Passive seismic makes sense for 4D reservoir monitoring

Stephen Wilson, Rob Jones, Will Wason, Daniel Raymer and Paul Jaques of Vetco Gray, Cornwall, UK (formerly part of ABB) describe how passive seismic monitoring technology is making its way into the mainstream as a value proposition for the management of hydrocarbon resources. The use of 4D seismic as a mainstream technology in the management of hydrocarbon reservoirs is now established. In contrast to the traditional perception of seismic technology as an exploration tool, the value of 4D seismic sits securely on the production side of oilfield technology. This shift in emphasis within the seismic industry to encompass both production and exploration work has recently taken on a higher profile as a result of the difficulty of increasing reserves purely by exploration. New production technology now offers an alternative path to increasing booked reserves. The widening scope of seismic applications and the increasing number of reservoir geophysicists is helping to bring forward another seismic technology capable of greatly improving our understanding of reservoir dynamics. That technology is passive seismic monitoring. During the past few years the implementation of passive seismic monitoring as a mainstream technology for the management of hydrocarbon resources has been gathering pace. Recent permanent passive seismic studies in Oman have shown the capabilities of this technology to provide information upon which reservoir management decisions can be made (Jones et al, 2004). Knowledge of the existence and capabilities of the technology within our industry is reaching a critical mass and the technological barriers to its uptake are disappearing. Perhaps the most critical of these barriers concerns the ability to monitor microseismic activity from within active wells during production or injection. Recent developments in downhole tool technology allow the deployment of downhole seismic sensors capable of a 30-40 db improvement in signal performance when compared with previous technologies (Jaques et al., 2003). In addition to improvements in tool technology, software applications capable of delivering automatic microseismic locations to the client’s desktop in real-time are now available (Jones and Wason, 2004). The advent of 4D has improved our ability to observe reservoir performance and make timely decisions about reservoir operations. The deployment of permanent ocean bottom systems provides scope for improving the speed with which reservoir management decisions can be made by reducing turnaround time. Passive seismic monitoring further supports this improved decision-making capability by delivering real-time information about the reservoir to the desktop within minutes. In terms of instrumentation, permanent downhole seismic sensors represent the cornerstone for the implementation of full-field continuous passive seismic monitoring. The prospect of permanent downhole seismic sensors for use during 4D studies offers the prospect of accurate well ties, wavelet characterisation and VSP on demand. The combined value proposition for passive seismic monitoring and 4D seismic using downhole instrumentation may now be sufficient to drive the deployment of these systems. Figure 1 illustrates this virtuous circle of 4D seismic promoting the take up of microseismic technology which in turn helps 4D to better resolve reservoir change. All of which is driven by the value proposition of new technology as a method of improving recovery, increasing NPV and booking reserves.