The first of a three-part series, this article addresses the challenges of managing water-soluble organics (WSOs) in offshore produced water. It provides a brief discussion of the chemistry, along with strategies and technologies that can be used to manage WSO levels in produced water, thus facilitating compliance with regulatory overboard water-quality guidelines. Part 2 will describe process design and strategies for the management of WSOs, and Part 3 will describe treatment technologies for WSO removal.
Overboard produced water contains both dispersed oil and water-soluble organics (WSOs), which are increasingly challenging to manage under tightening regulations. This article explains WSO behavior from a process-engineering perspective, including their chemistry, phase equilibrium, and sensitivity to pH changes driven by temperature and pressure reductions in the production train. It also briefly reviews treatment technologies and emphasizes that effective WSO management depends on sound process design, appropriate water-treatment selection, and careful handling of recycled oily water streams.
Produced water from offshore oil-processing facilities worldwide is treated for overboard disposal in accordance with applicable regulatory specifications in their respective locations. In the Gulf of Mexico (GOM), overboard disposal of produced water must be authorized by a permit issued under the National Pollutant Discharge Elimination System program and must meet guidelines detailed in the permit (NPDES General Permit, GMG290000). These guidelines include limits on oil and grease levels, the absence of a visible sheen on the receiving waters and produced-water toxicity requirements.
Produced-water chemistry differs among reservoirs and can vary over the life of a field, even within the same reservoir. Overboard produced water contains both dispersed and WSOs. In some cases, WSO levels are very high and create challenges in meeting regulatory guidelines.
This discussion focuses on oil and grease in treated produced water. The regulatory procedure for measuring total oil and grease (TOG) is US EPA Method 1664. Dispersed and WSOs are present naturally in produced water and can also be introduced through produced-fluid processing or using production chemicals in facilities and well-completion and remediation operations.
Production chemicals (mostly organics) are used in facilities in oil/water separation processes (e.g., demulsifiers), flow assurance (management of hydrates, waxes), and asset integrity (corrosion inhibitors). Different production chemicals will partition into the oil and water phases and may contribute to TOG as measured by US EPA 1664. It is also important to account for the contribution of all production chemicals to both dispersed and water-soluble organics. These contributions should be considered when developing an overall strategy for managing dispersed oil and WSOs in produced water.
The difference between dissolved and dispersed organics is important in the design and operation of the water-treating system. Technologies required for dispersed oil removal are different from the removal of WSOs, although some of the WSOs will be removed indirectly from the water by using the different produced-water treatment technologies. Contaminant material is considered dissolved if it passes through a 0.45-micron filter. Particulate solids and/or dispersed oil are retained on the filter.This is an arbitrary criterion but universally accepted in the E&P industry (Walsh, Produced Water, Vols. 1 and 2, 2019).
Technologies for the removal of WSOs are predominantly non-regeneratable processes, considerably more expensive than conventional produced-water treatment technologies, heavy, and require large footprint. The non-generatable technologies typically generate a large mass of material that cannot be discharged overboard and must be removed from the platform or FPSOs for onshore management and disposal. Most of these technologies are also not specific for WSOs removal. They remove both the WSOs and dispersed oil. The challenge is to minimize the use of technologies for removal of WSOs but still control levels of WSOs in certain facilities to meet overboard water quality specifications.
WSOs Constituents in Produced Water
In the US-based oil and gas industry, TOG is defined in terms of the US EPA 1664 analytical method that includes both the dispersed oil and WSOs. Total petroleum hydrocarbon (TPH) is the measurement of only the dispersed oil part in the produced water. Regulatory requirements in the US require reporting of TOG in overboard produced-water discharges. By comparison, in the North Sea, under the OSPAR (Oslo/Paris Convention for the Protection of the Marine Environment of the Northeast Atlantic) convention, reporting requirements for produced-water discharges focus on dispersed oil concentration and not specifically on TOG.
The table below lists seven classes of compounds typically identified as WSOs.
| WSO Class | Characteristics |
| Phenols: C6 to C9 | Highly water soluble and somewhat volatile Typically do not extract into hexane and do not contribute to WSO. |
| Phenols: C9+ | Relatively nonvolatile and less water soluble Significant partitioning into dispersed oil phase; soluble in extraction solvent hexane; these compounds when present typically do contribute to WSOs |
| BTEX | Benzene, toluene, ethylbenzene, and xylene. Benzene has high solubility in water but also in extraction solvent; however, benzene is volatile and therefore does not typically contribute to WSOs.TEX is less volatile and does contribute to WSOs. |
| Acids: C1 to C4 | These carboxylic acids are dissolved organics. They have high water solubility and high volatility. Even at pH2, have low solubility in hexane Phase partitioning (water/hydrocarbon) strongly dependent on pH Because of their volatility, they are not normally detected in WSOs. |
| Acids: C5+ | Somewhat water soluble depending on pH and carbon number Extract into hexane at low pH Phase partitioning (water/hydrocarbon) strongly dependent on pH High adsorption tendency onto silica gel Main contributor to WSOs in produced water with high WSOs |
| Naphthenic acids (NA) | Typically complex, aromatic, multifunctional, have a carboxylic group or several Typically soluble in crude oil, where they are in the protonated form. May have some solubility in water at mildly acidic pH values (pH 4 to 7)Could contribute to WSOs |
| Polycyclic aromatic hydrocarbons (PAH) | Low water solubility Moderate solubility in hexane (but not completely soluble in hexane). Will contribute to WSOs Most-abundant components are the NPD compounds They are defined here in terms of their abundance, solubility, and volatility. |
PAH – Polyaromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene.
WSOs include aromatics (BTEX and PAHs), acids (fatty acids, naphthenic acids), and phenols.
Chemical and Phase Equilibria
The quantity of organics in a sample measured as TPH or TOG is the result of a complex set of chemicals (acid dissociation) and phase equilibria (partitioning across phases).For organic acids’ contribution to WSOs, chemical equilibrium plays a more significant role than the phase equilibria.This is not the case for other classes of compounds.
In developing a treatment strategy for removing WSOs from produced water, the chemical and phase equilibria involved must be considered These treatment options differ significantly in their approach (chemical vs. mechanical or some combination, regeneratable and non-regeneratable process). A treatment that works well for BTEX may not work at all for acids.Thus, it is important to understand what class of compounds are present in produced water. The chemical and phase equilibria tell us why certain treatment options are effective for one class of compounds but not another (Walsh, Produced Water, Vols. 1 and 2, 2019; SPE 170806).
Chemical and phase equilibria are discussed below.
The water phase has various components dissolved in it including ions (inorganic salts), acids, phenols, and dissolved organics. Dispersed oil has various components dissolved in it as well. Dispersed oil does not have ions, but it does have phenols, protonated acids, and various organics. Components in the free oil and the dispersed oil phase are assumed to be in chemical and phase equilibrium with the components in the water phase (Fig. 1).
The important chemical equilibrium is shown in the middle of the figure where the protonated acid (HA) is shown on the left-hand side, and the dissociation is shown on the right-hand side of the equilibria. The dissociated acid is comprised of the anion (A-1) and the proton H+. HA partitions between the oil and the water phase. This partitioning is not equal. If the acid is a low-molecular- weight acid such as acetic acid, the partitioning strongly favors the water phase. Due to the very limited partitioning of these small-molecular-weight acids in an oil or hydrocarbon phase, their contribution to WSOs is minimal. If the acid is a higher molecular weight such as decanoic acid, the partitioning favors the oil phase. The ratio of the concentration of the organic acid in the oil phase to that of its concentration in the water phase is defined as the partition coefficient.The ions (right-hand side) do not partition into the oil or gas phase. They are only soluble in the water phase and, therefore, do not contribute to WSOs.
Partition coefficient is a measure of how a compound distributes between two immiscible phases, such as oil and water, and in this case the two phases are produced water and dispersed oil.It implies that each compound in each of the seven classes of organics discussed above will partition between the produced water and the dispersed oil phase and attain an equilibrium (has a specific partition coefficient). The partition coefficient is often treated as a constant under specific conditions at equilibrium.Based on this very important concept of phase equilibrium, the removal of dispersed oil can result in lowering WSOs concentration in produced water.
The volumetric amount of dispersed oil depends on external factors in the facilities—processes such as the extent of shearing, the separation efficiency of water-treating equipment, and the recycling of recovered oily water phase back to the process system and ultimately back to the produced-water system. The water-treatment system design should be designed to maximize the system efficiency for the removal of dispersed oil.
Chemical and phase equilibria are relevant in all stages of production process, in the produced water sampling, and analysis process. During the production process, as the fluids emerge from the reservoir, they experience shear and temperature reduction. Shear causes mixing of phases, drop size reduction, and an increase in the surface area of the droplets. New equilibria are rather quickly established during the fluid flow in the flowlines and separators. These new equilibria result in volatile compounds going to the gas phase, which increases the pH of the water phase (due to carbon dioxide equilibria). An increase in pH deprotonates the organic acids and pushes some of them out of the oil phase and into the water phase. Thus, the concentration of the organic acids dissolved in produced water will depend on the flashing of CO2 and the dissociation constants of the acids.
Similar phenomena occur during the sampling and analysis phase. During the sampling phase, mineral acid is added to bring the pH of the sample to a value of 2 or less. This is to standardize the effect of CO2 flashing and to preserve the sample and dissolve any precipitated minerals. But it will still result in a dependence between the concentration of the acid in the water and its concentration in the oil phase of the sample.
The analysis phase of the sample will result in additional modifications in the chemical and phase equilibria of the sample.For example, when hexane is added as an extractant, it increases the volume of the oil/hydrocarbon phase. This pushes oil-soluble compounds into the oil phase. The technical name for these compounds is hexane extractable material (HEM). The oil and water phases are then separated. The water sample, after hexane extraction, is no longer required and is typically disposed of. The oil phase containing the hexane is then heated so that the hexane can evaporate. What is left is HEM.
In Fig. 2, McFarlane et. al. give an example of the combined effect of chemical and phase equilibria as a function of pH for a particular sample of naphthenic acids. The y-axis is the total acid concentration in a produced-water sample after hexane extraction. This concentration is the sum of the protonated acid (HA) and the ionized (dissociated) acid (A-1). The y-axis does not include the concentration of organic acid that has partitioned into the oil (hexane) phase.
There are two important considerations. The first is the effect of adding mineral acid to a protocol sample of produced water as required by EPA 1664. (A protocol water sample is a laboratory-prepared quality-control sample). Mineral acid is a source of protons. When mineral acid is added, pH decreases to a value of 2 or less. This protonates a large fraction of organic acids. The protonated form is more soluble in hexane than the ionized form, so the protonated acid partitions into the hexane. As a result, the concentration of organic acid in the water phase decreases (red line).
The second consideration is the effect of adding a mineral acid in a facility to reduce the concentration of organic acids in the water phase. Just as in EPA 1664 protocol analysis, the concentration of organic acid in the water goes down as the protonated acid partitions into the oil phase. This is an effective strategy for removing WSOs from produced water, but obviously it only works if the WSOs are mostly composed of organic acids. Thus, recognizing the type of WSOs present in produced water is important.
Fig. 2 shows the effect of pH on the concentration of organic acid in the water phase. This is analogous to the chemical and phase equilibria involved in the EPA 1664 test. Mineral acid is added to adjust the pH during sampling, and hexane is added to remove the organics during testing (via liquid/liquid extraction). Note that at low pH most of the acids are in the oil phase, and at high pH most are in the water phase.
Conclusions
- WSOs and dispersed organics occur naturally in produced water. Chemicals used for flow assurance, oil/water separation, asset integrity, and in well completion and remediation operations also contribute to both the dispersed and WSO’s.
- EPA 1664 is the regulatory procedure for the measurement of WSOs in the GOM.
- Seven classes of organic compounds can contribute to WSOs: phenols (C6–C9); phenols (C9+); BTEX; acids (C1–C4); acids (C5+); napthenic acids; and PAHs.
- Chemical and phase equilibrium should be carefully considered in developing treatment strategies for removing WSOs. For organic acids, chemical equilibrium plays a more significant role than the phase equilibrium. This is not the case for other classes of organic compounds.
- Based on the phase equilibrium considerations, removal of dispersed oil also results in the removal of WSOs.
For Further Reading
Walsh, J.M. (2019). Produced Water, Volume 1: Fundamentals, Water Chemistry, Emulsions, Chemical Treatment. Petro Water Technology LLC.
Walsh, J. M. (2019). Produced Water, Volume 2: Equipment, Process Configuration, Applications. Petro Water Technology.
SPE 170806 Understanding Water-Soluble Organics in Upstream Production Systems by J.M. Walsh, J. Vanjo-Carnell, and J. Hugonin.
EPA-821-R-98-002 (1999) US EPA Method 1664, Revision A: N-Hexane Extractable Material and Silica Gel Treated N-Hexane Extractable Material by Extraction and Gravimetry by US EPA, National Service Center for Environmental Publications.
Mastering Produced Water Management in Deepwater GOM–Part 2 by K.M. Bansal and J.M. Walsh, SPE Oil and Gas Facilities, 6 March 2025.
SPE 159713 Produced-Water-Treating Systems: Comparison Between North Sea and Deepwater Gulf of Mexico by J.M. Walsh and W.J. Georgie.
