WO1996033296A1

WO1996033296A1 - Method for inhibiting microbially influenced corrosion

Info

Publication number: WO1996033296A1
Application number: PCT/US1995/009131
Authority: WO
Inventors: Edward Daniel Burger
Original assignee: Bio-Technical Resources Lp
Priority date: 1995-04-17
Filing date: 1995-07-19
Publication date: 1996-10-24
Also published as: AU3102395A

Abstract

A non-biocidal method for inhibiting microbially influenced corrosion of susceptible metal surfaces having an anaerobic biofilm containing active sulfate-reducing bacteria comprising contacting the biofilm with a liquid dispersion of anthraquinone compound.

Description

TITLE

Method for Inhibiting Microbially Influenced Corrosion

FIELD OF INVENTION

The invention is directed to a method for inhibiting microbially influenced corrosion. In particular, it is directed to a method for inhibiting microbially influenced corrosion of susceptible metals in contact with aqueous liquid systems.

BACKGROUND OF THE INVENTION

In the oil industry, uncontrolled microbial growth and activity can create severe operational, environmental, and human safety problems. Problems caused or intensified by microbial growth and activity include corrosion, solids production, and hydrogen sulfide (H₂S) generation.

The microorganisms primarily responsible for corrosion in an anaerobic environment within the oil industry are sulfate-reducing bacteria.

These organisms are ubiquitous and can grow in almost any environment. They are routinely found in waters associated with oil production systems and can be found in virtually all industrial aqueous processes, including cooling water systems, paper-making systems, and petroleum refining.

Requirements for sulfate-reducing bacteria activity and growth include an anaerobic (oxygen-free) aqueous solution containing adequate nutrients, an electron donor, and electron acceptor. A typical electron acceptor is sulfate, which produces H₂S upon reduction. A typical electron donor is a volatile fatty acid (e.g., lactic, acetic, or propionic acids), although hydrogen can also function as the electron donor. Conditions in an oil reservoir subject to seawater flooding are excellent for establishing sulfate-reducing bacteria activity. Seawater contains a significant concentration of sulfate, while connate, or indigenous formation, water contains volatile fatty acids and other required trace nutrients (e.g., nitrogen and phosphorus). Mixtures of the two waters in a reservoir provide all of the essential conditions for sulfate-reducing bacteria activity. This condition will result in sulfide generation within the reservoir, which is referred to as reservoir souring. Hydrogen sulfide is corrosive and reacts with metal surfaces to form insoluble iron sulfide corrosion products. In addition, H₂S partitions into the water, oil, and natural gas phases of produced fluids and creates a number of problems. For instance, oil and gas which contain high levels of H₂S have a lower commercial value than low sulfide oil and gas. Removing biogenic H₂S from sour oil and gas increases the cost of these products. Hydrogen sulfide is an extremely toxic gas and is immediately lethal to humans at even small concentrations. Thus, its presence in the oil field poses a threat to worker safety. The discharge of produced waters containing high levels of H₂S into aquatic or marine environments is hazardous because H₂S reacts with oxygen and lowers the dissolved oxygen levels in the water.

Waters produced from a reservoir in association with oil production, especially those resulting from a seawater flood, will typically contain sulfate-reducing bacteria and required nutrients. Conditions in surface facilities (e.g., pipelines, vessels, tanks) are usually quite favorable for sulfate-reducing bacteria activity. Furthermore, they are capable of activity and growth in a wide range of temperatures found in the oil field. Reduced temperatures in surface facilities many times enhance microbial growth as compared to elevated temperatures within the reservoir. Oil production operations favor the growth of anaerobic sulfate-reducing bacteria since those environments are usually kept oxygen-free to avoid oxidation corrosion of steel vessels, pipelines, and tanks. However, even if systems are aerobic, localized anaerobic conditions are maintained on the metallic surface (the substratum) beneath the biofilm due to oxygen consumption by aerobic bacteria.

Bacterial corrosion is actually caused by sessile anaerobic bacteria living under a thick biofilm composed of aerobic and facultative bacteria enmeshed in a fibrous anionic ion exchange resin that severely limits the penetration of charged molecules. (Costerton, J. W. and E. S. Lashen, "Influence of biofilm on efficacy of biocides on corrosion-causing bacteria," Materials Performance, 23, No. 2, p.13, 1984.) Waters used for cooling in heat exchangers are normally not deaerated, but sulfate-reducing bacteria growth flourishes on tube bundles unless extreme preventative measures are taken.

Sulfate-reducing bacteria activity in surface facilities is a source of H₂S production, which causes corrosion, and results in the production of solid corrosion products which may cause operational problems such as plugging of water-injection perforations in injection wells. (Produced waters are frequently reinjected into the formation for secondary oil recovery purposes or may be disposed of by injection into a different portion of the reservoir.) Inhibition of sulfate-reducing bacteria activity will reduce H₂S production and will halt anaerobic corrosion of the steel surfaces, thereby reducing solids formation.

Corrosion (pitting) caused by sulfate-reducing bacteria frequently results in extensive damage. Pipe systems, tank bottoms, and other pieces of oil production equipment can rapidly fail if there are areas where microbial corrosion is occurring. If a failure occurs in a pipeline or oil storage tank bottom, the released oil can have serious environmental consequences. If a failure occurs in a high pressure water or gas line, the consequences may be worker injury or death. Any failure involves repair or replacement costs.

Potential methods for mitigating sulfate-reducing bacteria activity include: temperature control, metabolite removal, pH control, Eh control, radiation, filtration, salinity control, chemical control (e.g., oxidizers, biocides, acids, alkalis), solids control (e.g., pigging or scraping the internal pipeline), and bacteriological controls (e.g., bacteria phages, enzymes, parasitic bacteria, monoclonal antibodies, competitive microflora). Some of these methods will kill the sulfate-reducing bacteria, while others stress or disturb them sufficiently to inhibit their activity.

Most of the above methods are not practical for oil field implementation due to their cost or potential effect on the downstream processes. For example, treating of large quantities of water by heating to sterilization temperatures, by filtering out the microscopic bacteria, or by removing a nutrient (e.g., sulfate) is prohibitively expensive due to large equipment and energy requirements. Removal or the killing of bacteria from a process stream must be 100% effective or else exponential growth of surviving bacteria will recolonize downstream surfaces. In addition, all downstream surfaces must be sterilized (i.e., bacteria-free) prior to implementation of a sulfate-reducing bacteria mitigation process upstream or else sulfate-reducing bacteria growth will continue within the biofilm. Two typical methods of controlling sulfate-reducing bacteria in oil field pipeline systems are pigging and biocide treatments. Pigging is required to remove or disrupt the biofilm on the pipe surface. Pigging can also remove many of the iron sulfide deposits which may be acting as cathodes to the corroding anodic areas. While pigging will be substantially effective where thick biofilms are present, thin biofilms and thin iron sulfide deposits are not appreciably affected by the scraping action of pigs. Subsequently, biocides and surfactant-biocide treatments are used extensively to control bacterial activity in oil field systems. Combination treatments in conjunction with pigging are more effective than the chemical treatments alone. However, treatments must be made routinely on a fixed schedule or else the bacteria population increases significantly and control becomes even more difficult. Monitoring the effectiveness of treatments must include the sessile bacteria, for the reason that the numbers of planktonic bacteria following a biocide treatment may have no correlation with the sessile bacteria involved with the corrosion process.

It has proved difficult to eradicate biofilms from pipelines because of their great resistance to bactericidal agents. The concentration of biocides required to kill bacteria in the sessile phase (in the biofilm) are often much higher than those required for bacteria in the planktonic or free-floating phase. (Blenkinsopp, S. A., Khoury, A. E. and Costerton, J. W., "Electrical enhancement of biocide efficacy against pseudomonas aeruginos biofilms," Applied and Environmental Microbiology, 58, No. 11, p. 3770, 1992.) This may be due to the role of the abundant exopolvsaccharide matrix of the biofilm. It has been suggested that the diffusion resistance in the biofilm mode of growth can be overcome by the imposition of a relatively weak DC electric field so that the biofilm bacteria can readily be killed by concentrations of biocides only one to two times those necessary to kill planktonic cells of the same organism. While this new technology may be technically effective, it appears to be impractical to apply it in a commercial pipeline system.

SUMMARY OF THE INVENTION

In the broadest aspect, the invention is directed to a non-biocidal method for inhibiting microbially influenced corrosion of microbially influenced corrosion-susceptible metal surfaces having an anaerobic biofilm containing active sulfate-reducing bacteria comprising contacting the biofilm with a liquid dispersion of anthraquinone compound by which the anthraquinone passes through the pores of the biofilm and is diffused within the biofilm to effect contact with the sulfate-reducing bacteria.

In a second aspect, the invention is directed to a method for inhibiting microbially influenced corrosion of susceptible metal surfaces by coating the metal surface with a solid organic polymer in which are dispersed finely divided particles of anthraquinone compound, the coating thereby forming a barrier between the metal surface and any anaerobic biofilm formed thereon.

In a further aspect, the invention is directed to a method for inhibiting microbially influenced corrosion of pipelines constructed from susceptible metals through which liquids are transported in turbulent flow comprising introducing a liquid dispersion of anthraquinone compound into the flowing liquid as a plug the volume of which is sufficient to provide contact with a given point within the pipe for at least one minute.

BRIEF DESCRIPTION OF THE DRAWINGS

The Drawing consists of two figures of which Figure 1 is a schematic representation of a typical biofilm and Figure 2 is a schematic representation of the valving involved in the use of pipeline scrapers (pigging).

DETAILED DESCRIPTION

Biofilm:

A biofilm is a heterogeneous accumulation of bacterial colonies attached to a substratum. Though characterized as a "biofilm" it is neither completely biological, nor is it continuous in the conventional sense of the word "film."

Recent studies indicate that a biofilm consists of discrete bacterial microcolonies immobilized on a substratum immersed in an aqueous medium, the microcolonies being separated by water channels though which convective flow can take place. The microbial cells are held together and to the substratum surface by extra cellular polymeric substances. Within the context of the invention, the medium (also called "the substrate") is an anaerobic liquid and at least a significant portion of the bacteria in the biofilm are sulfate- reducing. However, the biofilm can contain other co-existing bacterial species. In addition, the biofilm can contain extraneous material such as exoenzymes, solutes and inorganic inclusions such as corrosion products, silt and clay particles.

Figure 1 is a schematic representation of a biofilm which is attached to a metal substratum 1. As shown, a continuous thin layer of bacteria 3 are directly attached to the substratum 1. However, this layer 3 is not always continuous and its continuity does not enter into the efficacy of the invention in this environment. Attached to the thin bacterial layer 3, and/ or directly to the substratum 1, as the case may be, are a series of bacterial cell clusters 5 having channels between them through which the aqueous medium 7 can flow. Because of the porosity of the bacterial cell clusters 5, the aqueous medium 7 and materials dispersed therein are able to enter the structure and contact bacteria within the structure.

Anthraquinone Functionality and Applications

While biocides are aimed at killing sulfate-reducing bacteria, anthraquinones inhibit their activity. Studies have shown that anthraquinone blocks the production of adenosine triphosphate by sulfate-reducing bacteria, thereby removing the bacteria's ability to respire via sulfate reduction.

Without sulfate reduction, H₂S is not produced by the bacteria.

Biocides are very reactive, a property which is likely responsible for their limited effectiveness in penetrating into biofilms at low dosages. The extraordinary effectiveness of various forms of anthraquinone lies in their non- reactivity. These products are transported into the biofilm, diffuse through the biofilm voids, and then diffuse or are randomly transported by Brownian motion into the bacterial microcolonies without reduction in concentration as a consequence of a reaction with biofilm constituents. These anthraquinone materials are unaffected by other bacteria or the exopolysaccharide matrix present in the biofilm. Even though solid particles of anthraquinone are required to inhibit the sulfate-reducing bacteria activity, the anthraquinone can be introduced into the microbial environment in several physical forms. The anthraquinone can be introduced as a dispersion of these solid particles while an ionic (sodium salt) form of anthraquinone will allow the anthraquinone to be solubilized in an anaerobic caustic solution with pH greater than 12 and preferably greater than 13. The salt stays soluble if the pH of the solution remains above about 12, with precipitation of solid anthraquinone taking place as the pH is reduced below this value. In the soluble form, or with a slight amount of precipitated anthraquinone (typically in colloidal form), anthraquinone is in molecular form or consists as extremely small (submicron- sized) particles. The anthraquinone molecules or colloidal particles will then be able to move freely in the biofilm, contacting sulfate-reducing bacteria cells easily. Contact of anthraquinone with the sulfate-reducing bacteria, and partitioning of the anthraquinone into the cell membrane, blocks the organism's adinosine triphosphate production. In addition, decreases in pH in the biofilm (due to acid production from other bacteria in the biofilm or due to a sweeping of lower pH fluid through the pipe) will precipitate more small anthraquinone particles from the solution within the biofilm. This will expose the sulfate-reducing bacteria within the biofilm to additional anthraquinone particles, furthering the efficacy of the anthraquinone treatment.

The NaOH solution, the carrier of the anthraquinone, also adds to the effectiveness of the treatment by functioning as a surfactant. The caustic solution helps to disrupt the biofilm and increases the tendency for the biofilm to slough from the pipe wall. The high pH solution also shocks all of the bacteria within the biofilm, reducing all activity even in the absence of the anthraquinone. Field studies in a wastewater treatment system have shown that biogenic sulfide production was mitigated with both caustic and soluble anthraquinone treatments, but the maximum degree of inhibition was higher with the soluble anthraquinone treatment and the restoration of the sulfide production to the original level occurred quicker with the caustic treatment.

The protocol for implementation of a treatment with soluble anthraquinone salt is relatively simple. The solution typically contains active anthraquinone at a concentration of about 10%. The solution is pumped from a storage tank into the pipeline transporting the water to be treated. Typically a slug dosage of solution is injected. Enough solution is injected to yield a slug in the pipeline at a concentration of about 250 ppm by wt. active anthraquinone for a contact time of about 10 minutes. In some cases the slug may only need to be 50 ppm for 1 minute, while other more difficult systems to treat might require 1000 ppm for 30 minutes to control corrosion adequately. The slug dosage requirement is a function of biofilm composition, thickness, and tenacity and the presence of hydrocarbon constituents associated with the biofilm. Velocity of the flowing water, pipe diameter and length, and the pH and buffering capacity of the water will also affect the soluble anthraquinone requirements. Dispersion of the slug as it travels down the pipeline tends to reduce the pH of the slug ahead of and behind the slug. Dispersion is a function of the pipe diameter, number of bends in the pipe, and distance the slug has traveled. (Perkins, T. K. and J. A. Euchner, "Safe purging of natural gas pipelines," SPE Production Engineering, p. 663, 1988.) The slug is injected so that dispersion is minimized and the high pH length (i.e., the bulk slug) is sufficient to give at least one minute of contact time at that high pH. High anthraquinone concentrations for short contact times are typically more effective than low concentrations for long times, but circumstances may dictate that the concentration of the injected slug be limited. One such circumstance is when the water being treated contains soluble metals (especially calcium) and sufficient bicarbonate ion such that increasing the pH of the water to above about 9.5 will cause scale formation. If too high an amount of the soluble anthraquinone salt is introduced into this water, then this will occur. In addition, the scale formation process will buffer the pH at a level which will cause anthraquinone to precipitate from solution. The combined precipitate of scale and anthraquinone will decrease the overall treatment effectiveness. Limiting the amount of soluble high pH solution injected into this water so that the final water pH is below about 9.5 will minimize the amount of scale formation while still maintaining adequate anthraquinone solubility. If the pH is less than about 9.0, then no scale will form. However, significant anthraquinone will precipitate due to the low pH, thereby reducing the overall effectiveness of the treatment.

The frequency of the anthraquinone slug injection is based on corrosion monitoring results. Injection needs to be only frequent enough to maintain the corrosion below a predetermined level. Typically, injection is at a one-week interval, although the frequency might be as often as every other day or as infrequent as once per month. The efficacy of the treatment is increased by maintaining a pipeline pigging program. Pigging the line prior to anthraquinone treatments significantly increases the anthraquinone effectiveness by disturbing the biofilm, reducing its thickness, and removing solid iron sulfide deposits.

Treatments with aqueous dispersions of solid anthraquinone are made comparably to those for soluble anthraquinone. Control of pH is not important in these treatments. As with soluble materials, treatments with insoluble anthraquinone should be such that a high anthraquinone concentration is maintained for an adequate contact time as the slug travels down the pipeline.

In long pipelines with numerous bends and/ or in pipelines in which the flow is laminar, dispersion of the anthraquinone-water slug may become significant. This will cause anthraquinone concentration in the slug to be reduced below intended levels as the leading and trailing edges of the slug mix with the flowing water. In the case of salt solution usage, the lower pH of the mixed anthraquinone-water in these "tails" will result in anthraquinone precipitation and possibly reduced treatment effectiveness. The leading edge "tail" can be eliminated by launching a pig into the pipeline immediately prior to beginning anthraquinone injection. The pig will act as a barrier for mixing of the anthraquinone-water slug with water even with low Reynolds Number flow and/ or numerous bends in the pipeline. In addition, the pig helps to reduce the thickness of the biofilm with its scraping action and will remove many of the iron sulfide and other solid deposits which contribute to corrosion. All of these factors will help increase the effectiveness of the anthraquinone treatment. However, a pig trailing the anthraquinone-water slug is detrimental to the treatment since it would remove anthraquinone which has penetrated into the biofilm.

Anthraquinone can also be introduced into the system indirectly by passing the aqueous fluid (water) containing planktonic sulfate-reducing bacteria through a vessel containing anthraquinone. The anthraquinone in the vessel can be impregnated or deposited onto a support material. For example, the anthraquinone as a solid can be deposited on plastic or ceramic packing material such as Raschig Rings within a vessel or upon a porous membrane or other support in the flow path. The solubility of anthraquinone in water is extremely low. However, the water containing the sulfate-reducing bacteria will dissolve a small amount of anthraquinone from the packing and consequently treat the sulfate-reducing bacteria to inhibit their activity. Possibly an additive may have to be injected into the water to regulate the solubility of the anthraquinone. Inhibiting the activity of planktonic sulfate- reducing bacteria prior to their colonization on downstream pipe surfaces or capture by existing biofilm will minimize buildup of biofilm and thus inhibit subsequent microbially influenced corrosion. However, cleaning the surfaces of the pipe by pigging or other methods prior to implementing anthraquinone treatment is extremely beneficial for increasing the effectiveness of the treatment.

Enhancement effectiveness of the anthraquinone treatment for some applications can result from the combined utilization of anthraquinone and a biocide or oxidizer. The biocide/ oxidizer might be needed to reduce the amount of biofouling on a surface, while the anthraquinone is responsible for long-duration inhibition of the sulfate-reducing bacteria activity. This is especially true for applications in which a biofouling problem or thick biofilm has been established prior to anthraquinone treatment. Anthraquinone alone will penetrate the biofilm, leaving the sulfate-reducing bacteria inactive, but other bacteria and their resultant biotic and abiotic products (especially iron sulfides) will still be present at the wall and possibly will contribute to additional corrosion. A combined anthraquinone-biocide application, such as alternating materials or periodically treating with a biocide in place of an anthraquinone treatment, is more effective than the use of either material separately.

External corrosion of buried pipelines and vessels is typically mitigated by use of tapes, coatings, paints, and/ or cathodic protection. If disbondment of the tape/ coating occurs (due to poor application or other circumstances), then corrosion can initiate even in the presence of previously adequate cathodic protection. This corrosion may be microbially-influenced, especially during the initiation phase. Incorporating anthraquinone into the formulation of the solid organic polymer (e.g., tape, coating, or paint) will allow these systems to continue to provide corrosion protection against microbially influenced corrosion even if they are damaged and/ or disbond from the metal surface. Incorporation techniques include: 1) fine dispersed anthraquinone powder; 2) an extremely thin bonded sheet; and 3) a soluble ionic salt formulation. The last has the further advantage that alkali solutions naturally inhibit corrosion. The anthraquinone incorporated in the tape, coating, or paint is available to inhibit sulfate-reducing bacteria in encroaching water that might come into contact with the metal surface due to failure of the protection system. The anthraquinone can also be applied to the pipe or vessel surface as a separate thin coating prior to application of the tape, coating or paint.

Anthraquinone Compounds and Formulation

A wide variety of anthraquinone compounds can be used in the method of the invention. As used herein, the term "anthraquinone compound" refers to compounds comprising the basic tricyclic structure shown below, including derivatives thereof substituted with up to four simple halogen, carboxyl, hydroxyl or amino substituents.

More particularly, both water-insoluble and water-soluble forms can be used. The non-ionic compounds are largely insoluble in aqueous systems, while ionic derivatives are largely soluble in water.

Typical insoluble anthraquinone compounds include 1,8- dihydroxyl anthraquinone, 1-amino anthraquinone, 1-chloro-anthraquinone, 2-chloro-anthraquinone, 2-chloro-3-carboxyl-anthraquinone, 1-hydroxyl- anthraquinone and unsubstituted anthraquinone. Various ionic derivatives of these materials can be prepared by catalytic reduction in aqueous alkali. One such ionic derivative is the reduction product of anthraquinone itself which is the disodium salt of l,4-dihydro-9,10-dihydroxyanthracene.

Unlike the use of biocides for treating sulfate-reducing bacteria, the anthraquinone compounds used in the invention do not kill the sulfate- reducing bacteria, but merely inhibit the corrosion activity. Interestingly enough, the active species of anthraquinone compound is believed to be water-insoluble compounds which apparently deactivate the corrosive action of the sulfate-reducing bacteria. In order for the water-insoluble compounds to be effective, they must be very finely divided to an extent that they can be dispersed into the biofilm.

Notwithstanding the fact that the active species seems to be the insoluble form of anthraquinone, it is nevertheless preferred to use the ionic (water-soluble) anthraquinone form because it diffuses into the biofilm and thus contacts the sulfate-reducing bacteria more readily. The activity of the ionic form of anthraquinone seems to be derived from its conversion from the ionic form to the non-ionic form by which it is precipitated as very fine particles which attach to the sulfate-reducing bacteria.

Whether the soluble or insoluble anthraquinone is used, it has been observed that the functional attachment of the anthraquinone particles to the bacteria is limited in time by metabolism of the particles by the sulfate- reducing bacteria. Thus, application of the treating medium must be repeated periodically in order to maintain inhibition effectiveness.

The invention is a method to inhibit corrosion caused by sulfate- reducing bacteria comprising adding finely divided anthraquinones to an aqueous medium containing the sulfate-reducing bacteria. The term "finely divided" means that the anthraquinone has an average particle size of less than 2.5 micrometers and preferably less than 2.0 micrometers. As a practical matter the minimum particle size is about 0.1 micrometer. As the average particle size is decreased within this range, the activity of the anthraquinone increases.

The activity of the anthraquinone is a function of particle size and not available surface area. An anthraquinone particle having an average particle size of 2.0 micrometers used at a concentration of 5 ppm provides satisfactory inhibition of growth of sulfate-reducing bacteria. Particles of the same anthraquinone that have an average particle size of 3.7 micrometers used at a concentration of 30 ppm do not provide satisfactory inhibition of growth of sulfate reducing bacteria. This difference in effectiveness due to particle size is readily apparent upon initial application of the anthraquinone but is even more pronounced a short time after application of the anthraquinone when the effectiveness of the larger particle size anthraquinones is effectively completely lost.

The compositions are added to the medium containing the sulfate- reducing bacteria in a quantity sufficient to inhibit sulfide production. As little as 1 ppm by weight in the aqueous medium gives significant inhibition for many uses. In the preferred method the concentration of active anthraquinone in the medium is at least 5 ppm, preferably 5-50 ppm. Greater concentrations, such as up to 1000 ppm, of course can be used, but in most cases with little advantage so long as the "finely divided" particle size requirement is met.

Because of the hydrophobic nature and low water solubility of the anthraquinone (<10 ppm for most compounds), the preferred compositions also contain a surfactant, or wetting agent. The surfactant can be applied by any of a variety of techniques. In the case of a normally solid surfactant, the surfactant and the anthraquinone can be dry blended, preferably prior to the milling step. In the case of a normally liquid surfactant, the surfactant can be sprayed onto the anthraquinone followed by mixing such as rumbling to ensure that an intimate well dispersed mixture is obtained. Alternatively, a liquid solution of the surfactant can be sprayed on the anthraquinone and the anthraquinone further mixed.

The choice of surfactant is not critical. Any of the commercially available surfactants that are inert to the anthraquinone and other composition ingredients and are compatible with the use environment are suitable. Such surfactants can therefore be nonionic, cationic or anionic with the nonionic surfactants being preferred. These include Triton® X-100, a nonionic octylphenoxypoly (ethoxyethanol), available from Rohm & Haas; Poly-Tergent ® SLF18, poly(oxyethylene)-poly(oxypropylene)-rnonohexylether; mono- octylether; mono-decylether, available from Olin Corp.; Morwet® D-425, an anionic sodium salt of condensed naphthalene sulfonate, available from Witco Chemical Co.; Stepsperse® DF-500, an anionic blend of lignin sulfonates, available from Stephan Co.; Stepwet® DF-90, an anionic linear alkylbenzene sulfonate, available from Stephan Co.; Stepsperse® DF-100, an anionic/nonionic blend including lignin sulfonate, available from Stephan Co.; Stepflow® 41, an anionic lignin sulfonate, available from Stephan Co.; and Stepflow® 24, a nonionic nonylphenolethoxylate, available from Stephan Co. The surfactant is present in the dry compositions in an amount to enable the anthraquinone particles to be quickly wetted and thoroughly dispersed in the aqueous bacteria use locus. Quantities in the range of 2-15%, based on the weight of anthraquinone, are generally preferred.

The bulk anthraquinones are normally obtained as particles or granules of comparatively large average particle size, as dry powders or slurries. Generally the finely divided anthraquinone is used in the form of an aqueous suspension containing 10-60 wt % of an anthraquinone.

Pigging Procedure:

Figure 2 is a schematic representation of a typical pipeline for transportation of liquids which has facilities for pigging (or scraping) operations. Liquid flow through the line is directed through the main pipeline 1 through upstream valve B and downstream valve G, both of which are open during normal pipeline operation. Valve C in starter line 3 and valve A in scraper outlet line 7 are closed and valve D in pressurization line 9 is open during normal pipeline operation.

When it is desired to launch the scraper (pig), valve C, which connects the main pipeline 1 with outgoing scraper barrel 5 via line 3, is opened slowly to raise the pressure in launch barrel 5 containing the scraper, to full pipeline pressure. After the launch barrel 5 has reached full pipeline pressure, valve D in pressurization line 9 is closed and scraper outlet valve A in scraper outline line 7 is opened. Then, by pinching down slowly on valve B, the differential pressure within the launch barrel rises and overcomes the friction between the scraper and the launching barrel. The scraper passes slowly through scraper outlet valve A and scraper outlet line 7 into the full flow of the main pipeline 1. After the scraper is launched, valve B is fully opened and valves A and C are closed. In addition, scraper return line valve F, main pipeline valve G and scraper receiver line valve H are opened. The scraper then proceeds through the pipeline 1, scraper return line 11 and valve F into receiving barrel 13.

As the scraper passes the juncture of inhibitor feed line 17 and the main pipeline 1, inhibitor feed valve E is opened to inject anthraquinone compound into the main pipeline. Valve E is then closed as soon as the chosen quantity of anthraquinone compound has been injected into the main pipeline 1.

As soon as the scraper reaches receiver barrel 13, main pipeline valve G remains fully open and valves F and H are closed. Upon venting the pressure within receiver barrel 13, it can be opened to remove the scraper.

Example:

Steel pipelines are used to transport seawater from treatment and pumping facilities to oil field water injection wells. The water is injected into specific regions of an oil-producing reservoir to provide secondary oil recovery. This provides additional oil recovery over that which results from primary, or natural, production due to the initial pressurization of the reservoir.

Treatment of the seawater prior to entering the pipeline is required to prevent corrosion of the steel pipeline and the steel tubing in the water injection wells and to improve injected water quality. The treatment process includes chlorination, filtration, deaeration, and processed-anthraquinone addition. The chlorination kills, via oxidation, the majority of the bacteria and algae entering the system with the water from the sea. The filtration removes most of the sea sediments, large particles, and biomass. Deaeration of the water is critical to remove oxygen, a key element involved in the corrosion process. Deaeration of the seawater to less than about 20 ppb oxygen essentially eliminates the potential for common oxygen-induced corrosion. Unfortunately, removing the oxygen results in an anaerobic environment, which increases the potential for anaerobic corrosion of the steel pipeline due to the activity of sulfate-reducing bacteria in the system. Addition of anthraquinone is thus needed to control the activity of the corrosion-inducing sulfate-reducing bacteria.

Treatment of the seawater with soluble anthraquinone is performed downstream of all other processes in the seawater treatment plant. The anthraquinone solution is stored in a nitrogen-inerted (oxygen-free) feed tank connected to the suction-side of a variable-rate injection pump. This pump is connected to the pipeline with standard connecting lines and valves, on the discharge-side of the mainline seawater pumps. A flow meter indicates the volumetric rate of injection of the soluble anthraquinone. A check valve is located between the anthraquinone-pump and the pipeline to prevent seawater from flowing back into the soluble anthraquinone feed tank.

A ten-mile long, 60-inch internal diameter pipeline transports 1 million barrels of water per day from the treatment plant to an intermediate injection facility. The average seawater velocity is 3.3 ft/ sec, and the flow is clearly turbulent with the Reynolds Number being 1.5x10°-

Prior to initiating treatments in this pipeline with processed- anthraquinone, significant corrosion of the steel pipeline was found to occur. Corrosion rate and presence of corrosion in the pipeline is determined by internal flush-mounted corrosion coupons, by thru-wall ultrasonic and radiographic inspections, and by various types of internally-transported 'smart' pigs. Inspection of corrosion coupons removed from the pipeline after four-months of contact with the flowing water without processed- anthraquinone treatments indicate an average corrosion rate of 5 mils per year (0.127 mm/yr.). In addition, small pits are present. The other non-destructive inspection techniques confirm that overall corrosion rate is low, but that deep pits (maximum depth up to 0.1 inch and 0.2 inch in circumference) are prevalent in certain areas of the pipeline. Both isolated pits and linked pits are found, especially on the bottom of pipeline near girth welds. These pits are characteristic of those attributed to microbially-influenced corrosion. In addition, analysis of specially internally-mounted coupons indicate the presence of about 5x10⁵ sulfate-reducing bacteria cells per cm² of surface and

2X10⁴ cells per cm² of other bacteria types. The presence of an ongoing corrosion process is also inferred by a high level of soluble ferrous ions and iron sulfide solids in the effluent water. 80 ppb soluble iron and no detectable solid iron sulfide are found in the influent water. However, 800 ppb soluble iron and 250 ppb equivalent iron as a solid is found in the effluent. The iron sulfide forms by the reaction of ferrous ions and sulfide ions near the surface of the pipe as the corrosion process removes ferrous ions from the steel. No sulfide is contained in the influent water. The sulfide is produced within the pipeline as a metabolic product of sulfate-reducing bacteria activity. Much of the formed iron sulfide, a solid, remains on the surface of the pipeline, but some is swept off by the flowing water. The total measurable iron represents a loss of 120,000 pounds of iron per year removed from the steel pipeline, or a corrosion rate of 3.6 mils per year (0.093 mm/yr.). The total sulfide associated with the solid iron sulfide in the effluent water represents 19,000 pounds of hydrogen sulfide produced per year.

A pH 13.5 anthraquinone solution contains 9.5 wt. % active anthraquinone and has a density of 9.2 pounds per gallon. The active anthraquinone is solubilized in an ionic form in the processed-anthraquinone solution to help increase the effectiveness of transport of the active anthraquinone down the pipeline and into the biofilm on the pipe wall. Soluble anthraquinone solution is injected as a slug into the pipeline twice per month for thirty minutes at a rate of 42 gallons per minute, yielding a concentration of 150 ppm by weight of active anthraquinone in the thirty- minute slug of flowing seawater. The injected anthraquinone increases the seawater pH from 7.8 to 9.4 within the slug.

After four months of twice per month soluble anthraquinone injections, the corrosion coupons indicate that the corrosion rate is reduced to less than 1 mil per year and that pitting is minimal. Radiographic inspections of heavily corroded sites indicate that minimal corrosion has occurred since the last inspection four months previously. Total iron concentrations (solid and insoluble) in water effluent samples taken 48 hours following a treatment are 120 ppb, indicating at least a 95% reduction in iron loss from the pipe. The sulfide associated with the effluent iron sulfide particles is reduced comparably. The concentrations of both the iron and the sulfide in effluent water increases slowly with time during the semi-monthly treatment periods such that the total iron concentration at the end of the period averages about

380 ppb. Coupons removed for microbial analyses indicate that the sulfate- reducing bacteria density is 4X10⁴ cell/cm² and that other bacteria are present at a level of 3x10⁴ cells/ cm². All of these monitoring techniques confirm that injections of soluble anthraquinone into the flowing seawater effectively mitigate anaerobic microbially influenced corrosion of the steel pipeline and maintain minimal iron sulfide solids formation.

Claims

Claims:

1. A non-biocidal method for inhibiting microbially influenced corrosion of microbially influenced corrosion-susceptible metal surfaces having an anaerobic biofilm containing active sulfate-reducing bacteria comprising contacting the biofilm with a liquid dispersion of anthraquinone compound by which the anthraquinone is passed through the pores of the biofilm and is diffused within the biofilm to effect contact with the sulfate-reducing bacteria.

2. The method of claim 1 in which the anthraquinone compound is in the form of solid particles having an average particle size no larger than 2.5 micrometers.

3. The method of claim 1 in which the anthraquinone compound is dissolved in an aqueous solvent.

4. The method of claim 3 in which the solution of anthraquinone compound has a pH of at least 12.

5. The method of claim 1 in which the biofilm is on the surface of the metal in contact with a turbulently flowing liquid in which the anthraquinone compound is dispersed.

6. The method of claim 1 in which the biofilm is on the surface of the metal in contact with a static liquid in which the anthraquinone compound is dispersed.

7. The method of claim 1 in which the surface of the microbially influenced corrosion-susceptible metal is coated with a solid organic polymer in which are dispersed finely divided particles of anthraquinone compound, the coating thereby forming a barrier between the biofilm and the metal surface.

8. The method of claim 1 in which the microbially influenced corrosion- susceptible metal is selected from the group consisting of ferrous metals, brass, bronze and Cu/Ni alloys.

9. The method of claim 2 in which the anthraquinone-compound is 9,10- anthraquinone.

10. The method of claim 3 in which the anthraquinone compound is the disodium salt of l,4-dihydro-9,10-dihydroxyanthracene.

11. The method of claim 5 in which dispersion of anthraquinone compound is introduced into liquid flowing through a pipe as a slug the volume of which is sufficient to provide liquid contact with a given point within the pipeline of at least one minute.

12. The method of claim 3 in which the slug is introduced into the pipe immediately following a pig.

13. The method of claim 5 in which an aqueous alkaline solution of the anthraquinone compound is added to the flowing liquid continuously.

14. The method of claim 1 in which the anthraquinone compound is metabolized by the sulfate-reducing bacteria.

Title: Method for Inhibiting Microbially Influenced Corrosion

Abstract: