Patent Application
20250122112
The present invention generally relates to methods, and desalination units employing methods, for generating high-quality desalinated water from produced water in an oil production facility. More specifically, for example, the pH of produced water is lowered prior to desalination by membrane or thermal desalination methods thereby ensuring no precipitation of salts or other inorganic matter occur prior to desalination and facilitating smooth operation of desalination, without significant precipitation of calcium and magnesium carbonate.
Produced water (PW) is water that comes out of the well with crude oil during crude oil production. Produced water can contain water naturally present in the rock, water injected into the reservoir in steam-based operations, and water from fracking operations. It also includes soluble and insoluble oil and organics, dissolved solids, and salts, and may include one or more of various other elements, chemicals, and materials used in the oil well completion or production process or which is a byproduct of such process(es). The composition of produced water will vary based on, among other things, geography, age of the oil well, chemicals used during hydraulic fracturing and production. Table 1 provides an example of a typical composition of produced water. High total dissolved solids (TDS) and high hardness with low bicarbonate is a common signature of many produced waters.
Significant volumes of produced water may be produced for every barrel of oil produced. As an example, ˜75,000 m3 to 135,000 m3 (˜475,000 to 850,000 bbl) of water is required to develop one well drilled 2,400 m (7,875 ft) deep, in a typical unconventional shale oil and gas development. A typical mid-size to large shale production facility has about 10-15 wells. Nearly 50% of the water is returned, or flows back, to the surface. Formation water contained within the geology of the producing formation will also be co-produced with the oil. In the Permian Basin, for instance, between 2-9 barrels of water per barrel of oil flows to the surface, requiring treatment, recycling and/or disposal. Reuse of produced water in completions (subsequent hydraulic fracturing operations) can consume a portion of this water but significant more is produced than is needed. Beneficial reuse, as an alternative to deep well disposal, of treated produced water in applications other than completions activities is therefore an important component of future disposal options.
Before produced water can be reused, however, it must be treated to remove oil and grease, heavy metals, dissolved salts and chemicals, suspended solids, etc. The type of treatment and the rigor involved depends on the location of produced water and is also heavily dependent on the end-use for the produced water. When reusing for hydraulic fracturing operations, the level of treatment required is less, compared to reusing for other beneficial purposes such as irrigation or groundwater recharge or many industrial uses. The more treatment required, the more expensive the process.
After treatment of the produced water to remove all suspended impurities, utilizing the water for purposes outside of oil and gas usually requires the capital-intensive step of desalination. Salinity of produced water ranges from nearly fresh (<1%) to about 40% salt content. As a comparative example, sea water only has a salinity of about 3.5%. The composition and salinity of produced water is influenced by several factors, chief among them is the geographical location of the facility. Salinity of produced water is expressed as total dissolved solids (TDS). Depending on the geographic location of the oil and gas facility, the TDS of the produced water varies from as low as 200 mg/L to as high as 400,000 mg/L. Following desalination, produced water must be further treated to remove any trace organic or inorganic impurities, and be re-mineralized to be stable in the environment.
Conventional desalination methods for water treatment include membrane desalination, like reverse osmosis (RO), nanofiltration, and electrolysis. Other methods include thermal distillation systems, multiple-effect distillation, multiple-stage distillation, vapor condensation, and falling-film evaporation.
In a simple RO system, desalinated water is pushed through a semipermeable membrane that limits passage of dissolved chemicals and salts. Thus, dissolved salts, colloidal and suspended solids and matter can be removed from contaminated water. Although RO is a commonly used method, it suffers from the disadvantage that the membrane used for RO often experiences fouling and thus may be associated with high maintenance cost.
The pH of produced water after pre-treatment to remove oil and grease, chemicals, and minerals, prior to feeding into membrane (e.g., RO) or thermal desalination unit is typically in the range 6-9. Basic salts such as carbonate and bicarbonate are typically found in the produced water, along with very high levels of hardness-forming ions such as calcium and magnesium. When fed into a desalination unit (to perform, for example, RO or thermal desalination), salts may precipitate out of solution when they reach their solubility limit, thus clogging up the desalination unit. Repeated scaling and membrane blockages by salts impact the operational efficiency of the desalination process and cause frequent cleaning, making the process more expensive and inefficient. Depending on the pH, dissolved ammonia may pass into the desalinated water and the resulting water contains high concentrations of ammonia, thereby requiring additional treatment prior to its use in most applications.
Thus, what is needed in the art are efficient and more cost-effective methods for desalination of produced water to produce water of high quality. The ideal method would be cost effective, have a low footprint, and provide a means to eliminate or significantly reduce the concentration of salts in the produced water. Consequently, the method would yield produced water of very high quality suitable for consumption and agricultural purposes. Such a method would also not harm the equipment at the production facility and would be easy to install, operate, reliable, and have high efficiency. The present invention addresses one or more of these needs.
Conventional methods for removing scaling salts that increase the risk in high salinity desalination processes require addition of large quantities of lime and soda ash to raise the pH, thereby precipitating CaCO3 and Mg(OH)2. These precipitates require sludge disposal, requiring transportation and landfilling. Due to the high calcium and magnesium concentrations in typical high salinity produced water, this conventional or excess lime softening method results in excessive operating costs due to both chemical addition and sludge disposal costs, and adds to the complexity and costs of high salinity produced water desalination. Lime softening is commonly used in brackish water treatment to remove hardness.
Described herein are methods, and desalination units, for desalinating produced water to produce water of high quality suitable for re-introducing in ground water stream, consumption, and irrigation. An exemplary method of the present invention involves primary and secondary treatments to de-oil and remove suspended solids and the like. Before desalination, however, the pH of the produced water is reduced to pH 5-6. This converts carbonate to CO2, which can be stripped out, and reduces the degree of saturation of calcium carbonate and magnesium carbonate. By doing this, the need for capital intensive lime softening can be lessened or eliminated prior to desalination. The remaining calcium and magnesium ions are filtered out in desalination, without the problem of scaling.
Existing desalination units on the production platform or in the field can be fitted with a unit to reduce the pH of water, thus increasing the quality of the cleaned produced water, and increasing the efficiency of the desalination unit. The desalination method and process described herein can also be applied in facilities with limited space and offshore operations.
In one example, produced water is treated to remove volatile and non-volatile organic solutes, contaminants such as fats, oil, grease (FOG), microorganisms, chlorides, sulfides, precipitated polymers, and other frac chemicals with appropriate methods.
This water is then pretreated by acidification prior to desalination. Water is acidified to obtain a pH about 5-6. At this pH, carbonate and bicarbonate species in the water convert to carbon dioxide, as shown in Equations 1a) and 1b) below. The released carbon dioxide is removed from the water via air stripping and can be fed into a carbon dioxide compression compartment and reused in the reservoir or even sequestered therein. The ready water is then desalinated, e.g., with a membrane desalinator or a thermal desalinator, and because the carbonate has been removed from the water, it is not available to precipitate and form scale in the desalination process.
HCO3−(aq)→CO2(g)+H+(aq) Equation 1a.
CO32−(aq)→CO2(g) Equation 1b.
Customizable pH adjustment units are commercially available that can be fitted into the produced water purification units. A typical pH adjustment unit for water treatment uses sulfuric acid, phosphoric acid, hydrochloric acid, or nitric acid. The application of the acidizing solution to adjust the pH of treated produced water depends on factors including the concentration of salts dissolved in the water, the pH of the produced water prior to desalination and chemicals in the water.
Sulfuric acid (H2SO4) solution is the most used and least expensive neutralization and acidizing solution for produced water. Concentrations of 25% to 96% in aqueous solution of sulfuric acid are typically used, although concentrations of 35% to 65% are also commercially available. Sulfuric acid is very potent and is generally safer to use than other acids like nitric acid or hydrochloric acid.
Phosphoric acid (H3PO4) is generally safe and inexpensive. It is a weaker acid than sulfuric or hydrochloric acid. The reaction of phosphoric acid to acidify a solution is relatively slower and a higher contact time with the solution is required.
Hydrochloric acid (HCl) is also a commonly used acid for large scale industrial scale reactions. It is inexpensive and effective. HCl has some safety concerns as fumes of HCl gas at higher concentrations can be corrosive and cause damage to the pH adjustment unit. It is usually used at well ventilated facilities or outdoors. However, the reaction with HCl is quick and the pH can be appropriately reduced to the desired pH relatively faster thereby reducing acid contact time. This also eliminates formation of scale-forming sulfate salt (as is the case with H2 SO4 addition).
Any commercially available pH adjustment chemical unit can be fitted to a RO unit, or any other membrane or thermal desalination unit used on site. The acid used in the skid would be dependent on the quality of water after the removal of chemicals and other waste. If the produced water has basic impurities including calcium and magnesium carbonates, silicates, fluorides, phosphates, sulfates, nitrates and ferrous compounds, the acid used in the skid would be a strong acid like hydrochloric acid. It is desirable to monitor the pH of the water after treatment and before water enters the desalination unit.
A typical RO filter for industrial membrane desalination and filtration are pressure-driven membranes. RO filters can remove contaminants as small as 0.0001 μm. The efficiency of the RO systems depends on the pre-treatment of the produced water. Although RO membranes are prone to clogging and fouling, with the pre-treatment by reducing pH of the produced water, the lifespan of the RO membrane can be increased by minimizing and eliminating fouling and clogging of the membranes by excess salts. RO membrane systems are applicable for treating water containing TDS in the range of 500-25,000 ppm. A typical industrial RO system has a lifespan of about 3-7 years, but with the pre-treatment by acidification as taught herein, the lifespan of the RO filters can be increased by 3-4 additional years. Importantly, they will provide increased uptime and require less chemical cleaning to remove calcite deposits on the membrane surface.
Industrial RO systems may contain several pre- and post-RO filter parts including multimedia prefilter, water softener or anti-scalant dosing system, dichlorination dosing system, RO unit with semi-permeable membrane and a post chlorination treatment. They may also be fitted with ultraviolet sterilizers. The RO system uses the reverse osmosis technology by transporting feed water through a multimedia prefilter to remove particles larger than a said particle size. Typically, the size is <5 micron. By pre-treatment with pH reducers, fouling of the RO membrane is reduced significantly, and the RO filters can continue to operate without removing and changing out membrane elements. Fresh, potable water is pumped out of the RO system, and collected salts, minerals and other impurities are discharged as brine stream. Optionally, water passes through a UV sterilizer to rid the water of any bacteria and microbes that may have passed through the filters. Note that conventional RO technology is not suitable for treating high salinity produced water, but can be used on waters with TDS levels below 60,000 mg/L. Alternatively, for treating higher salinity produced water, alternative RO membrane technology may be applied including osmotically-assisted RO or high pressure RO.
In another example, the pH adjustment skid is installed before the produced water is fed into a thermal desalination treatment on site. Thermal desalination units are typically larger in size and require more energy to operate, but they can also handle higher salt loads. A thermal desalination unit can be operated for water with TDS levels 500 to about 200,000 ppm. The cleaned water from a thermal desalination unit usually contains about 0.002 to about 0.0014 ppm (10-1000 mg/L) TDS. But with robust pre-treatment with pH adjustments, the final TDS after thermal treatment can be as low as 0-0.1 ppm.
In produced water containing ammonium salts and dissolved ammonia, thermal desalination is a preferred method. If the pH of the produced water is high, ammonia is in a gaseous state and ends up in the distillate. If the pH of the produced water is low, ammonium ions typically remain dissolved in the water.
As used herein, pH, by definition, is the measure of free hydrogen activity in water, thereby assessing how acidic or basic a substance is. pH of 7 is considered neutral, pH 0-7 is acidic and 7-14 is considered basic.
As used herein, “TOC” is total organic carbon, usually in mg/L or ppm.
As used herein, TSS is total suspended solids. These can be removed by settling, with or without coagulants and flocculants, filtration, and the like.
As used herein, TDS is total dissolved solids. It is typically measured in ppm or mg or g/L of salt in water. The TDS of acceptable drinking water is regulated by the EPA and less than 300 mg/L is generally considered excellent drinking water.
As used herein, the “Langelier Saturation Index” (sometimes Langelier stability index) is a calculated number used to predict the calcium carbonate stability of water. It indicates whether the water will precipitate, dissolve, or be in equilibrium with calcium carbonate. Wilfred Langelier developed a method in 1936 for predicting the pH at which water is saturated in calcium carbonate (called pHs). The LSI is expressed as the difference between the actual system pH and the saturation pH:
LSI=pH(measured)−pHs
For LSI>0, water is super saturated and tends to precipitate a scale layer of CaCO3. For LSI=0, water is saturated (in equilibrium) with CaCO3. A scale layer of CaCO3 is neither precipitated nor dissolved. For LSI<0, water is under saturated and tends to dissolve solid CaCO3.
As used herein, “de-oiling” treatment reduces the small amounts of oil still in the produced water after the oil-and-water separator separates the bulk of emulsion into oil and water (and gas). Methods of de-oiling include hydrocyclones, centrifuges, corrugated plate separators (CPS) or corrugated plate interceptors (CPIs), passive gravity separation, skimming, flotation, carbon filters, nutshell filtration, and combinations thereof.
The oil-and-gas separator typically does not remove all oil and a secondary treatment usually uses chemicals to remove emulsified and the dissolved oil by flotation. Small gas bubbles (nitrogen, air, or fuel gas most common) to attach and float O&G with a bottom solids removal device. These methods may also use chemical (coagulant and/or flocculant) for better removal efficiencies. Methods include induced gas flotation (IGF), dissolved gas flotation (DGF), dissolved air flotation (DAF), dissolved nitrogen flotation (DNF) or a compact flotation unit (CFU) or microbubble flotation.
The conventional method for heavy metal removal from industrial wastewater generally involves a chemical precipitation process. Hydroxide precipitation is the most common treatment technology. Multi-valent ions and heavy metals are removed by adding alkali such as caustic, lime or soda ash to adjust the wastewater pH to the point where the metals exhibit minimum solubility. Then a proper solid-liquid separation technique removes the metal precipitation such as sedimentation and filtration. The conventional heavy metal removal process has some inherent shortcomings such as requiring a large area of land, a sludge dewatering facility, skillful operators and multiple basin configurations.
High silica and hardness are usually managed by the “lime softening process” wherein the addition of limewater (calcium hydroxide) removes hardness (deposits of calcium and magnesium salts) by precipitation. However, the acid removal of carbonates, as taught herein, allow us to omit lime softening, thereby avoiding the costly and environmentally unfriendly process for many produced waters due to the amount of lime and sodium carbonate required and the mass of solids resulting from the softening process that must be hauled and landfilled. Silica is undersaturated as water goes through the desalination, so is not impacted by lowering the pH.
As used herein, “stripping” is a method of stripping dissolved gases from produced water to alleviate scaling and corrosion concerns. Such stripping could be, for example, steam, air, or natural gas stripping. Its purpose is to drive off H2S, CO2, and O2 without vaporizing significant amounts of water. Produced water is first acidified to pH 4.0 using e.g., sulfuric or hydrochloric acid and dissolved gases are removed in the stripping process before feeding into the evaporation unit and heated to its bubble point. The distillate or water vapor generated is condensed and collected. An additional benefit of stripping is that it has the potential to greatly reduce VOC and other gas (e.g., ammonia) concentrations in the product streams, especially in the distillate. It can be thus sometimes used as a pretreatment step before desalination.
As used herein, “ready-water” is water that has been acidified so as convert carbonate to CO2, reduce the Langelier saturation index, thus being ready to enter the desalination unit without the need for expensive hardness removal pre-treatment step. The gases can be removed or scavenged and used for storage, or even just allowed to collect at the top, and the ready-liquid sent to the desalinator.
As used herein, “desalination” is a method of removing salts from PW. Desalination can utilize either thermal processes (involving heat transfer and a phase change) or membrane processes (using thin sheets of synthetic semipermeable materials to separate water from dissolved salt).
As used herein, “membrane desalination” technologies used for PW desalination, include reverse osmosis (RO), forward osmosis (FO) and membrane distillation (MD).
As used herein, “thermal desalination” uses heat and evaporation to separate water and salts. Technologies include multistage flash (MSF) distillation, multieffect distillation (MED), vapor compression (VC), thermal vapor compression (TVC), falling-film evaporator, and mechanical vapor compression (MVC).
Other desalination processes include solar still distillation, humidification-dehumidification, membrane distillation, and freeze/thaw desalination.
As used herein, “polishing” or “tertiary” treatments are typically a post desalination treatments, used as needed to bring the cleaned PW to code. Polishing methods include various filtration methods, absorbance methods and chemical or biological oxidation methods.
Filtration methods include nutshell or walnut shell filters, various filter media, multimedia filters (MMF), deep bed multi-filtration media, activated carbon filters and ceramic and polymeric Microfiltration (MF) or ultrafiltration (UF).
“Adsorption” is commonly used for the treatment of produced water, as it can remove more than 80 percent of organics and results in nearly 100 percent product water recovery. A variety of materials are used for adsorption, including zeolites, organoclays, activated alumina, and activated carbon, which can remove iron, manganese, TOC, and other contaminants. Chemical use is minimal. However, the adsorbent can be easily overloaded with large concentrations of organics, so this process is not always ideal for primary treatment. The media also eventually become consumed with contaminants and must be disposed or regenerated using chemicals. Regeneration creates a liquid waste product that must be disposed. Media may require frequent replacement or regeneration depending on type and feedwater quality.
Oxidation can be used to remove organics and some inorganic compounds like iron and manganese. Oxidants like chlorine, chlorine dioxide, permanganate, oxygen, and ozone are also frequently used to treat produced water. No pretreatment is required, but solid separation post-treatment is often necessary to remove oxidized particles. Oxidation can sometimes be a more expensive method, as chemical costs may be high and the purchase of chemical metering pumps is required for dosing.
The use of the word “a” or “an” in the claims or the specification means one or more than one, unless the context dictates otherwise.
The term “about” means the stated value plus or minus the margin of error of measurement or plus or minus 10% if no method of measurement is indicated.
The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or if the alternatives are mutually exclusive.
The terms “comprise”, “have”, “include” and “contain” (and their variants) are open-ended linking verbs and allow the addition of other elements when used in a claim. The phrase “consisting of” is closed and excludes all additional elements. The phrase “consisting essentially of” excludes additional material elements, but allows the inclusions of non-material elements that do not substantially change the nature of the invention, such as instructions for use, buffers, and the like. Any claim or claim element introduced with the open transition term “comprising,” may also be narrowed to use the phrases “consisting essentially of” or “consisting of,” and vice versa. However, the entirety of claim language is not repeated verbatim in the interest of brevity herein.
The following abbreviations are used herein:
In an oil and gas facility with a medium to high TDS produced water, a water desalination system as discussed in the present disclosure could be attached. The water typically has been at least through the oil-and-water separators at the CPF, then to a deoiler, flotation tank, and any other treatments needed before desalination. Such treatments can include any described herein or known in the art, except lime softening, which is omitted from the inventive methods. Operators can select suitable techniques given the degree of contamination of the original produced water, as well as the efficacy of the applied technology in reducing organics and solids.
A pre-desalination step using a pH adjustment skid is carried out wherein the resulting water is acidified to a pH of about 5-6, thus converting carbonate to CO2, which can be removed. The de-carbonated produced water exhibits a lower Langelier Saturation Index and lower degree of saturation of calcium carbonate. This lower pH also forces most of the ammonia to remain in an aqueous state, during thermal desalination, thereby lowering the amount of ammonia carry-over into the distillate. In membrane desalination applications, the produced water may undergo additional pH adjustment prior to desalination to improve removal of soluble organic constituents and specific inorganic salts, if needed, to result in a high-quality pretreated water stream ready to enter the desalination unit. This can be referred to as “ready-water” herein.
Recent laboratory testing to develop a titration curve for raw produced water from a Permian Basin formation provides a basis of understanding of pH adjustment of the complex chemistries of a typical produced water.
From
The ready-water is then fed into either a thermal or a membrane desalinator. In this example, an RO system with a multimedia prefilter is used. TDS of the produced water would typically be measured before any water purification treatment, and then measured again before feeding produced water into the desalination unit and finally after desalination.
We anticipate that the medium to high TDS of the produced water in the field when fed into the desalination unit after the pH adjustment would produce water with 0 or very low (<1000) water. The predicted results of this application are shown below in Table 2.
Testing has confirmed successful decarbonation and desalination of high salinity PW. Table 3 provides results for a thermal desalination system and Table 4 for membrane desalination treatment.
The examples herein are intended to be illustrative only and do not limit the scope of the appended claims. Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the scope of the disclosure as defined in the claims.
| Number | Date | Country | |
|---|---|---|---|
| 63543991 | Oct 2023 | US |
