systems retained an
amazing 95% coating integrity [8]. In 1970, Pacific Gas and Electric drilled 60
closely spaced gas storage wells to 5,400 feet. The casing was coated to depth with
coal tar epoxy. Cathodic protection current requirements have averaged 0.5
amps/well indicating a 95% to 98% effective coating system. No leaks have been
experienced in an area where leak history with bare casings is significant [8]. During
the early 1980's, Sun completed nine of its 11,500 foot wells with FBE coated
casing. Two of the casings were pulled due to non related operational problems and
the coating was examined. Other than a few scuffs, the FBE coating was found to be
in excellent condition. In 1981, two Aramco wells were installed with FBE coated
surface and intermediate casing to a depth of 4300 feet. The current requirement was
reduced over six times that of similar wells with bare casing strings. Two additional
wells in a closely spaced “drilling island” were installed with coated surface casings
to a depth of 4,800 feet. Results from the study showed that the closely spaced wells
were protected with less than 10% of the current required for a similar bare steel
casing despite:
1. The production casing (4,800 ft to 6,150 ft) being run bare.
2. The collars were bare.
3. No effort was made to repair coating damage sustained during transport or
handling [8].
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External coating has not caught on as a standard operating practice in the oilfield,
but is no longer an experimental technique. These case histories have shown that an
externally coated well casing with a good epoxy system such as FBE makes good
operating sense, especially in congested areas, deep wells, or corrosive areas. In the
last 20 years the standard industry practice has been to apply an external organic
coating to all new pipelines.
20. Conclusion :
A well casing CP installation can be summarized as follows:
1. Identify the candidate wells.
a. Search well files.
b. Record date and depth of leak (and other related well data).
c. Use open hole logs to correlate leaks with specific geologic formations.
d. Create a cumulative leak plot and estimate current/future repair cost.
e. Verify cause of leaks with inspection logs.
2. Determine casing current requirement.
a. Use table 2 or Dabkowski's model.
b. Install a pilot anode bed and energize.
c. Use Schremp-Newton's model to adjust current on test wells.
d. Log test wells 90 to 180 days after final adjustment.
3. Design and install the field-wide system.
a. Determine the anode bed type and depth.
b. Decide on the number of wells per anode bed.
c. Calculate the number of anodes and amount of backfill.
d. Estimate the total installed cost and run economic justification.
e. Establish a time table and begin system installation.
4. Monitor and adjust the system.
a. Energize anode beds and adjust to design current density.
b. Check for interference on foreign wells and pipelines.
c. Optionally run a CPP log to determine effects from the field wide system.
d. Establish a monitoring, adjustment, and maintenance program.
21. Acknowledgement :
The head of technical inspection and corrosion department in East Oil and Gas
Production Company, Mr. Javad Mostowfi is gratefully acknowledged for his
technical and financial assistance in this project.
22. References :
1. “Corrosion Prevention Manual,” Chevron Research and Technology Company,
Richmond, CA, December 1997.
1. “Application of Cathodic Protection for Well Casings,” RP-01-86, National
Association of Corrosion Engineers.
2. “Casing Evaluation Services,” Western Atlas International, Atlas Wireline
Services Catalog, 2004.
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3. “Corrosion Evaluation,” Schlumberger Product Catalog, January 2005.
4. “Instruments for In-Place Evaluation of Internal and External Corrosion in Casing
and Tubing,” 1C190, National Association of Corrosion Engineers2003.
5. Hamberg, A. “Well Casing Cathodic Protection Current Requirement Tests,”
COFRC, Report TM88000494, April 2006.
6. Townley, D. “Well Casing Cathodic Protection: Modeling, Interference, and
Protection Criteria,” COFRC, Report TM86001548, November 2005.
7. Orton, M.D., Hamberg, A., and Smith, S.N. “Cathodic Protection of Coated Well
Casing”, Corrosion 2005, Paper 66, San Francisco, CA.
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