Seismic monitoring of an old underground blowout – 20 years later

When Saga Petroleum drilled a deep exploration well in the southern part of the North Sea in 1989, it encountered a high pressure zone, and the well developed into an underground blowout that lasted for 326 days. A relief well was spudded 11 days after the blowout, and the underground blowout was successfully killed by pumping drilling mud of high density, 2.25 g/cc, into the blowing well. During this period, Saga Petroleum decided to use shallow seismic data to monitor the underground blowout. Ten monitor surveys were acquired during and after the blowout, and this campaign is probably the first successful time-lapse seismic acquisition done in the Norwegian part of the North Sea. In 2009 some of these 2D lines were repeated, using approximately the same acquisition parameters as in 1989 and 1990. Comparing the seismic data from 2009 with the data from 1990 and 1989, most of the gas appears still to be in the same layers as in 1990. There has been some lateral migration of gas, and only a minor amount of vertical gas migration.