Controlling Corrosion

Inhibiting or minimizing corrosion is extremely important when planning a well. This section provides a brief overview of the topic. A more complete discussion can be found in Chapter 8 in the "Corrosion Control" section.

Objectives

• Protect casing, liner, and downhole tools
• Minimize corrosion promoters
• Render corrosion products nondamaging to the formation
• Protect packer and production tubing

Factors Affecting

• Temperature
• Brine acidity (pH)
• Surface aeration and air entrainment
• Acid formation gases (CO₂, H₂S)
• Metallurgy
• Bacteria

Discussion

The factors affecting corrosion are very complex. The information presented here is intended as a brief introduction. TETRA has done extensive testing in the area of corrosion, especially as it relates to environmentally assisted cracking (EAC) in HPHT wells. A TETRA fluids specialist will be happy to assist in developing solutions aimed at reducing the probability of corrosion in your well.

Temperature. Most chemical reactions proceed more rapidly at higher temperatures. This is also true for the various reactions involved in the corrosion process. Temperature conditions in the well will provide the basis for choosing a corrosion program. With proper protection, by means of a thoroughly planned corrosion control program, brines are routinely used at temperatures as high as 350°F with corrosion rates of less than 15 mils per year (mpy). Recommended application rates for TETRAHib™ and CORSAF™ corrosion inhibitors, OxBan™ oxygen scavengers, and biological control additives are provided in Table 7, "Typical Corrosion Control System Applications," at the end of this section.

Brine Acidity (pH). Single salt fluids range from neutral to slightly basic when they are manufactured. They can be treated to increase alkalinity and reduce the presence of the corrosion promoting hydrogen ion (H+) with the careful addition of a base such as sodium hydroxide or lime.

Two salt calcium chloride/calcium bromide fluids are neutral to slightly basic in nature. Alkalinity can be adjusted to further reduce the presence of the corrosion promoting hydrogen ion (H+). This adjustment, however, is not easily accomplished in the field. Because of this, all TETRA two salt fluids are carefully blended to eliminate the presence of corrosion promoters.

Three salt fluids are prepared using calcium chloride (CaCl₂), calcium bromide (CaBr₂), and zinc bromide (ZnBr₂). When zinc is dissolved, it has a tendency to create acidic conditions. If a solution is made more alkaline, then zinc may begin to precipitate as zinc hydroxide (Zn(OH)2). In order to maintain the physical properties of zinc bromide brines, the pH must be kept at a relatively low level. Because corrosion is accelerated by low pH, special attention should be given to minimizing corrosion in three salt fluids. TETRA has a long history of manufacturing zinc bromide and formulates all of its zinc products to minimize acidity and corrosion.

Contact a TETRA fluids specialist if you have concerns about zinc precipitation. TETRA has developed a number of solutions to address this problem.

Surface Aeration and Air Entrainment.

Oxygen derived from the air is a major corrosion accelerator. Oxygen solubility in concentrated salt solutions is extremely low and becomes even lower as brine temperatures rise. Oxygen can, however, be introduced into the circulating system if fluids are allowed to freefall into tanks. Other possible sources for oxygen are leaking pump seals, agitators, and suction pumps. Small air bubbles can be entrained in more viscous brines and carried down into the well. With increasing pressure, the entrained air will eventually dissolve and react with casing, tubing, or downhole tools. To reduce the impact of surface aeration, it is prudent to add a small amount of oxygen scavenger.

OxBan HB can be used at the level of five gal/100 bbl and up, depending on oxygen entrainment. If not supervised, this course of action can lead to overtreatment. Preventative measures should be taken to eliminate air entrainment to reduce such overtreatment.

The presence of trace amounts of oxygen with sulfur containing species can be a dangerous combination with respect to EAC. For more information, see the "Corrosion Control" section in Chapter 8.

 

Acid Formation Gases. More common in a completion fluid situation, gases such as carbon dioxide (CO₂) and hydrogen sulfide (H₂S) can accelerate corrosion. Both gases are slightly acidic in nature and will contribute to the acidity of a brine.

Metallurgy. It is essential that information concerning the metallurgy of casing and tubing be considered in the planning and design of any completion. If carbon steel tubing is to be used, the issue of general corrosion must be adequately addressed. If CRA tubing is to be used, the issues of EAC must be addressed, with the compatibility between the fluids and tubing being carefully evaluated, especially if the fluid is to be used as a packer fluid. Through participation in extensive scientific test studies in the area of CBFs, metallurgy, and EAC, TETRA has developed a software program called the MatchWell fluid compatibility selector. It can be used to predict tubing/fluid compatibility and performance and make fluid recommendations based on specific well conditions. For more information about EAC, read the "Environmentally Assisted Cracking" section in Chapter 8.

Consult your TETRA representative to take advantage of this technology and receive a customer recommendation report from the MatchWell fluid compatibility selector to assist you in planning your next HPHT well completion.

Bacteria.

In spite of the salinity and high temperatures found in the subsurface environment, bacteria have been found to exist in some of the world's most extreme environments. Especially adaptable are iron bacteria, sulfur oxidizing bacteria, and sulfate reducing bacteria. The presence of these microorganisms can dramatically increase the corrosivity of the environment, especially if H2S is generated from the bacteria. Brines that are properly formulated with biocides can eliminate these bacterial problems.

 

Recommendations

1. Use a properly formulated TETRA clear brine fluid that has been manufactured to the highest specifications.
2. Select a corrosion inhibitor package that is compatible with the metallurgy at the expected bottomhole temperature.
3. Try to reduce all sources of entrained air such as freefalls, excessive agitation, leaking pump seals, and suction vortices.
4. Do not run jet hoppers unless a polymer is being added.
5. Whenever possible, minimize the contact between CBFs and acidic gases such as carbon dioxide (CO₂) and hydrogen sulfide (H₂S).
6. Select a brine formulation to help neutralize acidic gases.

Table 7 provides recommended application rates for TETRAHib™ and CORSAF™ SF corrosion inhibitors, OxBan™ oxygen scavengers, and biological control additives for different brine density ranges.

---

TABLE 7. Typical Corrosion Control System Applications

 

¹¹¹ Fluid Density  Temperature  Corrosion Inhibitor Dose  Oxygen Scavenger Dose BiocideDose

KCl, 3%	8.6	200ºF	TETRAHib	10	OxBan	1.5	Antimicrobial Biocide	1
		300ºF	TETRAHib	15	OxBan	1.5	Antimicrobial Biocide	1
KCl	9.7	200ºF	TETRAHib	15	OxBan	1	Antimicrobial Biocide	1
		300ºF	TETRAHib	20	OxBan	1	Antimicrobial Biocide	1
NaCl	10.0	200ºF	TETRAHib	15	OxBan	1	Antimicrobial Biocide	1
		300ºF	TETRAHib	20	OxBan	1	Antimicrobial Biocide	1
NaBr	12.0	200ºF	TETRAHib	15	OxBan	1	Antimicrobial Biocide	1
		300ºF	TETRAHib	20	OxBan	1	Antimicrobial Biocide	1
CaCl2	10.0	200ºF	TETRAHib	15	OxBan HB	10	Antimicrobial Biocide	1
		300ºF	TETRAHib	20	OxBan HB	10	Antimicrobial Biocide	1
CaCl2	11.6	200ºF	TETRAHib Plus	5	OxBan HB	10	Antimicrobial Biocide	1
		300ºF	TETRAHib Plus	7.5	OxBan HB	10	Antimicrobial Biocide	1
CaCl2 + CRA 2	11.6	200ºF	CORSAF SF	20	OxBan HB	10	Antimicrobial Biocide	1
		300ºF	CORSAF SF	30	OxBan HB	10	Antimicrobial Biocide	1
CaCl2/Br2	14.5	200ºF	TETRAHib Plus	10	OxBan HB	10	Antimicrobial Biocide	1
		300ºF	TETRAHib Plus	15	OxBan HB	10	Antimicrobial Biocide	1
CaCl2/Br2 + CRA 2	14.5	200ºF	CORSAF SF	20	OxBan HB	10	Antimicrobial Biocide	1
		300ºF	CORSAF SF	30	OxBan HB	10	Antimicrobial Biocide	1
CaCl2/Br2	15.2	200ºF	TETRAHib Plus	10	OxBan HB	10	Antimicrobial Biocide	1
		300ºF	TETRAHib Plus	15	OxBan HB	10	Antimicrobial Biocide	1
CaCl2/Br2 + CRA 2	15.2	200ºF	CORSAF SF	20	OxBan HB	10	Antimicrobial Biocide	1
		300ºF	CORSAF SF	30	OxBan HB	10	Antimicrobial Biocide	1
Zn/CaCl2/Br2	16.0	200ºF	TETRAHib Plus	15	OxBan HB	10	Antimicrobial Biocide	1
		300ºF	TETRAHib Plus	20	OxBan HB	10	Antimicrobial Biocide	1
Zn/CaCl2/Br2 + CRA 2	16.0	200ºF	CORSAF SF	20	OxBan HB	10	Antimicrobial Biocide	1
		300ºF	CORSAF SF	30	OxBan HB	10	Antimicrobial Biocide	1
Zn/CaCl2/Br2	19.0	200ºF	TETRAHib Plus	15	OxBan HB	10-15	Antimicrobial Biocide	1
		300ºF	TETRAHib Plus	20	OxBan HB	10-15	Antimicrobial Biocide	1
Zn/CaCl2/Br2 + CRA 2	19.0	200ºF	CORSAF SF	20	OxBan HB	10-15	Antimicrobial Biocide	1
		300ºF	CORSAF SF	30	OxBan HB	10-15	Antimicrobial Biocide	1

¹

Dose quantities are in U.S. gallons per 100 barrels of brine, gal/100 bbl

²

Corrosion Resistant Alloy (e.g., 13 Chrome)