Microseismicity - so what?

Abstract

Phillips licence 018 group is testing the use of microseismic activity to understand compaction and subsidence. Monitoring in 1997 in the Cl 1a well bore showed activity rates of 5 events per hour. If part of this activity is in association with waterflood fronts, then such a high event rate, combined with good event positioning, shows the potential to define the waterflood fronts inthe field. At least 16 oil and gas fields have been described with microseismicity which may be associated with hydrocarbon production and waterflooding, so this phenomenon could be more widespread than expected. At Ekofisk, positioning the microseismic activity has the strong added benefit that the faults interpreted in the crestal area are obscured from normal seismic view by gas in the overburden. Microseismic events have been monitored at Ekofisk with downhole geophones on a wireline in four separate short-term periods over the last 10 years. In all cases, rates of activity of between 5 and 15 events an hour were observed. Subsidence of the sea bottom has been continuing throughout that period at the average rate of c. 37 cm per year. It is significant that there was no microseismicity in the upper overburden because it indicated the absence there of seismically active faults. The 1997 work was the first to provide accurate event positioning. This was due to two elements: the tool was largely resonance free and the analysis, performed by the team from Keele University, used more sophisticated analysis tools.