Measuring oil content in wastes is nothing new to the petrochemical industry. Whether it’s produced water from onshore or offshore sites, effluents from refineriers or drill cuttings and drilling mud, limits on hydrocarbon levels need to be met. With the increase of hydraulic fracturing in the U.S., more public attention has been focused on the need for regulations and limits. In addition to increasing where tests are required, there’s also a need for more flexible testing equipment that can handle different oil concentration ranges as well as liquids, slurries and solids. Infrared analysis has been used by oil producers for over 40 years for quantitative measurements of oil in water and oil in soil. Improvements in instrumentation now allow for systems that are compact, affordable and rugged enough for oil production sites.
Infrared analysis of oil and grease in water or soil
EPA Methods 418.1 and 413.2 were widely used infrared (IR) methods for measuring oil in water and soil prior to the banning of Freon. IR continues to be widely used today with replacement solvents such as hexane, perchloroethylene and S-316.
EPA Method 1664 using hexane as the extraction solvent and gravimetric analysis is now the standard method in the U.S., replacing Freon methods. This gravimetric procedure requires a skilled laboratory technician and is a time- and equipment-intensive process. The ASTM passed an alternate method, D7066, using a Freon replacement solvent (S-316) and simplified IR analysis. For those that perfer to use the same solvent as the EPA method, there are IR analyzers available that use hexane extraction.
The measurement by infrared absorption makes use of the fact that hydrocarbons, such as fats, oil and grease, can be extracted from water or soil through the use of an appropriate solvent. The extracted hydrocarbons absorb IR energy at a common IR wavelength, and the amount of energy absorbed is proportional to the concentration of the oil/grease in the solvent. This can be directly calibrated or converted to the amount of oil in the original sample. Because only one wavelength is required for the analysis (3.4 microns or 2930 cm-1) a full spectrum FTIR (Fourier transform infrared) spectrometer is more than needed for the measurement. For measuring oil content, IR analyzers using single-wavelength fixed filters allow for compact, portable systems for on-site measurements in rugged environments. Improvements in components from circuitry to sources and detectors have lowered detection limits, as well as expanded the customer interface options making oil content testing easier and more versatile.
Filter-based IR oil in water/soil analyzers, as shown in Figure 1, have been used in the oil industry for more than 40 years, predominantly on offshore rigs for produced water testing where equipment needs to be reliable. A test can be done on-site in less than 15 minutes without incuring the cost and delay of off-site laboratory analysis. An added benefit is this simplified technology does not require a skilled laboratory technician to do the analysis.
Hydraulic fracturing wastes
At a hydraulic fracturing site, water and wastewater management practices are driven by both cost and regulations. If the cost of fresh water is low, there’s little impetuous to reclaim the flowback or produced water for reuse. In arid areas where fresh water costs are high and there are concerns of depleting aquifers, water reuse is becoming an economic option.
Wastewater management options include removal to an off-site treatment facility, evaporation ponds, reuse for hydrofracking and treatment for surface discharge. Each option has maximum acceptable levels of free or dissolved oil making oil removal the first step in wastewater handling. The wastewater is tested after the oil removal to ensure the required levels have been attained.
Drill cutting and mud wastes
Solid wastes from drilling are typiclly handled by overboard disposal, reinjection, land application, pit burial, off-site treatment or reuse for road surfaces or filler in concrete. Again, each option usually requires testing oil contaminant concentrations. Checking initial oil levels or verifying the efficiency of the oil removal system can be done on-site with the same IR analyzer the measures oil in wastewater. The levels for cuttings are typically in the 0.5 to 10% range, well above the ppm requirements for oil in water.
Going from ppm levels up to percent levels with the same sampling system can be an analysis challenge. A horizontal ATR (attenuated total reflection) sample stage using hexane, pentane or cyclohexane as the extraction solvent can be used for oil in water levels from 0.3 to 5000 ppm range. The hydrocarbon solvent is evaporated off and the IR absorbance due to the residual oil film is measured. The same ATR sample stage can be used for the higher 0.5 to 10% range found in drill cuttings or muds. The sample is diluted by using a solvent that does not have an IR absorbance at the hydrocarbon wavelength, such as tetrachloroethylene or S-316 (dimer/trimer of chlorotrifluorethylene), and therefore doesn’t require evaporation. This method can also be used for TPH in soil if a spill or pond leak occurs to determine the extent of contamination.
Offshore produced water
Any effluent that’s dumped overboard from an offshore oil rig is tested for it’s oil content. Each country has different regulations, and for the U.S. the average must be under 29 ppm. Most operators perform several tests a day to ensure that their pretreatment systems are operating correctly. Newer IR systems not only have lower detection limits, but also allow for expanded ranges into the percent levels found in drilling wastes using the ATR sampling method described above.
A quick and simple on-site infrared oil and grease measurement gives the petrochemical industry a useful tool for:
- Optimizing frac water treatment procedures.
- Maximizing evaporation pond efficiency.
- Meeting off-site treatment facilities requirements.
- Complying with drill cuttings disposal regulations.
- Ensuring that produced water is under the regulatory limit.
- Assessing soil contamination.
- Oil content measurements in under 15 minuteseliminating the wait for off-site results.
Expanded capabilities now give operators the ability to cover the range of oil and grease measurements on-site using one portable instrument with enhanced field-proven technology.