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The Lease Pumper’s Handbook

CHAPTER 16

CORROSION, SCALE, AND CATHODIC PROTECTION

A. CORROSION AND SCALE.1. Introduction to Corrosion.2. Carbon Dioxide Corrosion.3. Hydrogen Sulfide Corrosion.4. Oxygen Corrosion.5. Electrochemical Corrosion.6. Problems with Scale.

· Stopping scale in the formation and chemical treatment.· Protective coatings.· Scale removal. Problems with scale caused by construction practices.

7. Other Types of Corrosion.B. CORROSION PROTECTION.

1. How to Prevent Corrosion Damage.· Rust.· Oxidation.· Painting.· Inside coating and linings.· Oiling the outside.· Insulating flanges.· Sacrificial anodes.· Electrical current.· Tarring and wrapping for underground black metal lines.· Conduits.· Fiberglass.· Galvanized bolted tanks.· Stainless steel and metal plating.· Plastic flow lines.· Chemical protection.· Mechanical barriers.

2. Locating Corrosion Damage Downhole.3. Protecting the Casing Long String.

· The use of insulating flange unions.· The use of sacrificial anodes.

4. Corrosion Protection at the Tank Battery.· Tank battery elevation and ground protection.· Lines protection.· Protecting vessels on the inside.

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16A-1

The Lease Pumper’s Handbook

Chapter 16Corrosion, Scale, and Cathodic Protection

Section A

CORROSION AND SCALE

The purpose of this chapter is to assist thelease pumper in understanding corrosion andhow to reduce the damage it causes in wellsand surface facilities. In addition, thischapter provides a basic overview of scaleand scale control for wells that produce a lotof water. Examples of corrosion and scaleproblems are shown in Figures 1 and 2.

Figure 1. Note corrosion on left item, adownhole pump part, and a parted rod.

Figure 2. Scale in a cut-out section oftubing showing how it is deposited inside

the unprotected pipe.

A-1. Introduction to Corrosion.

Corrosion is a general term for a reactionbetween a metal and its environment thatcauses the metal to breakdown. While thereare many types of corrosion, they all involveeither a chemical reaction or anelectrochemical reaction. In chemicalreactions, chemicals in the environmentreact with the metal to create differentchemicals. Thus, atoms or molecules of themetal combine with other atoms ormolecules that contact the metal to formdifferent, generally weaker, materials. Rustis an example of this type of corrosion.

In electrochemical corrosion, theenvironment around the metal results in thecreation of an electrical current, which issimply a flow of electrons. The metalcorrodes by giving up electrons to create theelectrical flow.

The oil field environment is filled withmetal pipes and other components that oftenexposed to chemicals that can causecorrosion, especially when the metal andchemicals are in a solution such as downholefluids. The pumper must understand how toreduce corrosive damage to the metal inwells, flow lines, tank batteries, andequipment.

There are four general types of corrosionof concern in the oil field. These involvethree chemicals of concern andelectrochemical corrosion. The types ofcorrosion include:

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· Carbon dioxide (sweet corrosion)· Hydrogen dioxide (sour corrosion)· Oxygen corrosion (oxidation)· Electrochemical corrosion

A-2. Carbon Dioxide Corrosion.

Carbon dioxide (CO2) is a corrosivecompound found in natural gas, crude oil,condensate, and produced water. CO2

corrosion, or sweet corrosion, is common inthe oil fields in southern Oklahoma, NewMexico, the Permian Basin of Texas, andthe Continental Shelf along the Gulf ofMexico because of the high CO2 content ofthe crude from these areas. Specialrefineries capable of refining high CO2 crudeoil are expensive to build and maintain.Most of these refineries in the U.S. areclustered in the Gulf Coast area. Much ofthe crude oil from Alaska must be refined inthis area because it also contains a largeamount of CO2.

CO2 is composed of one atom of carbonwith two atoms of oxygen. When combinedwith water (H2O), carbon dioxide producescarbonic acid (H2CO2). Carbonic acidcauses a reduction in the pH of water andresults in corrosion when it comes in contactwith steel. When iron (Fe) combines withcarbonic acid, it produces iron carbonate(FeCO2). Iron carbonate is not as strong asthe refined iron or steel used to make thewell components.

The damage caused by sweet corrosion inoil wells usually results in pitted sucker rodsand the formation of hairline cracks.Corrosion test coupons can be inserted intothe lines to indicate the level of iron removaland other corrosive conditions. Calipersurveys can also be used to determine theextent of tubing damage. Chemicalinjection, alloys, and protective coatings areused to combat the problem.

A-3. Hydrogen Sulfide Corrosion.

Hydrogen sulfide (H2S) occurs inapproximately 40% of all wells. Theamount of H2S appears to increase as thewell grows older. H2S combines with waterto form sulfuric acid (H2SO4), a stronglycorrosive acid. Corrosion due to H2SO4 isoften referred to as sour corrosion. Sincehydrogen sulfide combines easily withwater, damage to stock tanks below waterlevels can be severe.

Solutions to sour corrosion problems aresimilar to those for sweet corrosion. Thisincludes the use of chemicals, alloys, andcoatings to combat the problem and reducethe damage. Circulating chemical down theannulus is a common practice to treatdownhole problems.

When sufficient amounts of H2S areproduced with the emulsion, the pumpermust wear a gas mask when gauging orworking oil—that is, testing oil at the thiefhatch to determine if it is ready for sale(Figure 3).

Figure 3. Where H2S gas is present, sourcorrosion is likely and breathing

apparatus is required.

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A-4. Oxygen Corrosion.

Oxygen corrosion or oxidation is the mostcommon form of corrosion. On steel andiron, oxidation typically takes the form ofrust. Painting the tank battery and othersurface equipment to eliminate this contactis basic oxygen corrosion protection.

Oxygen corrosion begins when equipmentmakes contact with the atmosphere andmoisture. Under these conditions, the ironand oxygen react with each other to formferric oxide (Fe2O3), which is commonlyreferred to as rust. Oxidation can also occurwith other metals, including aluminum.Although the compounds formed will bedifferent, the results are the same in that themetal is weakened, usually undergoingembrittlement in which the metal becomesbrittle. Oxidation can also accelerate thedamage from sweet corrosion.

An oil blanket is often deliberatelymaintained on produced water to block theatmospheric oxygen from contacting thewater. A system that prevents theatmosphere from making contact with theproduced water is referred to as a closedsystem. A system that allows the air tocontact the produced water is referred to asan open system. Stripper wells are oftenproduced with the casing valve open to theatmosphere, which promotes downholecorrosion. Water flood can also injectoxygen into the formation, promotingoxidation. Open systems are sometimespreferred because the formation of rust canactually help to protect against some formsof corrosion, such as electrochemicalcorrosion described later.

A-5. Electrochemical Corrosion.

There are two common types ofelectrochemical corrosion. One is the result

of electrical current leaking into theenvironment. Electrical current may beplanned to power electrical equipment or itmay be generated accidentally, such as thestatic electricity produced by the windblowing. Corrosion can occurs wherever theelectrical current leaks into the environment.For example, poor grounding may allowstray current to enter a pipe. When thecurrent reaches a wet area, the current mayflow from the pipe. Electrochemicalcorrosion is likely to occur at that point.

The second type of electrochemicalcorrosion is more common. It occurs whenmetal in water, such as downhole parts orpipe laid in moist soils, becomes part of anelectrical cell. Such a cell is essentially anacid battery and will be formed just aboutany time two different types of metals areplaced in an acidic solution. Electrons fromone metal will flow to the other metal. Thisresults in the metal that gives up electronsbeing eaten away and the other metalbuilding up a brittle coating. The metal thatgives up electrons and corrodes away isreferred to as the anode, and the metal thatcollects electrons is called the cathode.

Techniques that are implemented to slowdown or eliminate electrochemical corrosionare referred to as cathodic protection. Incathodic protection, the flow of electricalcurrent is altered to prevent the metal to beprotected from serving as an anode.

A-6. Problems with Scale.

Scale, or gyp, is carried with water in asolution and migrates toward the well bore.Problems with scale can begin as itapproaches the matrix or well bore area. Itcan plug the formation, casing, and tubingperforations; make tubing in the hole stick;fill tubing to the point that the pump cannotbe pulled without stripping; and plug flow

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lines. At the tank battery it can fill lines andvessels and accumulate in the bottom as asolid. Scale can accelerate electrochemicaldamage by acting as a cathode to the steel,resulting in deep pitting.

Stopping scale in the formation andchemical treatment. The first place tosolve problems with scale is in theformation. Chemicals are available that canbe blended with water and pumped into theformation to stabilize the scale and highlyreduce its accumulation in the system. Itmay be necessary to acidize the wellsperiodically to clean up the immediatereservoir area and to reopen the casingperforations. Special small materials thatare water- or oil-soluble can be blended intothe fracing compounds to control the processand increase the efficiency of the treatment.

Figure 4. The chemical tank(background) and the pump and injectiontee (left of wellhead) are used to combat

scale with chemicals.

Protective coatings. Special coatings canbe applied to reduce the ability of scale tocling to the inside of the tubing. Withflowing wells, this coating may be appliedlike paint. On pumping wells the rod actionwould damage the coating. Consequently,special chemicals can be circulated down the

casing that are produced back up through thetubing to protect perforations, tubing, andthe downstream flow line and tank battery.This treatment can be a continuous orperiodic batch treatment.

Scale removal. When systems arepermitted to fill with scale, it may benecessary to disassemble, clean, and rebuildof the system. A good and efficient scalecontrol program must be maintained.

Scale can be reduced and removed by slowand expensive chemical processes. Scaleaccumulation in tubing can also be scrapedor drilled out. In vessels, it may require theremoval of the manway plates, entering thevessel with the proper safety equipment, andphysically shoveling the accumulation out.

Problems with scale caused byconstruction practices. Good constructionpractices should always be followed toprevent building systems that trap scale andcause unexpected problems. As an example,the line from the wellhead to the tank batteryshould not contain any 90-degree bends.Even an ell in the line may not besatisfactory. Pipe can be bent or slow curvesinstalled to eliminate any scale trapsbetween the well and the tank battery.

A-7. Other Types of Corrosion

Many environmental factors can influencethe effects of corrosion. The chemicalcontent of the soil and the production fluid,the climate, the materials used for wellcomponents, and other factors have aneffect. The presence of microorganisms canaccelerate corrosion, and microbiologicalcorrosion in which organisms eat thematerials or chemically transform them isalso common.