Monitoring of an Uncontrolled Gas Flow by Sea Bottom Seismic Instruments – A North Sea Case Study

While drilling a deep target Saga Petroleum had an uncontrolled situation were gas from the target were injected into sandlayers above through a leak in the casing. Over a period of approximately two weeks seismic data were registered from a small array of geophones located on the sea-bottom close to the platform.<br>Several periods of high underground seismic activity were identified. These are most likely induced small earthquakes. With such a sea bottom seismic it is possible to determine the direction to the underground acoustic source, but one has only limited control on the distance from the receivers, and thereby also limited control on the source depth. It is, however, possible to discriminate between activity close to the surface and genuine underground seismic events. The observed events are not located to the well path. Almost all of the major events taking place in the recording period are located west of a line going South-southeast to North-¬northwest. There is possibly a tendency for the events to move outward with time. The events could basically be explained as being caused by the moving front of hydrocarbons into shallow sandlayers. Systematically repeated phenomena not hitherto understood are, however, also observed.<br>