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Water treating chemicals
Chemicals play an important role in the oil producing operation. They assist oil/water/gas separation, aid in fluid transport, protect treating equipment, and improve the quality of the gas, oil, and water.
In water treating, chemicals aid in producing suitable water for discharge or injection. A wide range of chemicals is available for water treating.
A chemical-injection package enables various types of chemicals to be dosed into the water stream to optimize the treatment process. In many operations, each chemical-injection stream is equipped with two dedicated pumps, both of which are rated for 100% capacity:
- a duty pump
- a standby pump
Storage-tank capacity is designed to allow the plant to run for several days between refills. Tank-construction materials can be:
- carbon steel
- stainless steel
- other material appropriate to withstand the action of the stored chemicals
General dosing rates and injection points for the main chemical classes are listed in Table 1. These rates provide guidelines for sizing injection pumps and chemical-storage tanks.
(Flocculants) The purpose of water clarification is to improve the water quality to meet discharge or injection requirements. Water-clarification chemicals aid in coagulating and flocculating the oil and solid particles into larger ones to enhance their separation from water.
Benefits of enhancing removal
Increasing the particle or droplet size significantly enhances the removal efficiency of:
- skim tanks
The commonly used water-clarification chemicals may be classified as inorganic coagulants or polyelectrolites, but polyelectrolites are used normally as a secondary coagulant and filter aid.
Most of the inorganic coagulants and polyelectrolites can be dissolved and ionized in water. Their ion charges attract oil droplets and solid particles with opposite charges. Oil droplets grow by coalescence, and solid particles grow by forming flocs. The larger oil droplets or larger solid flocs are easier to separate. Particle separation from water follows from Stokes’ law in that larger particles separate more quickly from water.
- copper salts
Except in the case of sodium aluminate, most of the common aluminum and iron coagulants are acid salts and require pH adjustment to reach the best operating range. For instance, aluminum coagulants require a minimum pH of 6 to 7, while iron salts are effective in a pH range of 5 to 11. These chemicals are very effective in promoting coagulation of oil and solids particles; however, excessive use or improper application of these chemicals will form undesirable oily gel-type settlements and occasionally cause malfunction of the monitoring/controlling instrument. The inorganic or low-molecular-weight coagulant is effective in the range of 10 to 45 ppm.
Ferric sulfate may be used as a coagulant. The large size of the positively charged cation, Fe3+, upsets the stability of the colloid, and the finest solids become entrapped by the precipitated ferric hydroxide. In the overall reaction, ferric sulfate consumes bicarbonate ions (or alkalinity) and can cause a pH reduction at higher dosage levels.
Polyelectrolites refers to all water-soluble organic polymers. The polyelectrolites are long chain molecules, frequently polyamines or polyacrylamides. Their overall charge and size destabilize colloids and provide agglomeration of solids (flocculation). In water treating, the term “polyelectrolyte” is generally used in reference to two types of chemicals.
- The first type is the polymeric primary coagulant; these chemicals are cationic, with relatively low molecular weight (<500,000).
- The other type of polyelectrolite is the polymeric flocculant or coagulation aid, which may be anionic, cationic, or approaching a neutral charge. Typical molecular weight may be as high as 20,000,000.
Polyelectrolites serve to bridge particles together. Only a small amount of this polymeric chemical is needed to produce a significant effect on the oil droplets and solids. The normal application concentration for this chemical is 1 to 5 ppm.
As the water stream flows through the treatment system, it will change in:
Pressure and temperature changes affect the solubility of the chemical components in water and may form scale. Depending on the composition of the water, various types of scales can be formed in the:
- control system
Types of scales
Scale formation leads to equipment failure, plugging, and contamination. The most common types of scales found in produced or seawater systems are:
- metallic-silicate complexes
Determining scale tendency
Several methods have been developed for determining the scale tendency of water systems. The current trend is to use several different approaches to calculate the water scale tendency and obtain a range of conditions in which scale could form in the system. Once the scale tendency is established, scale inhibitors are selected for treating the system.
Various chemicals have been developed to inhibit scale formation in water. Selection of scale inhibitors is dependent on the type of scale to be inhibited and the operating conditions, such as the temperature and pressure of the system. The commonly used scale inhibitors are classified into the following types.
Chelant compounds form soluble complexes with either of the following:
- divalent compounds, such as calcium and magnesium
- trivalent metals
The most common chelants are EDTA and NTA. These compounds are used frequently in steam-generator treatment because they are thermally stable at elevated temperatures. Stoichiometric levels are necessary for the use of chelants in controlling scale formation.
The polyacrylates contain the carboxylic acid group. Depending upon its composition, it is thermally stable to a relatively high temperature and can be used to control scale as well as suspended material.
These organic phosphorous compounds have been used for controlling iron or hardness salts and form inhibitive films along metal surfaces. It is normally thermally stable to 250°F but can go higher in the absence of oxygen.
Polyphosphates of various chain lengths are used for controlling hardness and iron scales. They are very effective chemicals; however, their thermal stabilities are lower than the phosphonates.
Depending upon the application, various methods are used for the selection of scale inhibitors. Among the factors which are tested to determine a chemical’s effectiveness at in-situ conditions are:
- thermal stability
- absorption characteristics of the environment
- time of exposure
Commonly used methods are the thermal-stability and dynamic scale-inhibition tests.
Thermal stability test
This method evaluates the thermal stability and scale inhibition at the same time. NACE TM0374-2001 gives the general test procedure.
Dynamic scale inhibition test
The ability of an inhibitor to prevent scale is its primary function in an application. Dynamic tube-blocking tests permit ranking of inhibitors to prevent scale. Different temperatures can be used to mimic changing field conditions.
Chemical effect mechanisms
Scale inhibitors vary in type and mode of action. Individual chemicals operate with one or more of the following mechanisms.
These compounds bind to one of the species that would precipitate, making it unavailable for precipitation. A common example is EDTA, which forms a claw-like arrangement attached to calcium ions called a chelate, preventing it from being precipitated as calcium carbonate or sulfate.
These compounds can actually disperse scale already attached to surfaces. They can be used to clean up scaled systems but must be used with caution because they could result in releasing suspended solids that could block the formation.
These compounds, such as complex phosphates, interfere with the crystal growth of the scale, preventing further growth.
This refers to the ability of many inhibitors to hold considerable quantities of scale-forming compounds in solution when present only in very small concentrations themselves.
Corrosion is defined as the destruction of metal by either chemical or electrochemical reaction in the given environment. Because piping and processing equipment are normally made of metals that are in contact with produced water or seawater, chemical or electrochemical reactions will occur.
The following factors all have a significant effect on the corrosion rate:
- the type of metal
- water pH
- dissolved oxygen, dissolved salts, and acid gases in water
- temperature, pressure, and fluid velocity
Corrosion inhibitors reduce the corrosion rate by interrupting the electrochemical corrosion cell setup between the metal and the liquid or by stopping the deterioration of metal by a chemical reaction. One method of interrupting the electrochemical cell is to form a film on the metal surface, which stops the transfer of ions. One method of chemical protection uses an absorption inhibitor, such as a surface-active amine, which forms a chemisorption bond to the metal, rendering it incapable of dissolving into the fluid.
There are two types of film-forming inhibitors
The aestivating inhibitors promote the formation of a passive film to protect the metal surface. The precipitating inhibitors react with the corroded metal and deposit a barrier film on the metal surface. Phosphates and silicates are precipitating inhibitors.
In addition, various scavenger chemicals have been developed for removing corrosion-aiding components from the produced-water systems. Oxygen and hydrogen sulfide scavengers effectively remove these respective components from the water by combining them into the scavenger chemicals (see Oxygen Scavenger).
Corrosion inhibitors are selected by laboratory or field tests. Laboratory testing and evaluation have the advantage of being quicker and less expensive; however, inhibitor selection should always be verified by a field test. The most commonly used laboratory and field methods are:
- the wheel, copper-ion displacement, and stirred autoclave tests
- electrical probes and corrosion coupons
The laboratory wheel test is the mostly commonly used method in the oil industry. This apparatus consists of an enclosed rotating disc that holds bottles containing the test water, each with corrosion inhibitor in various concentrations, and a steel coupon in each bottle. The disc is spun at 25 to 30 rpm to cause agitation of the fluid and coupons; the temperature inside the enclosure can be varied to match the process conditions. The tested metal coupons are weighed before and after exposure to determine the weight loss.
Copper ion displacement test
Used to determine the effectiveness of a protective film created by a corrosion inhibitor, in this test, the minimum concentration of inhibitor needed to form a persistent and protective film is determined. The test procedure requires first dipping a shim stock iron metal sample (1/4 × 6 in.) into a 5% NaCl solution for 5 minutes. This metal is then dipped into an inhibitor solution of known concentration for 5 minutes with stirring and finally into a 10% CuSO4 solution for 30 seconds. The results are observed for copper plating with a magnifying glass. This method is subject to temperature limitation, and its results are semiquantitative.
Stirred autoclave test
For high-pressure and high-temperature applications, it is necessary to use a high-pressure method, such as the stirred autoclave test. A stirred autoclave can be tested to 500°F and 5,000 psia. In addition, it can be used to test various oil/water ratios and concentrations of H2S and CO2. The test may be performed under either static or dynamic conditions. A schematic diagram of the autoclave test unit is shown in Fig. 1.
These provide a direct reading of the corrosion rate. Both polarization resistance and electrical resistance-type probes can be used for these evaluations. An electrical probe is the most widely used device for field evaluation. These probes can be applied at any point in the produced-water system to evaluate inhibitor performance and compare it with the background corrosion before application. The probe application is very quick, but it provides a corrosion rate only at the point and time of measurement. In most cases, the probes are used in combination with the corrosion-coupon measurement.
Corrosion coupons can be put into the operating process to directly measure the corrosion rate. The advantage of the corrosion coupon is that it is exposed to the true condition of the water system and measures the accumulated results for a time. It is especially valuable when the system has multiphase- or transient-flow conditions. The coupons can be placed at various points of the produced-water system; they are made from the metal of interest and placed in the system for a time (usually 1 to 6 months) to measure the accumulated corrosion of the system. The coupons are weighed before and after the exposed period.
Both surface and produced water contain biological constituents (primarily bacteria) that can contaminate the water-treatment or -injection systems. Because bacteria have the ability to multiply rapidly into colonies, they can cause plugging of surface and downhole equipment and injection-well formations, promote corrosion of surface piping and downhole tubulars, and generate H2S that can cause pitting corrosion. Therefore, it is essential to develop means to control bacteria growth in surface-water-treatment systems.
Bacteria are unicellular, microscopic organisms that can be found in a wide variety of conditions, in both:
- oxygen-containing systems (aerobes)
- systems without oxygen (anaerobes)
The one factor common to all bacteria is their requirement for water, which provides a means of carrying food to the bacterial cells and taking away waste products.
Seawater has predominantly:
- aerobic bacteria, such as iron-oxidizing bacteria
- some slime formers
- other bacteria, such as the hydrocarbon oxidizers
The growth of these species will be reduced by the dosing of chlorine (itself a biocide) to the intakes and by the deaeration process. However, anaerobic bacteria, such as sulfate reducers and other slime-forming bacteria, are present in seawater and, following the removal of oxygen and chlorine in the residence/deaeration tank, may proliferate rapidly, causing associated problems including corrosion and biofouling.
API Spec. RP-38 presents two culturing techniques for counting both aerobic and sulfate-reducing ( anaerobic) bacteria, and both techniques count planktonic bacteria. “Planktonic” are the bacteria found swimming or floating in the water system. API Spec. RP-38 states that aerobic bacteria greater than 10,000 bacteria/mL and anaerobic bacteria levels greater than 1 bacteria/mL are considered significant if they are associated with physical changes in the system, such as:
- increasing injection pressures
- decreasing water quality
Current studies indicate that a count of all bacteria types at key points in the system is necessary to determine the extent of the problem and to monitor the treatment progress.
The types of biocide available are varied and are selected on the basis of:
- kill rate to anaerobic bacteria
- kill rate to aerobic bacteria
- ability to penetrate surface films
Some of the typical compounds used as biocides include:
- quaternary ammonium compounds
- chlorinated phenols
- organometallic compounds
- sulfur organic compounds
In seawater systems, chlorine is the primary biocide used.
Many years of operational experience have shown that chlorine is very effective in controlling bacterial growth. Chlorine in water takes on various forms according to the pH of the water. The types of chlorine present in the water include Cl2, HOCL, and OCl–. In a seawater system, it is possible to generate hypochlorite (OCl–) in situ through electrolysis of the seawater, as discussed previously.
Sodium hypochlorite is dosed to the inlet of the lift pumps at such a level as to give a free chlorine residual of 0.5 to 1 ppm Cl2 upon exiting the filter. Chlorine is a very strong oxidizing agent and reacts with many materials. Once it reacts, it is no longer available to kill bacteria. Chlorine may react with:
- ferrous iron
- hydrogen sulfide
- organic compounds
- sulfite ions (present in the scavenger chemicals)
To evaluate a biocide, a bacteria culture is first obtained from the field. The culture is usually a mixed strain of the organisms that can grow in the media used. If a field culture is not available, cultures of bacteria can also be obtained from a commercial laboratory. One or more of the following methods are then used to select a biocide.
This test is used for preliminary screening of biocides and involves exposing a bacteria sample to a known concentration of biocide. The test is performed in broth bottles with a range of biocide concentrations that are incubated for 15 days. Afterward, the bottles are observed for bacterial growth. The minimum concentration that shows no bacteria growth is the effective concentration.
Planktonic time kill test
This test involves putting the field sample into a known concentration of biocide for a specific contact time, then counting the bacteria that were not killed. The effectiveness of the chemical is determined by comparing the results to a blank determination in which no biocide is present. A biocide concentration that results in a 99% or better kill is considered to be effective. The kill rate is calculated with the lowest number in the population range for the blank and the highest number in the bacteria population range observed for each biocide concentration.
Sessile bacteria time kill test
This test uses a mild steel coupon that has been exposed to a bacteria culture for at least 14 days. This period is sufficient to allow for the development of a sessile population on the surface of the coupon. The contaminated coupon is then used in the time kill test.
Once a biocide has been selected, a field test is conducted. In general, batch treatment is more economical than continuous treatment. However, the effectiveness and economics of chemicals are dependent on field producing conditions.
It should be noted that generally, because of the enormous complexity and density of microorganisms, there is no single biocide that is completely effective. For this reason, and because bacteria can adapt themselves (i.e., build up an immunity) to a biocide, it is customary to use alternate slugs of different biocides or biocide blends.
Chemical scavengers are used to remove dissolved oxygen from water-flow streams of less than 10,000 B/D. There are a number of compounds commercially available to remove dissolved oxygen, and all can be considered as a source of sulfur dioxide or sulfite. These include:
- sodium sulfite (Na2SO3)
- sulfur dioxide (SO2)
- sodium bisulphate (NaHSO3)
- sodium metabisulfite (Na2S2O5)
- ammonium bisulfite (NH4HSO3)
Ammonium bisulfite is commonly used on river and aquifer water systems because it is relatively stable to the atmosphere (solutions in storage tanks will not absorb oxygen from the air), and its rate of reaction with dissolved oxygen is catalyzed by the trace of metals, particularly iron, in the water. It is, thus, the most convenient chemical to use. To speed the reaction rate, a catalyst such as cobalt is often required.
Some water systems (e.g., seawater) have a tendency to create foam when subjected to high pressure-drop or turbulence conditions. This foaming is very unpredictable and can cause enormous problems, particularly in deaerator columns. In the deaerator, foaming may result in failure of the level control on the base of the column and reduced oxygen-removal efficiency.
Antifoam chemicals are polyglycols or silicones contained in a solvent that is fully water-soluble or water-dispersible. At dose levels usually less than 1 ppm, they act by decreasing the surface tension of bubbles so that they expand and coalesce. They are dosed to the inlet of the deaerator before the inlet distributor. In water-injection systems, the polyglycols are preferred because silicones produce fine precipitates, which can cause injectivity loss. However, silicones may be required in difficult applications, usually in low-temperature vacuum systems or when the foaming tendency is very high.
Surfactants are normally water-based alkaline cleaners that are biodegradable. These chemicals are used as detergents in cleaning oil films from equipment surfaces. A common use for surfactants is to aid in cleaning media filters. Because of its physical properties, oil is not easily removed from media filters during normal backwashes. To properly remove oil, a surfactant is slug-dosed into the media filter inlet and flushed into the bed. The combination of detergent and air scour breaks the viscous bond between the anthracite and oil and allows the particles to separate. The oil is removed during the subsequent backwash steps.
- NACE TM0374-2001, Standard Test Method Laboratory Screening Tests to Determine the Ability of Scale Inhibitors to Prevent the Precipitation of Calcium Sulfate and Calcium Carbonate from Solution (for Oil and Gas Production Systems). 2001. Houston: NACE.
- API RP-38, Recommended Practice for Biological Analysis of Subsurface Injection Waters. 1975. Washington, DC: API.
Noteworthy papers in OnePetro
Hart, P. R. 2003. Removal of Water Soluble Organics from Produced Brine without Formation of Scale. Presented at the Society of Petroleum Engineers International Symposium on Oilfield Chemistry, 5-7 February, Houston, Texas, USA. SPE-80250-MS. http://dx.doi.org/10.2118/80250-MS
Frankiewicz, Ted. 2012. Diagnosing and Resolving Chemical and Mechanical Problems in Produced Water Treating Systems. https://webevents.spe.org/products/diagnosing-and-resolving-chemical-and-mechanical-problems-in-produced-water-treating-systems
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