The Oil & Gas industry uses and generates enormous quantities of this commodity. On average, for every barrel of oil produced there are eight barrels of associated wastewater. Increasing the efficiency of water usage and improving its management is both a high priority among E&P companies and a subject of intense scrutiny for the communities in which they operate.
To illustrate the scale of water resources required, typical quantities needed for the various stages of operation are: 4,000-5,000 barrels (bbl) for drilling, 300-400 bbl for completion fluids, and 100,000 bbl for hydraulic fracturing. Efforts to secure dependable and affordable sources of water for E&P operations drove the first attempts to recycle wastewater. In the Barnett Shale a prolonged drought that raised questions about long term, reliable access to fresh water, pushed some operators to investigate and invest in methods of reusing water (Dale, 2009). In parts of the western US the main source of water, the Colorado River, is already stretched beyond its capacity (Finley, 2010). Other states such as Arkansas have significant annual precipitation (40-58 inches), but can experience periods of acute seasonal drought (National Atlas – Arkansas Maps, 2006). In the Northeast, the Marcellus Shale is situated in a region with more abundant water resources. However, the permitting process to tap these can be lengthy and complex, and can change on relatively short notice. In areas like these another option for water is desirable, and the recycling of wastewater can help fill that need.
Management of the wastewater produced is an equally important issue. In areas such as Texas, where the Ellenburger formation allows for injection of large volumes into wells far beneath any freshwater aquifer (9,000 to 11,000 ft deep), disposal is usually preferred. However, underground injection disposal is not always an attractive choice. For example, the entire state of Pennsylvania has a total of seven disposal wells compared to more than 50,000 found in Texas (Saltwater Distposal Wells FAQ, 2010). The geology of this area cannot accommodate the amounts of wastewater necessary for the industry. For lack of local disposal options, operators often find themselves hauling wastewater as far as Ohio and West Virginia to be injected into disposal wells. The cost per barrel of water trucked in this manner can be as high as $15, without taking in consideration the wear and tear on road infrastructure.
Alternative disposal methods include land farming and evaporation ponds. Both methods have been under increasing attack as of late from communities and regulators. A court ruling from the state of Montana in 2010 declared coal bed methane evaporation pits unconstitutional because the process causes “water to evaporate and be lost from any and all beneficial use” (NPRCCommunications, 2010). Under this ruling, the evaporation ponds violate Article IX, Section 3 of the Montana Constitution. Land farming operations in Arkansas have been used in the past for disposing of oilfield wastewater. Increasing concern for the safety of ground water has severely limited if not eliminated the amount of wastewater that can be absorbed from E&P operations by these disposal methods.
The problem with oilfield wastewater is that it contains a varied array of contaminants, many of which make it unsuitable for discharge or reuse without some form of treatment. Management of wastewater is highly dependent on its quality and intended use. For example, concentrated brine will be difficult to transport via temporary pipeline over areas with cultivated fields because of the potential harm of a spill. The composition of the fluid varies widely depending on the type of operation, location, and geology, just to name a few factors. The combination of these makes almost every situation unique. While existing technologies offer a wide selection of treatment options, cost remains the determining factor. Ultimately, all treatment methods must compete with the cost of disposal. The analysis followed water quality starting with the generation of wastewater, possible contaminants, continuing with typical compositions encountered, and then the content of discharged or reused wastewater.
First, the most common sources of wastewater in E&P operations were identified as shown below.
- Spent mud or water from mud dewatering
- Rig runoff
- Rainwater catch
- Rig wash
- Grey water and black water from the drilling camp
- Diluted completion and drill-in fluids
- Acidizing and chemical stimulation waste fluids
- Frac flowback – In hydraulic fracturing operations, after pumping the frac fluid downhole, there is a variable percentage of water that flows back to surface as the well is brought online. This water is called frac flowback. While there is no formal time period, flowback is typically classified as water from the first two to four weeks of a hydraulically fractured well. As a general rule, the quality of water decreases with time as it flows from the well. So, late stage flowback will often be a poorer quality than early stage.
- Produced water (Produced Water Facts; citation – Produced Water Society)
Second, the components of the wastewater were identified and classified. Contaminant is defined as “a substance that is either present in an environment where it does not belong or is present at levels that might cause harmful effects to humans or the environment” (Contaminant citation – greenfacts.org). Some of the contaminants found in the categories below occur naturally in minute quantities; their toxicity comes from higher concentrations (Example: Calcium, Sodium). Other species were introduced by human activity (Example: acids, biocides). For the purposes of this paper, contaminants are classified into four categories: organics, dissolved solids, suspended solids, and bio-contaminants.
Organics category includes liquids, solids, and semisolids insoluble in water (hydrophobic) or partially insoluble in water:
- Oil and grease
- 2-Butanone, 2,4-Dimethylphenol, Phenol
- Anthracene, Xylenes
- Benzene, Benzo(a)pyrene, Chlorobenzene, Ethylbenzene
- Di-n-butylphthalate, Naphthalene, p-Chloro-m-cresol
- n-Alkanes, Steranes, Toluene, Triterpanes
- BTEX, includes Benzene, Toluene, Ethylbenzene, and Xylene
This category also includes volatile organic compounds (VOCs), which are organic chemical compounds that have enough vapor pressure to vaporize and enter the atmosphere. VOCs are legally defined in the various laws and are regulated in the US by the United States Environmental Protection Agency in the air, water and land.
Dissolved Solids are inorganic and organic substances molecularly dispersed in water; they are reported as Total Dissolved Solids (TDS). Some of these substances are salts that make up for the salinity level.
A. Inorganic compounds:
- Metallic Ions (cations): Aluminum, Arsenic, Barium, Boron, Cadmium, Calcium, Copper, Iron, Lead, Magnesium, Manganese, Nickel, Sodium, Titanium, Zinc. This class includes heavy metals.
- Inorganic anions: Carbonate, Chloride, Sulfate, etc.
B. Organic soluble/ partially soluble compounds (not included in the separate organics category). Although the organic substances cited in this category are not solids per se, they are still included for lack of a better, separate category.
- Low molecular weight (C2-C5) carboxylic acids (fatty acids), ketones, and alcohols.
- Partially soluble components include medium to higher molecular weight hydrocarbons (C6 to C15): aliphatic and aromatic carboxylic acids, phenols, and aliphatic and aromatic hydrocarbons.
- Treatment chemicals: scale removers (acids), biocides, reverse emulsion breakers, corrosion inhibitors.
C: NORM (naturally occurring radioactive materials), such as Strontium, Radium (Radium 226, Radium 228), and Uranium. The sources of most of the radioactivity are isotopes of uranium-238 (U-238) and thorium-232 (Th-232), which are naturally present in subsurface formations from which oil and gas are produced. NORM is regulated by U.S. states regarding admissible levels, licensing, equipment contamination, worker protection, and waste disposal.
Suspended solids are solids that float on the surface, are suspended as a colloid, or are suspended due to the motion of the water (not in solution). Included in this category are:
- sand and silt, carbonates, clays, proppant (resin-coated sand, high strength ceramic materials introduced to help the fracturing of the shale), corrosion products;
- scales formed by precipitation of dissolved solids in the water: calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, and iron sulfate;
- colloidal organics such as coal or polymers.
Bio-contaminants are essentially microorganisms present in water – including algae, fungi, and bacteria (sulfur reducing bacteria, acid producing bacteria, and aerobic bacteria). Bio-contaminants can cause microbial influenced corrosion (MIC), create toxic substances such as hydrogen sulfide gas, plug the pores of the hydrocarbon-bearing formation, and damage or render equipment ineffective.
Comprehensive, complete analyses of water from any field-based operation are yet to be compiled. Some partial data available – direct from operators as well as from literature (Veil, Puder, Elcock, & Redveik, January 2004) – is presented in Table 1. These examples are not meant to cover the entire spectrum of wastewater, but more of a demonstration of how varied the composition can be. In addition, analyses are not always performed to identify the same contaminants. Hence, direct comparisons of samples are difficult to achieve.
Discharge and reusable composition
Finally, some of the limiting factors for reuse and discharge were identified. The various contaminants that can appear in the wastewater can have adverse effects on E&P operations and the environment. For example, suspended solids such as clay can plug and damage equipment like pipelines or pumps. They can also increase turbidity to a point that, if discharged into a body of surface water, they will block sunlight and harm aquatic plant species. High concentrations of dissolved solids in the wastewater can lead to precipitation downhole, reducing the production of hydrocarbons. Salts and other substances can harm aquatic life and crops and lead to the deterioration of groundwater supplies.
Regulations cover the required concentration of potentially harmful components of water for discharge. A recently approved Pennsylvania law will impose on the O&G industry a discharge limit of 500 ppm total dissolved solids (250 ppm chlorides) beginning in January 2011. The U.S. offshore discharge limits for wastewater’s oil and grease content is 29 ppm. There is a continuous non-specific pressure of communities on E&P operators to abide by stricter limits and cleaner methods.
WASTE MUD TREATMENT METHODS
Physical and chemical treatment can provide “Engineered Water” for all levels of operations. The exact solution composed of one method or several methods chosen for contaminant removal has to be in tune with the quality of influent water and the quality of effluent water desired. There is no “perfect” or absolute separation method. For example, reverse osmosis works for particles that are about 1 nm in size and that seems to cover most of the contaminants, but it is economical only at lower values of dissolved solids (less than 65,000 ppm dissolved solids). For comparison, the Marcellus shale wastewater can show concentrations as high as 275,000 ppm dissolved solids, which will need some other form of treatment in order to reuse. Through distillation one can obtain pure, contaminant-free water, out of virtually any aqueous feed but the cost of energy required for this process is high. To produce one pound of steam at normal pressure 1000 BTU are required.
The question is: what are the most economically viable treatment methods to obtain the quality desired? The answer to this dilemma seems to be the use of an array of methods, in a customizable, interchangeable setup that would fit better with the needs of the Operator. In the following we present briefly the principle of various existing technologies that could be used by themselves or combined and partially summarized in Table 2.
Blending or directly diluting wastewater with fresh sourced water is the simplest method of recycling. It is used by some operations in the Marcellus Shale area, where demand for drilling and frac fluid is high. The frac flowback is blended with fresh water in proportion of one part of flowback to several parts (4-5) of fresh water. The mixture is then used for the next hydraulic fracturing.
Filtration is the physical removal of particles by size as shown in Fig. 1. The separation can be passive (letting the gravity and or the volumetric flow do the job), or active (energy is used to exercise pressure and force “clean” water through a filter or membrane, leaving the contaminants behind).
- Particle filtration (particle size ≥ 1,000 nm) separates by size particles that are larger than one micron.
- Microfiltration (particle size ≥ 60 nm) is generally referred to the filtration at less than one micron level. Industrial microfiltration is a separation process for suspended solids or emulsified oils in liquids like water or organic solvents. The separation takes place based on size exclusion only. It is independent of temperature and specific weight of components. It is a low pressure-driven process with high volume processed.
- Ultrafiltration (particle size ≥ 10 nm) is a selective separation process using pressures up to 145 psi (approximately 10 bar). It is done with membranes with pore sizes in the range of 100 nanometers to 1 nanometer. It concentrates suspended solids and solutes of molecular weight greater than 1,000 Daltons. Permeate will contain low-molecular-weight organic solutes and salts. This method can successfully remove bacteria, viruses, and proteins from the feed.
- Nanofiltration (particle size ≥ 1 nm) is selected when Reverse Osmosis and Ultrafiltration are not the correct choice for separation. For example, if solids concentrations are too high for RO, Nanofiltration can be an alternative. The membranes have nanometer-size pores. Nanofiltration can perform separation applications such as demineralization and desalination, concentration of organic solutes, suspended solids, and polyvalent ions. Permeate contains monovalent ions and soluble low-molecular-weight organics together with water.
- Reverse Osmosis (hyperfiltration) (particle size ≥ 1 nm) is an efficient technique for dewatering process streams, concentrating/separating low-molecular-weight substances in solution, or cleaning wastewater. It has the ability to concentrate all dissolved and suspended solids. Permeate contains a very low concentration of dissolved solids. Reverse Osmosis is also used for the desalination of seawater. The method forces water through the membrane against the osmotic pressure and requires energy. This method is efficient at TDS concentration lower than 65,000 ppm. Higher concentrations would require too high of a pressure to overcome the osmotic pressure of the solution.
- Forward osmosis is the nature’s way of transporting fluid through membranes
(shown in Fig. 2). It is not really a filtration although fits the description of separating contaminants from the influent and letting through only water but the separation is done under osmotic pressure on each side of the membrane, not physical pressure. One of the advantages of this method is the energy efficiency, while the downfall is that you have to have in the “draw” solution a solute that will create a big enough osmotic pressure gradient to trigger the transfer of water. The solute needs to be either usable with the water, such as the NaCl, which produces clear brine; or it has to be relatively easily removable, such as the NH3 and CO2 gas mixture. This mixture of gases dissolves relatively easily and is removed by simply heating up the draw solution at about 65oC. Gases can be then re-circulated in the system.
- Membrane distillation is arguably a future valid and efficient way of obtaining clean water out of wastewater. We wanted to mention the method because of its popularity in the science and academic media (Walton, Lu, Turner, Solis, & Hein, 2004) (Bolto, Tran, & Hoang, 2007). Warm wastewater and cooler pure water are circulated one side and the other of a hydrophobic membrane which will permit only vapors of water to pass through the nano-sized pores from the contaminated side to the clean side under vapor pressure influence. The vapor passed through the membrane condenses on the clean side. The heat sink between the two membrane sides is about 5oC and process takes place at temperatures below 80oC.
Selecting the type of filtration depends on the water that is being treated. Different filtration systems have unique advantages and vulnerabilities when exposed to specific contaminants. All filtration methods are prone to fouling. Microorganisms can create biofilms that clog the pores of the membrane. The pores can also be blocked by suspended solids or precipitates. Some types of membranes are sensitive to specific chemicals and pH. For example, most RO membranes are sensitive to Chlorine and Chlorine derivatives. Against all these deterrents, filtration is still one of the cheapest separation methods.
Gravimetric Separation technologies use the specific weight of the materials to separate them from a watery mixture. The simplest method is the natural settling of the heavier materials at the bottom of the liquid and decanting (skimming) the top part of the wastewater which contains the lower gravity suspended contaminants and all dissolved particles (shown in Fig. 4). This requires space and time and it is at the core of the floc and drop separation method. Induced gas flotation is a popular gravimetric separation method that uses the ascensional force of gas bubbles to lift hydrocarbon droplets to the surface of the fluid. The technique increases the efficiency of the process by speeding the natural rise of lower density substances to the top of the mixture.
By adding centrifugal force, the heavy particles are pushed towards the circumference of the vessel while the water with the dissolved solids and lighter suspended solids are separated close to the shaft. In the category of centrifugal-enhanced gravimetric separation enter also the hydrocyclones. Highly engineered hydrocyclones might be a valid way of separating water and oil downhole or subsea and use the pumping capacity exclusively for oil. The main downfall of hydrocyclones is the fact that for optimum operation, they require steady inflow of fluid, without fluctuations.
Flocculation is a process where colloids come out of suspension and aggregate in the form of floc or flakes. These flakes are result of a process that agglomerates some compounds in “flocs”, such a polymerization. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution. The resulting sludge or the precipitate is subsequently separated from the cleaner liquid via filtration, decanting, etc.
Evaporation is nature’s way of circulating and purifying water. Various types of systems emulate natural evaporation but in an accelerated environment. The main problem with any evaporation technique is the energy necessary to evaporate water. The use of waste heat and enhanced heat recovery methods have rendered some existing methods to lower the energy necessary to fraction s of the thermodynamic requirement of 1000 BTU for one pound of steam to as less as 50 BTU for one pound of steam (Dale, 2009). An evaporation/re-condensation system is otherwise called distillation (shown in Fig. 5). Types of evaporation methods:
A. Vapor-compression evaporation – a compressor is added to enhance efficiency and capacity of distillation (Fig. 6)
- Mechanical vapor recompression (MVR) – Uses a mechanically-driven compressor (centrifugal) or blower (roots blower) to compress the vapor.
- Thermo-compression or steam compression – Uses a high pressure steam ejector to compress the vapor.
B. Enhanced natural evaporation processes manipulate the simple evaporation/ condensation process by playing with the other factors that influence and increase efficiency of the evaporation process such as surface area or concentration. A simple example is the use of misting devices in the evaporation ponds.
“Crystallization is a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs, equals precipitation” (Crystallization, 2010). Crystallization is also known as a procedure for purifying compounds. As an unrelated example, crystallization is used in obtaining salt by crystallizing sea salt out of sea water. Crystallization can be enhanced by seeding the solution with start-up crystals, providing extra surface for reaction, changing the equilibrium by changing the pH. When cradle to grave approach is targeted, crystallization of salts contained in heavy brines can be performed, freeing the last bit of water contained in the solution.
Sublimation of an element or compound is a transition from the solid to gas phase with no intermediate liquid stage (exactly like evaporation is from liquid to vapor). Sublimation is a rather rare quality and it happens naturally for water at normal pressure and (sub)freezing temperatures (phenomenon is responsible for drying clothes on a line during wintertime). These methods freeze the water and separate it via sublimation. The contaminants’ separation occurs due to the freezing point difference between the various species present in the influent.
Advanced Oxidation is a set of chemical treatment processes designed to remove organic and inorganic materials or bio-contaminants in wastewater by oxidation. This results in the development of hydroxyl radicals. Depending on the dosage of the oxidant, the bio-contaminants can be either inactivated (prohibited from replication) or killed. The procedure is particularly useful for cleaning biologically toxic or non-degradable materials such as aromatics, pesticides, petroleum constituents, volatile organic compounds, and multiple valence ions in wastewater. The contaminant materials are converted to a large extent into stable inorganic compounds such as water, carbon dioxide, and salts, i.e. they undergo mineralization. The source of oxidation dictates the specifics of the process. Some of the most popular oxidation sources are: ozone, sonoluminescence, ultraviolet irradiation, hydrogen peroxide, chlorine, sulfur dioxide. Some successful techniques combine two oxidation sources such as ozone and ultraviolet radiation, or ozone and other form of energy (ultrasound). Ultraviolet (UV) radiation can be used by itself as an efficient way of inactivating or killing bacteria. UV light is not used very often by itself for disinfection of oilfield wastewater because the latter presents usually high values of turbidity which would prevent the transmittance of radiation through the fluid.
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference (Electrodialysis). The cell consists of a feed (diluate) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes (a cell) as shown in Fig. 7. In almost all practical electrodialysis processes, multiple cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation exchange membranes forming the multiple electrodialysis cells. Electrodialysis processes are unique compared to distillation techniques and other membrane based processes (such as reverse osmosis) in that dissolved species are moved away from the feed stream rather than the reverse. Because the quantity of dissolved species in the feed stream is far less than that of the fluid, electrodialysis offers the practical advantage of much higher feed recovery than regular filtration techniques. The quantity of ions a cell can carry from solution to one of the electrodes is dependent of the amount of electricity applied. The energy cost generally prevents the use of ED on water with high TDS concentrations.
Adsorption is the accumulation of atoms or molecules on the surface of a material (Adsorption). Absorption refers to adherence of particles of gas or liquid within the volume of a liquid or solid material (Absorption). Molecules undergoing absorption are taken up by the entire volume, not just the surface (as in the case for adsorption). Different materials can be used: surfactant-modified zeolite (SMZ), granular activated carbon (GAC), or ion-selective resins, to name just a few. The ad- or absorbing material can be packed in a static or fluidized bed.
The dynamic nature of the market and the new regulations that seem to appear daily are going to enhance the need for solutions to treat oilfield water. The authors decided to investigate what factors operators take into consideration when choosing a technology.
- Economics is the biggest driver. The cost per barrel of water treated has to be compellingly competitive with the costs that the company might incur with the disposal, procuring, and/or storing that quantity of water.
- Volumetric capacity of the installation has to be sufficient to keep up with the flow rates necessary, and that number is sometimes as high as 5,000 bbl per day. Machines have to be scalable and adjust to sudden variations in flow.
- Quality of the feed can be variable while the quality of the output has to be fairly stable. The solution has to be flexible enough to handle sudden spikes in contaminants with minimum adjustments, preferably able to optimize in real time.
- Efficiency of treatment has to be high. The higher the recovery rate (quantity of clean water per quantity of raw feed), the better. Unlike seawater desalination, operators are tasked with disposing of every barrel of water not treated by the system.
- Mobility of the technology presents a definite plus. In many instances, an ideal system would be deployed right at the wellhead.
- If an on-site system is being considered, the footprint has to be small. Well sites offer a limited space that is already crowded.
The demand for clean water in E&P operations is high and increasing, while finding sources is becoming more and more of an obstacle. Challenges include economics, climate, geology, geography, and regulations. One valid solution to this problem is recycling water obtained from the various phases of exploration and production and reusing the treated water. Wide varieties of technologies exist and continuously develop to meet the quality output requirements of operators and regulators. The range of contaminants found in the feed and the needed quality of output will dictate the actual method or methods used. Operators will select solutions that are economical, have adequate capacity, are flexible, efficient, mobile, and have a small footprint.