In-situ measurements of velocity-stress sensitivity with cross-well CASSM
Abstract
The ability to characterize time-varying reservoir properties, such as the state of stress, has fundamental implications in subsurface engineering, including for the geological sequestration of CO2. Stress variation, here in the form of changes of pore fluid pressure, is one the factors known to affect seismic velocity. Induced variations in velocity have been used in seismic studies to determine stress and monitor changes in the stress state. Previous studies conducted to determine the change of velocity, as a function of varying stress at reservoir conditions, rely primarily on laboratory measurements of core samples or theoretical relationships. We present a novel field-scale experiment designed to study the in-situ relationship between pore fluid pressure and seismic velocity using a Continuous Active-Source Seismic Monitoring system (CASSM) configured in a cross-well geometry. At the Cranfield, Mississippi, CO2 sequestration field site, we actively monitored seismic response for 5 days with a temporal resolution of 10 minutes; the target was a 21 m-thick injection zone at 3200 m depth in a fluvial sandstone formation (lower Tuscaloosa Fm). The variation of pore fluid pressure was obtained during discrete events of fluid withdrawal from one of the two wells and monitored with downhole pressure sensors. The results show a correlation between decreasing CASSM time delay (i.e. velocity change for a raypath in the reservoir) and periods of reduced fluid pore pressure. The correlation is interpreted as the velocity-stress sensitivity measured in the reservoir. This observation is consistent with published lab studies documenting velocity increase with effective stress increase. A travel time change of 2.5 x10-5 ms is measured as the consequence of a change in pressure of 3 MPa (dPeff). For 15 ms total travel time, the velocity-stress sensitivity is dt/t/dPeff = dVel/Vel/dPeff = 5.6 x10-7 / MPa. We also investigated the change in RMS amplitude of the wave first arrival during the fluid withdrawal events. Data show an increase in seismic amplitude with increasing effective stress presumably due to an increase in seismic quality factor (Q). The overall results suggest that CASSM measurements represent a valid technique for in-situ determination of velocity-stress sensitivity in field-scale monitoring studies.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.S44A..03M
- Keywords:
-
- 0599 General or miscellaneous;
- COMPUTATIONAL GEOPHYSICSDE: 1699 General or miscellaneous;
- GLOBAL CHANGEDE: 7290 Computational seismology;
- SEISMOLOGYDE: 7299 General or miscellaneous;
- SEISMOLOGY