What is TOC?
The Total Organic Carbon (TOC) content of water is the total mass bound by organic carbon. This can include carbohydrates, both filterable and dissolved, residual hydrocarbons, and bacterial biomass. Flowback water from a typical gel-based frac fluid may contain as little as 1,000 mg/L TOC to as much as 3,000 mg/L TOC.* By contrast, TOC levels in municipal waste water are typically less than 300 mg/L TOC. There is a substantially higher TOC loading in flowback water from gel-based frac fluids than found in spent slickwater flowback. TOC contributions from living microorganisms can be determined from the ATP (adenosine triphosphate) measurement where 1mg of ATP has been found equivalent to about 250 mg of carbon in living microbiota. The EIA (Energy Information Administration) has reported that in an assessment of 137 shale plays in 41 countries outside the US, that the amount of organic carbon measured in a petroleum formation is 20,000 mg/L minimum. However, most of this TOC is not present in produced water to be treated for frac re-use.
How is it measured?
There exist several analytical methods for measuring TOC in water. The most common method for analyzing high loads of TOC in oilfield water is the High Temperature Combustion Method 5310-B, which is described in Standard Methods for the Examination of Water & Wastewater (2005).
Why is TOC important and what do you risk by ignoring it?
The portion of TOC present in the form of Assimilable Organic Carbon (AOC) is the primary food source of bacteria. Oilfield bacteria can be detrimental to the quality and performance of frac fluids, especially if the frac fluid is guar or derivatized-guar based. Sulfate-reducing bacteria (SRB’s) contribute to the formation of hydrogen sulfide (H2S) and are a source for microbially induced corrosion (mic). Left untreated where high levels of TOC are present, the regrowth potential for bacteria is great.
How then, can you treat it?
Methods in use in oilfield water treatments today to reduce TOC include chemical and electrochemical oxidation, aeration-enhanced aerobic digestion and activated-carbon filtration. The most commonly used methods are chemical and electrochemical oxidation, and aeration-enhanced aerobic digestion. Other potential methodologies, but perhaps more costly, include reverse osmosis, activated-carbon filtration, and distillation.