CATHODIC PROTECTION OF ONSHORE OIL

WELLS

Prepared by : Abinash Padhy

Email: abinash.cp@gmail.com

Introduction

CP is 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 of Well Casings Vs. Pipelines

The pipe is vertical to the surface rather than parallel.

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.

Well casings are connected by threaded collars rather than welded connections, which may increase the resistance of the metallic path.

Soil/formation changes with length/depth.

Well casings are typically installed without an organic coating on the OD (outside diameter), although most have a partial cement coating.

Long lengths of the production casing are shielded from CP by surface and intermediate casing strings.

Limitations

CP can mitigate corrosion only on the exposed external surfaces of a well casing. A conventional CP system has no effect 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.

Notes

A metallic well casing that is not installed through a corrosive formation and does not have a permanent flow-line does NOT require cathodic protection. However, if a foreign impressed current anode bed is within 200 meters of this type of well, electrical bonding shall be implemented to minimize the probability of Downhole interference.

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.

Notes

A metallic well casing that has not been coated with Fusion Bonded Epoxy (FBE) and is installed within a metallic or non-metallic cellar and backfilled shall be provided with two prepackaged 27.5 kg (60 lb) magnesium anodes installed inside the wellhead cellar to provide supplemental cathodic protection for the landing base area.

Supplemental galvanic anodes for permanent protection of the landing base area are not required for well casings that:

*Do not have cellars *Are installed in cellars that are not backfilled, or * Are FBE coated through the cellar area. .

Corrosivity increases in deeper layers of soil where formation is restricted by salt and concentration cells may formed.

Coated Casing

A well casing with an external non-conductive coating (typically Fusion Bond Epoxy or FBE). The coating must be applied to all sections of the casing in contact with soil or formation, from surface to the bottom of the casing or to a depth determined to facilitate external corrosion mitigation with cathodic protection through the relevant down hole corrosive aquifers.

Casings that have been coated over the upper two or three joints of casing only are not “coated casings”. Coating applied to well casings is not applied as a corrosion barrier. It is applied to reduce the total amount of CP required or to extend the influence of the applied CP.

Coated well casings do not require a dedicated negative cable and are not restricted by the 2 km separation imposed on CP systems with multiple uncoated well casings.

Uncoated Casing

A well casing that is either bare (no external coating), or may have an external coating applied to the shallow sections of casing to minimize corrosion in the landing base or surface soils.

Uncoated well casings separated by less than 2 km may utilize a single CP power source.

Investigation for Corrosion Damage

1. Exact depth of the screw socket connections2. Reduction from the average wall thickness3. Attach by internal corrosion penetration.

** Methods 1 & 2 are electromagnetic or eddy current While Method 3 is a mechanical system similar to caliper pig.

CP For Onshore Well Casings

DEDICATED ICCP SYSTEM

SHARED ICCP SYSTEM

GALVANIC CP SYSTEM

CP POWER SUPPLY

Photovoltaic Galvanic AC Powered

Air Cooled/Oil

Cooled

Single Phase/Three

Phase

Remote monitoring unit

Signal Transmitters, or

A Remote Monitoring Unit (RMU)

Operating parameters to be monitored Communication system to be decided Software to be decided

Design

Estimate current requirement.

Design test groundbed for E log I test.

Determine anode-well separation.

Size photo voltaic power system.

Design anode suited to power source.

Design

Soil Resistivity Survey

Site Information- Buried structures within 200 m of the proposed anode bed & well casing.

Field verified operating data for existing CP equipment within 200 meters of the proposed anode bed or associated well casing.

Site tests on each relevant type of well casing to determine the respective resistance to ground are required.

Design

Permanent buried flare lines and blow down lines on a well pad shall be made permanently electrically continuous with the well casing or the negative circuit of the well casing CP system.

The use of a photovoltaic CP power supply shall be assessed from an economic perspective for cathodic protection of new wells located more than 3 kilometers from the nearest 4.16 kV or 13.8 kV power line.

Resistors shall not be designed into a multi-well CP system, however, if during commissioning, adequate current distribution is not achieved, If resistors are approved as stated above, they shall be welded tap adjustable or fixed (non-adjustable) resistors and shall be 0.15 ohms or less

Distance from Anode Bed-Well casings & Other protected Structures

For an anode bed discharging 25 amperes or more, a minimum distance of 150 meters shall be maintained between the nearest anode and the well casing.

For an anode bed discharging less than 25 amperes, a minimum distance of 75 meters shall be maintained between the nearest anode and the well casing

Mathematical Modeling for Current Requirement

Schremp-Newton. The technique is a modification of the Pope attenuation model used for pipelines. The Schremp- Newton 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.

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.

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

Interference

In oilfields with high paraffin content, the production tubing has to be cleaned by heating every week to remove the paraffin deposits. The heating of the pipe takes place while a DC of about 1.2 kA is fed into the production tubing from the wellhead.

During the heating period the lower part of the casing is anodically polarized, this can interfere with the wells within a radius of 1000 m.

CP Monitoring for well casings

Protection Criteria

Incase of separate protection current supply for flow lines, the pipe/soil On potential should be -1.5 V CSE.

Interference can only be suppressed by an equal potential bond to the cathodically protected structure.

Conclusion1. 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 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