HR: 16:50h
AN: S34A-06 INVITED     [Abstracts]
TI: Microseismic Monitoring and Seismic Anisotropy in Oilfields
AU: * Kendall, J
EM: kendall@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AU: Al-Anboori, A
EM: anboori@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AU: Caddick, J
EM: jen@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AU: Teanby, N
EM: n.teanby@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AB: Passive seismic monitoring in oil fields provides insights into spatial and temporal variations in the stress field. Microseismic activity can be induced by production, injection and regional tectonic processes. Such earthquake activity can delineate faults, identify reservoir compartmentalization, and monitor the progress of injection fronts. Microseismic data can be also used to estimate seismic anisotropy in hydrocarbon settings. Anisotropy is an indicator of order within a medium (e.g., aligned cracks, crystals or layering) and as such offers information about the stress field and dynamic processes within the reservoir. Here we present evidence of seismic anisotropy based on observations of shear-wave splitting in two quite different oilfields, one in the North Sea and one in Oman. In each experiment data were acquired by downhole arrays of three-component sensors. The datasets are very large as 10-20 events are normally recorded each day and the experiments can last many weeks (North Sea) to many months (Oman). We have therefore developed automated techniques to analyze shear-wave splitting in large datasets. The magnitude of the anisotropy varies considerably throughout the reservoirs, ranging from near isotropy to almost 10% anisotropy. Variations in anisotropy magnitude are often sharp in transition, being controlled by faulting or lithology. In both fields we find the dominant orientation of the fast shear-wave parallels known fracture trends and the maximum compressive stress directions. More detailed investigation of spatial variations in the anisotropy reveal that it is also controlled by the intrinsic crystalline anisotropy of the rock. Collectively, our results suggest that the anisotropy is controlled by both rock-type and fault-related fracturing. We also consider the frequency-dependent nature of shear-wave splitting in an effort to better constrain the length-scales of fractures or microcracks responsible for the anisotropy. We estimate the fractures in the competent carbonate reservoir rock of the Oman field to be on length scales of meters in size, whilst in the less competent siltstones on the North Sea field, we estimate the cracks to be cm or smaller in length scale. These results are consistent with geological information for these reservoirs and suggest that passive seismic monitoring can be used to study the fracture character of reservoirs.
DE: 0935 Seismic methods (3025)
DE: 7205 Continental crust (1242)
DE: 7294 Instruments and techniques
DE: 8010 Fractures and faults
DE: 8168 Stresses--general
SC: Seismology [S]
MN: 2005 Joint Assembly