Improved microseismic event location by inclusion of a priori dip particle motion: a case study from Ekofisk

Abstract

A B S T R A C TMicroseismic monitoring in petroleum settings provides insights into induced and naturally occurring stress changes. Such data are commonly acquired using an array of sensors in a borehole, providing measures of arrival times and polarizations. Events are located using 1D velocity models, P-and S-wave arrival times and the azimuths of P-wave particle motions. However in the case of all the sensors being deployed in a vertical or near-vertical borehole, such analysis leads to an inherent 180 • ambiguity in the source location. Here we present a location procedure that removes this ambiguity by using the dip of the particle motion as an a priori information to constrain the initial source location. The new procedure is demonstrated with a dataset acquired during hydraulic fracture stimulation, where we know which side of the monitoring well the events are located. Using a 5-step location procedure, we then reinvestigate a microseismic data set acquired in April 1997 at the Ekofisk oilfield in the North Sea. Traveltimes for 2683 candidate events are manually picked. A noise-weighted analytic-signal polarization analysis is used to estimate the dip and azimuth of P-wave particle motions. A modified t-test is used to statistically assess the reliability of event location. As a result, 1462 events are located but 627 are deemed to be statistically reliable. The application of a hierarchal cluster analysis highlights coherent structures that cluster around wells and inferred faults. Most events cluster at a depth of roughly 3km in the Ekofisk chalk formation but very little seismicity is observed from the underlying Tor chalk formation, which is separated from the Ekofisk formation by an impermeable layer. We see no evidence for seismicity in the overburden but such events may be too distant to detect. The resulting picture of microseismicity at Ekofisk is very different from those presented in previous studies.

Influence of a velocity model and source frequency on microseismic waveforms: some implications for microseismic locations

“…• ambiguity in azimuth and incorporation of dip is often needed (e.g., Jones et al, 2010). For the hodogram approach, travel time phase picks and a velocity model are required.…”

“…Jones et al, 2010) and velocity model. Probably least well understood is the impact of velocity model error on location accuracy.…”

Microseismic Monitoring of Fracture Networks During Hydraulic Stimulation: Beyond Event Locations

“…The first step in the analysis is event locations. Various location algorithms exploit the array-properties of downhole recordings on sensor strings (cf., Jones et al, 2010). The second step involves shear-wave splitting measurements and their inversion for fracture parameters.…”

Characterization of fractures and faults: a multi‐component passive microseismic study from the Ekofisk reservoir

“…Although challenging, with an array of geophones in a single borehole, microseismic events can be constrained in space and time to a degree of precision sufficient to discriminate between reservoir and overburden deformation (e.g., Jones et al . , ). The identification of clusters of seismic events with similar waveforms but different origin times, known as multiplets, allows for the precise mapping of asperities or locked zones along a fault (e.g., Geller and Mueller ; Poupinet, Ellsworth and Fréchet ; Arrowsmith and Eisner ; De Meersman, Kendall and van der Baan ; Dyer et al .…”