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Difference between revisions of "Surface water treatment for injection"
Revision as of 08:36, 17 September 2013
In many operations worldwide, surface waters are injected into producing formations to enhance oil recovery. The types of surface waters used range from seawater (salt water) to lake water (brackish) to river water (fresh water).
Surface water injection is an attractive option for the following reasons:
- In many cases, surface water is easily accessible and readily available without high-cost well-drilling and well-completion activities.
- Surface-water supplies are considered inexhaustible.
- Most surface-water supplies can be used without having to pay fees or taxes.
- The use of surface water creates very little environmental impact or concern.
Common types of contaminants
Surface water must be treated to remove undesirable components before injection.
- solids (sand)
- dissolved gas (oxygen)
- biological material (plankton and bacteria)
- dissolved solids (sulfate)
Treatment of surface water for injection requires a specially designed system made up of various components to remove or control any contaminants in the water. The system is engineered to perform the required treatment in the most cost-effective and environmentally sensitive manner. A typical system is shown in Fig. 1. Commonly used methods for removal or control of these contaminants are discussed in this section.
Separating suspended solids from injection water
Surface waters normally contain suspended solids particles that, if injected into the producing formation, will plug the injection well. The type, concentration, and particle-size distribution of suspended solids in water will vary depending on the source of the surface water. For example, river-water sources tend to have higher concentrations of suspended solids (100 to 1,000 mg/L), whereas deep offshore water sources tend to contain rather small amounts (5 to 50 mg/L). Furthermore, the suspended solids found in river water tend to be inorganic (silica-based), whereas the suspended solids found in the oceans tend to be organic (primarily bacteria). Hence, the treatment methods also vary depending on the source. Solids-removal equipment generally may be classified as primary (coarse) or secondary (polishing) removal devices. Table 1 lists the solids-removal devices commonly used to treat surface water for injection.
Primary (coarse) solids removal
The term refers to the amount and the size range of suspended solids to be removed. Primary removal considers solids concentrations greater than approximately 100 mg/L and solids particle sizes greater than 50 μm.
Solid or Liquid hydroclones
Solid or liquid hydrocyclones, or desanders, can be classified as primary or bulk-removal devices designed to handle larger particle sizes and higher solids concentrations. These units provide an inexpensive first pass at removing 50- to 100-μm solids. They are more commonly used with river waters to remove silt and sand.
Coarse strainers are devices designed for applications that require the removal of large solids (> 250 μm). Strainers mechanically remove or screen out solids particles based on size alone. Many of the strainers commonly used in water-injection systems employ either a basket-style straining device or a wire-wound cylindrical element. During filtration, the strainer screen fills up with material, which leads to a gradual increase in pressure drop. This increase in differential pressure means that each strainer must be backwashed periodically to remove accumulated solids. This period can be varied, and the program time will be established during commissioning. Backwashing occurs while the strainer remains on line without interruption in the forward flow. Either the basket or wire-wound design can be automated for online self-cleaning of the solids that build up. This is a useful feature because it affords continuous operation of the plant, even during the cleaning cycle.
Secondary (polishing) solids removal
The term refers to the amount and the size range of suspended solids to be removed. Secondary removal refers to suspended solids concentrations of less than 100 mg/L and solids particle sizes less than 50 μm.
Solid or Liquid hydroclones
Solid or liquid hydrocyclones also can be used for polishing. Desanders in this application can practically remove solids up to 10 μm in diameter, the lower limit for hydrocyclone technology.
Granular media filters
The majority of large-flow-rate polishing filter applications involve the use of granular-media filtration. Granular-media filters, also called sand filters, contain a be of:
- graded sand
The beds may be of a single medium or may be graded from coarse to fine media to allow for greater solids loading. Sand filters are good for separating 25-μm particles, but some manufacturers claim that their filters are good for 5- to 10-μm separation.
During filtration, the particulate matter carried by the water is trapped within the filter media. Because of the carefully selected grades of media, this entrapment occurs right through the top two layers. The increase in pressure drop across the filter is gradual because the solids collection occurs through the filter bed. It also means that the filters can easily cope with sudden increases in the solids content of the seawater without blinding. This increase in differential pressure, however, means that each filter must be washed to remove the accumulated solids, normally achieved by washing each filter in rotation (e.g., in a 24-hour period, one filter backwash would start every 8 hours).
The media are arranged in a pressure vessel for either downflow filtration and upflow backwash, as shown in Fig. 2, or for upflow filtration and upflow backwash. Conventional downflow filters are limited to flow rates of 2 to 5 gal/min-ft2 and total solids loads (before backwashing) of 1/2 to 1 1/2 lbm/ft2. With appropriately designed distribution systems, high-rate filters can be operated at 7 to 15 gal/min-ft2. This higher loading forces the solids farther into the bed, allowing for solids loadings of between 1 and 4 lbm/ft2. Upflow filters have a greater capacity for solids loading; flow tends to loosen the bed, allowing for greater penetration of the solids (up to 6 lbm/ft2 of solid loading). However, because of the danger of losing the bed, upflow filters are limited to flow rates of 6 to 8 gal/min-ft2 and require longer backwashing time and more backwash fluid.
Two modes of filter-backwash control are available:
- manual (used during commissioning or troubleshooting)
- automatic (normal operating selection)
When a filter is selected to manual, the operator can:
- select any stage in the backwash sequence (i.e., a drain-down stage)
- initiate that stage
- run it for the required amount of time
When a filter is set to automatic, the filter will be backwashed automatically when the backwash interval time starts it, when a high differential pressure occurs, or when the operator starts an autobackwash.
Cartridge filters are simple to install, require no backwash, and are capable of removing solids particles 2 μm or larger in diameter. Their drawback is that they can take only very low solid loadings, and the cartridges must be disposed of after use. The filter vessel must be taken out of service and depressurized, and the cartridges must be replaced whenever the volume of solids trapped causes the differential pressure to exceed a predetermined maximum (usually 25 psi). Some modern cartridge filters can be backwashed.
Fig. 3 shows a typical cartridge filter. The cylindrical filters are encased in a pressure vessel. Flow enters the vessel and flows from the outside of the cartridge to the center, where it enters a perforated pipe that is open on the bottom. A bypass mechanism is included that will automatically allow flow to pass from the inlet to the outlet chambers if the differential pressure exceeds the capacity of the cartridges.
Table 2 indicates the particle size that can be separated and the recommended flow rate through various standard-size cartridges. Molded fiberglass has the least solid-storage area, and pleated wire screen or paper has the most.
Dissolved gas removal
Surface water (fresh or saline) will contain dissolved oxygen that must be removed by the water-treating facility. Oxygen in concentrations of 0.5 ppm in hydrogen-sulfide-free water and 0.01 ppm in water containing hydrogen sulfide is generally considered to be sufficient to cause corrosion problems in the facilities and bacteria-plugging problems in an injection reservoir. For this reason, attempts are made to exclude oxygen from produced-water systems by maintaining gas blankets on all tanks. However, these systems sometimes must be designed to handle rainwater, which may introduce dissolved oxygen in sufficient quantities to require removal. All seawater contains oxygen, and while the location of surface-water intakes can be arranged to minimize the oxygen content, oxygen will have to be removed in almost all cases.
Some water sources contain ammonia, H2S, or CO2, which must be removed. The following are capable of removing these dissolved gases:
- chemical scavengers
- gas stripping
- liquid extraction
The basic principle used in gas stripping is that the quantity of oxygen dissolved in the water is directly proportional to the partial pressure of the gas that is in contact with the water (Henry’s law). Because partial pressure of the gas is a function of the mole fraction of that gas, the addition of other gases to the solution will decrease the partial pressure of oxygen and, thus, the concentration of oxygen in the water.
In a typical gas-stripping column, natural gas or steam is introduced in the base of a packed or trayed column (similar to a glycol contactor used in gas dehydration) and flows upward countercurrent to the water. The water is introduced in the top of the column and flows downward.
If natural gas is used, the oxygen-contaminated gas from the top of the tower can be used for fuel, compressed for inclusion in the sales gas stream, or vented, depending on the process design, environmental regulations, and gas sales contract. Stripping-gas usage of 2 to 5 scf/bbl is common.
It also is feasible to strip oxygen from water with a concurrent flow. This is common in cases in which lift gas is used as the artificial-lift mechanism for obtaining the water from a reservoir or subsea source. The gas is sometimes injected into the water with a static mixer in concurrent flow in a pipe. While this may require more stripping gas, when the value of the stripping gas is low, it may be more economical from the standpoints of:
- equipment cost
Stripping-gas usage in concurrent flow can be in excess of 10 scf/bbl.
Before entering the contactor column, water (from the media-filter package) is treated with an antifoam chemical. Seawater has a high foaming tendency, which can seriously affect the performance of the column. The seawater is then fed to the mass-transfer section (containing the packing or trays) of the column for oxygen removal. Water enters the column near the top through an inlet-distribution header, which ensures an even flow across the full-tower cross section. From the distributor, the water flows down through the packing or trays. The objective of the mass-transfer section is to create a large surface area over which the water forms a thin film, promoting intimate contact with the fuel gas and enhancing mass transfer. After passing through the mass-transfer section, the water falls into the column sump, where a chemical oxygen scavenger is added to reduce the oxygen content to acceptable levels. During normal operation, when the filters are not backwashing, the level in this sump is used to control the flow rate of the water entering the top of the contactor.
Because the partial pressure of oxygen in water is a function of the total pressure of the system, applying a vacuum to the water/gas system can reduce the partial pressure of oxygen. Vacuum deaerators can be combined with either countercurrent or concurrent stripping gas to provide very low oxygen concentrations in the water. Stripping-gas usages of a fraction of a cubic foot per barrel are common. Vacuum stripping towers are used:
- where no stripping gas is available
- where the available stripping gas contains contaminants, such as CO2 and H2S
- where stripping gas has a high value
Disadvantages of vacuum-deaeration systems
The disadvantages of vacuum-deaeration systems include high power costs (to operate the vacuum pumps) and high maintenance to the system to ensure that oxygen does not enter the system through:
- pipe joints.
The weight and space allotted to production equipment are of major concern to operators, especially in offshore applications. In response to this sensitivity, oilfield equipment vendors have developed compact deoxygenation systems. At present, there are two systems in commercial use.*,
Wet combustion catalytic process
One system uses a wet combustion catalytic process to consume the dissolved oxygen in the water. The operating principle of this system is simple—hydrogen (produced and injected into the water stream) and oxygen (contained in the water) react in the presence of a palladium catalyst (contained in a pressure vessel) to produce water molecules. The major system components are:
- The palladium catalyst.
- The inline mixer.
- The hydrogen generator skid.
- The liquid-filled catalyst vessel.
Oxygenated water enters the inlet piping of the system and is measured with an accurate flowmeter. The measured-water flow-rate value is registered by the system controls, and a signal is sent to the hydrogen generator. Based on the inlet water flow rate, a proportional amount of hydrogen is produced and injected into the water upstream of the mixer. It should be noted that the hydrogen reacts with any free chlorine in the water (i.e., from the electrochlorinator); therefore, an additional amount of hydrogen is produced to make up for this loss. The static mixer ensures good dissolution of the hydrogen gas into the water. The water/hydrogen mixture is then routed to the catalyst vessel, where contact with the palladium catalyst is achieved. The oxygen in the water is reacted with the dissolved hydrogen gas in the presence of the palladium catalyst to produce water molecules.
Hydrogen is produced by the electrolysis of an ultrapure freshwater stream to produce H2 and O2 gases. The H2 gas is separated for injection into the water stream, while the O2 gas is vented to a safe location. A local programmable logic controller (PLC) controls the process for preparing the ultrapure water. The same PLC is used to regulate the production of hydrogen in proportion to the measured flow rate of the incoming oxygenated water.
An alternative compact deoxygenation system uses a high gas/water ratio stripping process in either a concurrent or countercurrent mode. The stripping gas used is nitrogen instead of natural gas. In either mode, the oxygen-laden stripping gas is regenerated in a catalytic purification vessel by means of a reaction of the oxygen with methanol in the presence of a palladium catalyst to produce CO2 and water. Compressed air is used as makeup gas to replace nitrogen losses from the system.
In the concurrent mode, deoxygenation occurs within two stages of mixers. Nitrogen gas is injected into the water stream in a concurrent manner upstream of the mixer. The mixer creates intimate contact between the oxygenated water and the stripping gas. As discussed previously, the oxygen will diffuse out of the water according to Henry’s law. Located downstream of each mixer is a partially liquid-filled disengagement vessel. Once inside the vessel, the oxygen-rich nitrogen gas is separated from the water and removed through the gas outlet at the top of the vessel. Stripping gas for the first-stage mixer is taken from the gas outlet of the second-stage disengagement vessel after compression. The oxygen-rich nitrogen gas from the first-stage disengagement vessel is routed to a catalytic deoxidizer to remove the oxygen. Regenerated nitrogen from the deoxidizer is then routed to the second-stage mixer.
In the countercurrent system, the oxygenated water is routed to the top of a partially liquid-filled deaerator column in the same manner as described previously. The column is equipped with:
- water inlet distribution piping
- mass-transfer packing
- inlet distribution piping for the nitrogen stripping gas
- a sump section
A major difference between the compact system and the traditional gas-stripping system described earlier is that the oxygen-rich nitrogen is deoxygenated and recycled to the stripping-gas inlet of the column. As mentioned previously in the description of the concurrent system, the oxygen-rich gas from the top of the column is routed to a catalytic deoxidizer to remove the oxygen.
Either system allows a substantial reduction (20 to 50%) in the weight and space of the deoxygenation equipment. The reduced weight and space requirements translate into reduced structural and support steel on the deck. Because either system can be provided as skid-packaged units, the amount of site work is reduced, and the need for special cranes or other special lifting requirements is minimized.
*Place, M.C. Jr.: “Catalytic Oxygen Removal—A Light Compact Water Deoxygenating System,” Shell Oil Co., unpublished internal document (1993).
Surface water contains biological constituents (primarily bacteria) that can contaminate the water-injection system. 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
- generate H2S that can cause pitting corrosion
Therefore, it is essential to develop a means to control the growth of bacteria in surface-water-injection systems. Bacterial growth is controlled mainly by chemical biocides, the most common of which is chlorine, which may be added directly or produced in-situ from seawater. Direct-added chemicals are covered in Water treating chemicals.
Because of its high chloride content, seawater can be electrolyzed with a hypochlorite generator or an electrochlorinator to produce hypochlorite (OCl–). Chlorine production in this way makes for a very convenient, inexpensive, and reliable source of bactericide. Chlorine from the electrochlorinator is continuously dosed into the seawater lift-pump intake to prevent marine fouling within the system, making up the injection-water-treatment system.
The hypochlorite generator produces chlorine in the form of sodium hypochlorite at a rate equivalent to a concentration of approximately 5 ppm. The electrolyzer is fed with seawater from the coarse-filtration outlet. The chlorine is generated by electrolysis in a single electrolytic cell. The amount of sodium hypochlorite formed is proportional to the amount of direct current passed through the seawater. The byproduct, hydrogen gas, is released and diluted to less than 2% mixture (less than the explosive limit) and vented to atmosphere.
Seawater contains approximately 2,800 to 3,000 mg/L of sulfate ion. Using seawater for injection into a producing reservoir for pressure maintenance or for waterflooding can cause problems if the formation water contains significant levels of:
Depending on the pressure and temperature of the system, these ions react with sulfate to produce either:
- calcium sulfate scale
- barium sulfate scale
- strontium sulfate scale
Barium sulfate and strontium sulfate scales
Both barium sulfate and strontium sulfate scales are extremely hard to dissolve in acid and equally hard to remove by mechanical means. Hence, once these types of scale deposit in either the production tubulars or the surface process piping, the likely result is that the well or platform may have to be shut in while the affected piping is replaced. One solution to this problem is to remove or reduce the amount of sulfate ion in the seawater before it is injected.
The process for removing sulfate ions from seawater is based on nanofiltration (NF) membrane separation. NF is a membrane process that selectively removes sulfate ions to produce reduced-sulfate seawater. The process is similar to reverse osmosis (RO), used extensively worldwide for seawater desalination; however, the NF membrane has a larger pore size and possesses a slight negative charge and, thus, can reject divalent ions (e.g., sulfate). Furthermore, NF membrane has a better feed-to-permeate conversion at 75% of the inlet flow rate; that is, for every 100 bbl of seawater fed to the system, 75 bbl of low-sulfate water are produced, and 25 bbl of high-sulfate water are rejected. *NF refers to a specialty membrane process that rejects particles in the approximate size range of 1 nanometer (10 Angstroms), hence the term “nanofiltration.”
The concept has been proven technically by its successful application in a west Africa seawater injection plant with a capacity in excess of 330,000 B/D. The design point for a sulfate ion in the treated seawater is 40 mg/L at 20°C seawater temperature. At this level of sulfate removal, it is expected that the amount of barium sulfate scale would be reduced from 50 to 7 kg per 1,000 bbl of produced water.
Simplified process diagram
A simplified process diagram is shown in Fig. 4. With a booster pump, pressurized saline feed water is continuously pumped to the module system. Within the module, consisting of a pressure vessel (housing) and a membrane element, the feed water will be split into:
- a low-saline product, called permeate
- a high-saline brine, called concentrate or reject
A flow-regulating valve, called a reject valve, controls the percentage of feed water going to the concentrate stream and the permeate that will be obtained from the feed.
Normally, the plant is divided into two membrane arrays staged in a 2:1 configuration. Each array is operated at 50% conversion, with the reject from the first array being fed to the second array for further treatment. The product streams from both the first and second arrays are then combined for injection into the formation, while the concentrate stream from the second array is rejected. Because each array is operated at 50% conversion, the combined product streams constitute 75% of the inlet flow rate (75% total conversion to product).
The seawater temperature is the most significant parameter affecting the design of sulfate-removal membrane systems. Lower sulfate levels in the product stream result in lower seawater feed temperatures; however, lower seawater feed temperatures require higher pressure drops through the membrane to maintain the desired flux rate and vice versa. Therefore, a balance must be established between the desired sulfate level of the product stream and the energy available for use to raise the seawater temperature or to increase the seawater feed pressure.
Proper pretreatment of the feed water supplied to the NF membrane is required to maximize the efficiency and life of the membrane elements and to ensure trouble-free operation.
- Removal of fine suspended solids that can plug or block the membrane surface.
- Prevention of biological growth on the membrane surface.
- Prevention of scale formation on the membrane surface during concentration of the feed water.
- Removal of any oxidizing biocides (e.g., chlorine) that can damage the membrane.
- Pressurization as required to achieve NF separation.
Sulfate reduction package
The sulfate-reduction package is part of a system that is designed to achieve all these objectives. For instance, both specially designed media-filter systems and cartridge filters are used to remove solids from the seawater upstream of the membrane elements. Furthermore, as sulfate is removed from the seawater along the length of each membrane element, the dissolved-solids content of the concentrate increases. As a result, the scaling tendency of the concentrate increases, as shown in Fig. 5. To avoid the deposition of scale on the membrane surface, antiscaling chemicals are injected into the seawater upstream of the membrane system. In addition, an organic biocide is dosed into the seawater to control the growth of bacteria in the membrane, and a dechlorination chemical (bisulfite) is injected into the seawater to neutralize any strong oxidizers (chlorine).
Despite these efforts to keep the membranes clean, the membranes will foul with time as the plant is operated. Therefore, membrane elements must be taken out of service on a routine basis for a deep clean with special cleaning chemicals. These chemicals are designed to remove deeply embedded scale and bacterial fouling with minimal damage to the membrane-element materials. It is possible to prepare a membrane-monitoring system to provide feedback to the operators that indicates the need for deep chemical cleaning of the membrane elements.
*Weston, R.: “Engineering Design of a Sulphate Removal Package,” Axsia Serck Baker Ltd., internal document (December 1995) 3.
- Snorre water injection tries new treatment technology. 1991. Ocean Industry (January): 22.
- Daniels, I. 1995. Developments in compact deoxygenation: the MINOX ‘compact tower’ process. Paper presented at the 4th International Water Management Offshore Conference, Aberdeen, 3–6 April.
- Henriksen, N., Nord, L., and Kjøberg, S.A. 1987. Efficient New Processing System for Water Deoxygenation. Presented at the Offshore Technology Conference, Houston, Texas, 27-30 May. OTC-5591-MS. http://dx.doi.org/10.4043/5591-MS.
- Heatherly, M.W., Howell, M.E., and McElhiney, J.E. 1994. Sulfate Removal Technology For Seawater Waterflood Injection. Presented at the Offshore Technology Conference, Houston, Texas, 2-5 May. OTC-7593-MS. http://dx.doi.org/10.4043/7593-MS.
- Vu, V.K., Latapie, D., and Davis, R.A. 1999. Barite Scale Prevention for Elf Angola’s Girassol Field Using Sulphate Removal Technology. Paper presented at the Deep Offshore Technology Conference, Stavanger, 19–21 October.
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