Produced water quality pre-treatment
Produced water is a mixture of elements which give it its characteristics. The physical and chemical features of produced water do vary significantly mainly on the basis of the location of the oil field, the geographical formation with which the water has been in contact with hitherto extraction, and the type of hydrocarbon product that is extracted (Argonne National Laboratory et al. 2004). As a result, it is possible for the properties of produced water to vary throughout the lifetime of the reservoir. In cases where waterflooding is done, then the properties if produced water will even vary more dramatically as injected water is added to the formation.
Understanding the characteristics of produced water has the benefits of increasing its production. Essentially, understanding the constituents of produced water allows producers to determine the appropriate application of scale inhibitors, to identify potential areas of problem in a well-bore, and the best suited produced water treatment method towards the desired purpose effectively and cheaper. Some of the features of produced water that are of importance and therefore from the focus of this study are; total dissolved solids, and other mineral constituents.
- Salinity/Total dissolved solids (TDS)
Salinity is the amount of total dissolved solids (TDS) in the water and it is most often determined by electrical conductivity (EC). EC is preferred because ordinarily, ion dissolved in water will conduct electricity and the actual TDS analysis alternatives are very expensive to conduct. Produced water with a higher concentration of TDS will be relatively conductive and TDS is measured in parts per million or mg/l and EC is measured in micro-Siemens per centime (µS/cm) (Argonne National Laboratory et al. 2004). Produced water that is high in TDS can reduce the availability of water for plant use if and when disposed of untreated or insufficiently treated into the vegetated field. If it is used as irrigation water, the high TDS levels reduce the ability of plant roots to uptake water thus diminishes crop yield.
Saline water in terms of EC is that which has more than 3000 µS/cm. in the United States, salinity threshold values of 1000 µS/cm have been established for the Little Bighorn and the Tongue Rivers and Rosebud Creek. In addition, salinity levels of 2000 µS/cm have been determined for the Little Powder and Powder Rivers and Mizpah Creek (ALSPACH 2014).
Various forms of water are used in the process of oil extraction. While these forms of waters all contribute to the volume of produced water as well as its properties, each has its specific properties. These forms of water are hydraulic fracturing water, hydraulic fracturing fluid, and the flowback water. The salinity of hydraulic fracturing varies with the specifications of the fracturing fluid injected into the well-bore. Hydraulic fracturing fluid on the other hand, which is a combination of sand water, and chemical additives has a greater salinity level than hydraulic fracturing water and this is dependent on the additives used. Formation water, which is the water present in the production zone and comes to the surface through the wellbore has a salinity level that varies from 100-400000+ mg/L (ALSPACH 2014).
Te salinity of formation water is as a result of biological and chemical reactions that start as soon as sediments are deposited are the formation of the reservoir. The reactions continue and accelerate as the formation is subjected to higher temperature and pressure before a drill finally enters the reservoir for extraction. According to Abdou et al. (2011), the combined effects of these biological, physical, and chemical processes are known as diagenesis. As a result, there are no significant variations in the salinity of produced water across oil-producing regions in the world, but rather, the salinity of produced water varies from one well to another depending on the salinity levels of the types of water used in the extraction process. Table 1 below shows the salinity levels of the various types of water that can be used in the process for the various uses.
|Water type||Salinity, parts per thousand|
|Average river water||0.11|
|Evaporate systems||35 to 350|
|Formation water||7 to 270|
Table 1. Salinity variation of types of water that may be used in the oil extraction process (adopted from Abdou et al. 2011)
The other feature of produced water that is of interest is the amount of sodium, which is referred to as sodicity. Produced water with the excess amount of sodium can adversely affect plant growth when disposed of in open vegetated fields. The standard measure for sodicity is sodium absorption rate (SAR. SAR is a calculated parameter that compares the concentration of sodium to the sum of calcium and magnesium concentrations; the higher the SAR of produced water, the greater the potential to diminish permeability thus reducing hydraulic conductivity, infiltration, and causes surface crusting. Produced water with SAR levels that are higher than 12 are considered sodic (ALL 2003; Argonne National Laboratory et al. 2004).
The sodic levels of produced water are mainly dependent on the nature of the oilfield with coalbed natural gas (CBNG) fields having the highest level of sodium. In the US, CBNG sodicity SAR varies from an average of 30 in the Powder River Area and the Pumpkin Creek Area to 43 in the Tongue River area. Similar to salinity, SAR averages are not specific to a region but vary depending on oilfield formation and the nature of the reservoir (Argonne National Laboratory et al. 2004).
- Other mineral elements
The other mineral of important to be discussed herein are calcium and magnesium. This is primarily as a result of their relevance in determining SAR, and secondary due to their effects with regard to the various water treatment methods particularly membrane filtration. The concentration of calcium and magnesium varies from oilfield to another depending on the nature and type of the reservoir. For example, CBNG wells have the highest concentration of calcium. When produced water is oversaturated with calcium, it precipitates to its calcite form. In the US, calcium concentrated averages vary greatly across three oilfields; 9.3mg/L in the Tongue River area, 31mg/L for the Powder River area, and 27 for the Pumpkin Creek area (Jackson & Reddy 2007; Norvell et al. 2009).
On the other hand, magnesium concentration varies significantly from one area to another. In the Tongue River Area, the average for magnesium concentration is 4.0mg/L, in the Powder River Area the average is 32mg/L, and an average of 33mg/L for the Pumpkin Creek area (Jackson & Reddy 2007; Norvell et al. 2009).
Select the best alternatives/methods to treat oilfield produced water depending on the quality of produced water (salinity and Calcium…etc.) in each country
Environmental and economic factors have necessitated the treatment of produced water for desalination, demineralization, and removal of oil and grease residues. The need for treatment of produced water has prompted enterprising equipment suppliers to develop innovatively produced water treatment process that varies in terms of treatment efficiency, energy consumption, and costs. The adoption of a treatment method is largely dependent on these factors and the treatment goal. For example, if the purpose of treating produced water is to desalinate it, then the most effective desalination method for achieving the required saline concentration will be chosen. As stated by Argonne National Laboratory et al. (2004), the treatment technologies available don’t offer compressive options for the removal of all foreign materials in produced water as to realize pure clean water. As a result, oil and gas producers will only opt for that which will result to the desired results at the minimal costs.
The most common process for treating produced water in the United States is reserve osmosis. This method is used for potable and reuse water applications, however, before the membrane-based reverse osmosis was invented, thermal technologies such as the multistage flash distillation and the multi-effect distillation methods were used. However, these methods were used mainly for desalination of seawater for municipal, agriculture, and industrial use in the remote and arid areas with water scarcity for example, in the middle east (Argonne National Laboratory et al. 2004). These methods, however, had the limitation of high energy costs. With reverse osmosis, the specific energy decreases significantly with TDS concentrations.
The advantages of reverse osmosis as compared to thermal alternatives for produced water treatment are readily apparent through the energy and cost requirements. Even though the specific benchmarks involved will vary with literature, the averages as per the Colorado School of Mines (2009) are as shown in Table 2 below. The figures of thermal options have the assumption of an available thermal heat source like a power plant, at minimal costs.
|Treatment technology||References costs (life cycle) $/kgal||Total specific energy consumption kW-h/kgal|
|Membrane processes||Reverse osmosis||2||11-16|
|Thermal processes||Vapor compression||1.90||-30|
|Multi flash distillation||4.4||70-112|
Table 2. Comparison of produced water treatment alternatives (Adopted from ALSPACH 2014)
The salinity, Calcium concentration, and other mineral concentration affects the cost and energy involved in that, the higher the concentration, the more the costs and energy required. Based on this tabular illustration, it is evident that, of the most popular produced water treatment methods, reverse osmosis is the most advantageous.
Abdou M, Carnegie A, Mathews AS, O’keefe M, Raghuraman B, Wei W & Xian C, (2011). Finding value information water, Oilfield review 23, no.1.
ALL (2003). Handbook on Coal Bed Methane Produced Water: Management and Beneficial Use Alternatives, prepared by ALL Consulting for the Ground Water Protection Research Foundation, U.S. Department of Energy, and U.S. Bureau of Land Management, July.
ALSPACH B, (2014). Produced water and salinity management: The desalination frontier, Journal: American Water Works Association, 106, 11, pp. 47-52.
Argonne National Laboratory, Veil JA, Puder MG, Elcock D & Redweik, Jr. RJ, (2004). A White Paper Describing Produced Water from Production of Crude Oil, Natural Gas, and Coal Bed Methane, U.S. Department of Energy National Energy Technology Laboratory Under Contract W-31-109-Eng-38.
Colorado School of Mines, (2009). Technical Assessment of Produced Water Treatment Technologies. (1st ed.). Colorado School of Mines, Golden, Colo.
Jackson RE & Reddy KJ, (2007). Geochemistry of Coalbed Natural Gas (CBNG) Produced Water in Powder River Basin, Wyoming: Salinity and Sodicity, Water Air Soil Pollut. 184:49–61. DOI 10.1007/s11270-007-9398-9
Norvell KL, Harvey KC, Brown DE, DeJoia AJ & Bembenek AJ, (2009). “LAND APPLICATION OF COALBED METHANE PRODUCED WATER: CHANGES IN SOIL CHEMISTRY THROUGH TIME,” Paper was presented at the 2009 National Meeting of the American Society of Mining and Reclamation, Billings, MT, Revitalizing the Environment: Proven Solutions and Innovative Approaches May 30 – June 5, 2009.