Meeting stringent slop water discharge regulations

For its 16-well operation, an operator sought to meet stringent slop water discharge regulations offshore Mozambique and Tanzania, where the discharge standard required that slop water not exceed a contamination level of 15 ppm. Achieving this standard would be difficult using conventional onsite treatment systems, which can be operationally hindered by solids content above 2%. Meeting treatment standards typically requires transporting slop water to shore for treatment before it can be discharged, reinjected, or disposed. The cost of shipping slop water to shore can exceed USD 1,500 per cubic meter.

Deploying ENVIROUNIT treatment system for waste minimization

M-I SWACO recommended the ENVIROUNIT offshore slop water treatment system to meet goals for waste minimization, fluid reuse, and cost reduction. The treatment system allows fluids with up to 10% solids and 35% oil to be treated to less than 15 ppm while recycling expensive oil- and synthetic-base drilling fluid for direct reuse. Despite varying amounts of drilling fluid in slop water, optimal recovery is ideal to save expensive synthetic-base and oil-base mud and reduce waste. With the ENVIROUNIT system, slop water oil content treated to less than 15 ppm is diverted to a storage tank before being released overboard after regulator-approved water testing, and recovered drilling fluid is routed back to the active mud system. Solids and waste removed from both waste streams are redirected to a cuttings box for onshore disposal.

After using a diluted stock sample for reference, the Horiba method reads a 20-mL sample of the water and solvent mixture to determine oil content. Sufficiently low readings will allow operators to discharge water. Water presumed not to have contacted oil is discharged without treatment, and all water that courses through the system is stored in a tank for testing before discharge.

Exceeding discharge requirements for hydrocarbon content

The ENVIRONUNIT system processed a total of 7,995-m³ [2,112,285-galUS] slop water offshore Mozambique and Tanzania. Throughout the 16-well project, the highest oil content of treated water was 14.4 ppm, the lowest was 1.60 ppm, and the average was 9.66 ppm. In each well, all water was tested against regulatory compliance and documented before discharge.