Hydrocarbon production reduces natural methane seeps in the Santa Barbara channel

Abstract

Abundant natural hydrocarbon seepage in the Santa Barbara Channel includes large amounts of gas, mainly methane. Numerous seeps are focused along a faulted anticlinal axis where in 1982 ARCO placed two large (900 m2 each) steel gas capture tents on the seabed adjacent to Platform Holly. The seep rate into the tent at high tide is about 5 percent less compared with low tide (2 m tidal range). An earthquake in 2013 (M = 4.8) did not affect seepage rates into the steel tents, indicating that gas was not released by fault dilation. The CO2 and H2S content of the captured gas are anomalously low compared to typical Holly gas due to bacterial and sea water-gas interaction at the seep tent. Platform Holly production had an overall effect of reducing gas seepage into the tents over time. Wells beneath the tents at 1-km depth had a much greater influence on the seepage rate than more distal wells. The nearby wells had a direct effect on seepage rate when they were put on production (seepage decreased) versus shut in (seepage increased). The response time was within hours, indicating a direct hydraulic connection between the reservoir at 1 km subsea and the seabed. Permeability of the approximate 1-km fault pathway between the reservoir and the seep tent is calculated to be about 3 × 10−14 m2 (30 millidarcys). Gradually, the seep rate declined from peak flow in 1990 (4.5 × 104 m3 dy-1) to less than 7.1 × 103 m3 dy−1 in 2013. In June of 2013, a horizontal well was completed high on the structural anticline between Platform Holly and directly beneath the seep tents. Within three weeks of turning on this new well, seepage into the tents completely ceased, ending more than 30 years of natural methane seepage captured by the tents.