Integrated Nitrogen Rejection Facility Produces Fuel and Recovers NGLs

Shortly after the discovery of the Block 31 field in 1945, ARC0 Oil and Gas Company, then Atlantic Refining Company, started searching for ways to improve the ultimate recovery from the Devonian formation. Laboratory research showed that natural gas would become miscible with the crude at 3500 psi. Therefore, injection of the produced gas began in 1949. A processing plant was built in 1957 to extract liquids from the gas before reinjection. Eventually, the produced gas volume was not adequate to maintain miscibility pressure and additional gas was purchased to make up reservoir voidage. Further laboratory research showed that flue gas injection would maintain miscibility at a higher injection pressure. ARC0 began flue gas injection in the Block 31 field in 1966 at a rate of 40 to 50 MMSCFD. In 1980, a favorable decision was handed down from the U.S. Department of Energy relative to the recoupment of investments made in tertiary recovery projects. This provided further incentive for an ongoing development program. Since 1966, continuous flue gas injection has sustained the miscible flood and, being predominantly nitrogen, caused a constant increase in nitrogen content of the produced gas. The inlet gas to the processing plant currently contains from 30 to 45% nitrogen. The processing plant consists of three lean oil absorption trains for propane and heavier recovery. Residue gas from the processing plant, which is used for fuel, has declined in BTU content to below 800 BTU/SCF. All the fuel users were designed for 950-1000 BTU fuel. The fuel had to be "spiked" with a high ethane stream to maintain a 950 BTU fuel stream. This blending was satisfactory for several years, but variations in nitrogen content and momentary imbalances in this blending cause operational problems in the gas-engines, power boilers, and flue gas generators. Three alternatives were evaluated for solving the fuel problem: purchasing outside fuel, retrofitting existing equipment for low BTU fuel or building a nitrogen rejection facility. Nitrogen rejection was determined to be the preferred solution from an economic and reliability standpoint. The nitrogen rejection facility (NRF), built in 1983, handles 94 MMSCFD of the highest BTU (lowest N,) gas. This facility was designed to produce up to 38 MMSCFD of fuel gas while recovering 300,000 gal/day of NGL's.