Prior to the recent EPA modification of the National Pollutant Discharge Elim-ination System (NPDES) general permit for the eastern portion of the Outer Continental Shelf (OCS) of the Gulf of Mexico, produced water handling options were limited to water injection. Under the March 14, 2001 modification of the permit, treating water for overboard discharge is now a feasible option for cost-conscious oil producers.
The environmental component in upstream production involves complying with water quality standards that are designed to meet appropriate health and safety requirements. In the region of the Gulf of Mexico, east of Louisiana, oil and gas is produced from different geologic strata than the Eocene and Miocene formations that are typical of production horizons offshore Louisiana and Texas. These new sources produce different chemical compositions of water, and as a result, require mechanical and chemical engineers to develop customized water treating systems for Gulf operators.
Handling the produced water within environmental limits requires careful consideration of the following produced water contaminants:
- Increased concentration of dissolved hydro- carbons in the water
- Increased scaling in water due to high concentrations of carbonate reacting to produce solid precipitates.
The presence of hydrogen sulfide (H2S)
The increasing need to treat produced water containing these contaminants prompts different methods of removal. Engineers work toward an accurate and fixed method in the design of a system that will meet the water quality standards by closely reviewing the water characteristics in the initial analysis:
- What is the water rate?
- What's the hydrogen sulfide concentration?
- Is there accessibility to sweet gas for stripping?
- What is the method of disposal for the sour gas?
- Is there hydrogen sulfide in the water, and if so, what is the method of its removal from the water?
- Does the water contain dissolved hydrocarbons?
- What is the scaling tendency of the produced water?
Water treatment options
There are two options for treating produced water containing these materials. The first option involves injecting the produced water (after some treatment) into a depleted reservoir. The second option involves stripping the produced water of contaminants to meet the water quality standards and discharging the treated water overboard. System designs will require specialized equipment in each case. The first option will need water injection treating equipment such as:
- Water surge tank - flashes produced water to achieve low levels of hydrogen sulfide in the water to avoid severe corrosion
- Solid filters - removes solid precipitates that could damage the injection pump and well
- Injection pump - increases pressure to allow injection to occur
- Injection well - repository for produced water.
The second option will need water discharge treating equipment such as:
- Skimmer - separates the free oil and flashes off the hydrogen sulfide from the water
- Flotation cell - separates the small oil droplets and purges the produced water to low hydrogen sulfide concentrations
- Solid filter - eliminates the carbonate (scale) precipitates from the produced water
- Activated carbon filter - lowers toxicity of produced water and removes dissolved hydrocarbons for overboard discharge.
Engineers evaluate how each option fits the design criteria based on the water toxicity, required equipment, and environmental and regulatory requirements. Each method is associated with a number of advantages and disadvantages. The water injection method advantages are:
- More reliable; less downtime
- Allows for a wide range of water rates and compositional analyses
- Reduced possibility of failure to meet the NPDES permit limits
- Operator friendly
- Minimum exposure of personnel to hydrogen sulfide
- Minimum treatment chemical requirements
- Sweet gas not required for stripping
- Lower personnel cost for operation.
The water injection method disadvantages are:
- Finding reservoir for injection
- High capital costs due to drilling injection well
- High cost and down time for repair/work-over of injection well
- High electric (energy) load
- Possibility of hazardous waste associated with the filters
- Possible requirement for second injection well
The water treatment method advantages are:
- Lower capital costs
- Standard industry practice
- Extensive industry experience.
The water treatment method disadvantages are:
- Increased possibility of failure to meet the NPDES permit limits
- Higher exposure to hydrogen sulfide and dissolved hydrocarbons, which leads to possible problems with NPDES produced water toxicity limits
- More deck space required
- Increased operator interface
- Requires availability of sweet gas to remove hydrogen sulfide from water
- Higher treatment chemical requirements
- Difficult to accurately design the process without water rates and analyses
- Requires redundant equipment to minimize down time while changing filters
- Possibility of hazardous waste associated with the filters
- Increased difficulty in meeting SOx/NOx requirements for air emissions due to the burning of acid gas (the by-product of stripping the hydrogen sulfide from the water).
Regulatory requirements are a significant determinant in the process of selecting a water treating system. The MMS (US Minerals Management Service) requires that if operators dispose of produced water that potentially contains hydrogen sulfide by a means other than subsurface injection, the operator must submit an analysis of the anticipated hydrogen sulfide content of the water at the final treatment vessel and at the discharge point.
A water analysis is necessary to be able to accurately predict the hydrogen sulfide content of the produced water. If hydrogen sulfide is present, the MMS district supervisor may require that the produced water be treated for removal of the hydrogen sulfide, although there are no numeric limitations on the hydrogen sulfide content stated in 30 CFR 250.417 (q)(12), Water Disposal.
The EPA overboard discharge (NPDES) permit does not directly limit hydrogen sulfide in overboard discharge of produced water, however, produced water discharges have limitations in the parameters of oil and grease, and toxicity. Therefore, the hydrogen sulfide is indirectly regulated by what direct effects it would have on the oil and grease levels and the toxicity levels of the samples tested. The oil & grease limitations are:
- 42 mg/L (ppm) daily maximum
- 29 mg/L (ppm) monthly average.
The toxicity limits are 7-Day NOEC (no observable effect concentration). Labs generally have not seen any effects from hydrogen sulfide in produced water samples because the gasses usually flash off during the sample taking or during the decanting and dilution of the sample contents in the lab.
Pragmatically, the hydrogen sulfide has a negligible effect on the toxicity of the sample.
The hydrogen sulfide may have some effect on the oil and grease test results, but only if the compound bonds with something in the produced water and does not flash off as described above. Typically, only a minor effect exists with regard to the oil and grease results.
The hydrogen sulfide may have some indirect effect on the oil and grease test results, but is much more likely to have an indirect effect on the toxicity test results. Soluble hydrocarbons including BTEX (benzene, toluene, ethylbenzene, xylene) are on the EPA priority pollutant list and can only be discharged overboard under the NPDES permit in "trace amounts." The term trace amounts is defined in the NPDES permit as follows: "Trace Amounts means that if materials added down hole as well treatment, completion, or workover fluids do not contain priority pollutants then the discharge is assumed not to contain priority pollutants, except possibly in trace amounts."
In other words, if these chemicals occur naturally from the producing formation and are not added downhole by the operator, the chemical concentration is considered to be in trace amounts and can be discharged overboard. However, if these chemicals occur in significant amounts in the produced water, there is a possibility that the contaminants can have an effect on the produced water toxicity as well as the oil and grease levels.
The recent changes to the general permit for NPDES for the eastern Gulf of Mexico can be summarized as follows:
- Modified the produced water toxicity limit multiplier from 100 to 10, which decreases the LC50 limit by a factor of 10 and increases the possibility of meeting the LC50 limits
- Allows use of a diffuser, seawater addition, or multiple discharge ports on produced water overboard discharge to increase mixing, which decreases the toxicity-testing limit
- Allows the operator to conduct Cormix (Cornell Mixing Zone Expert System) modeling for the produced water toxicity limit rather than EPA model
- Allows a new option to pursue an individual permit in lieu of a general permit, with coverage under the expired 1986 permit being effective until final determination is made on the individual permit application (chemically treated seawater was added to the miscellaneous discharges, without the restriction of it being used for hydrostatic testing of new pipelines)
- Shorter notice to drills (NTD) notification period (from a 60-day notice prior to activity to a 14-day notice after activity commences). EPA accepted and used a higher current speed (15 cm/sec vs. 5 cm/sec) for the produced water toxicity limitation for deepwater, defined as 200 meters, which reduces the LC50 limit by almost 50% compared to the shallow water limit.
The recent EPA modifications, in essence, allow produced water to be treated and discharged overboard with decreased LC50 limits and the permissible use of a diffuser. Previously, the probability of discharging the produced water overboard within toxicity limits was extremely small.
Overboard water discharge in the eastern Gulf of Mexico should be addressed independent of areas, which fall under existing general NPDES permits. Overboard discharge of produced water is now an option that must be fully investigated for each location, specifically balancing the long-term and short-term costs and effects.
The combination of problems presented for proper treatment of produced water cannot be minimized. Hydrogen sulfide and soluble hydrocarbons make any design problematic for the long term. It is necessary to diligently monitor the produced water streams and test for toxicity as well as oil and grease content regularly. In addition, personnel must be trained in safe handling of these materials.
Treating produced water continues to provide challenges as the Gulf of Mexico is further developed. The steady increase in activity forces the industry to balance the long-term costs with the short-term expediency.