Abstract
Time-lapse (4D) seismic is often used for monitoring hydrocarbon fields during depletion and injection processes (hydrocarbon production, enhanced oil recovery, or CO2 storage operations). When the reservoir pore pressure changes during production or injection, the formation stresses in reservoir and overburden change accordingly. Since velocities in rocks are stress dependent, the stress-induced velocity changes result in detectable time-shifts on 4D seismic data. Knowing the stress sensitivity of rocks, it is possible to invert time-lapse seismic data for stress, strain, and pore pressure changes. This invaluable information can be used for finding undepleted pockets of hydrocarbon, as well as assessment of reservoir and caprock integrity.
To quantify the stress sensitivity of shales in the field, systematic laboratory experiments are very useful. Here, field rocks are brought to the in-situ stress state and probed with different stress changes, whereupon the associated (typically ultrasonic) velocity changes are measured (Holt et al., 2018). The stress sensitivities are most often assumed to be independent of frequency and applicable for time-lapse seismic data interpretation. In our work, based on multi-frequency measurements (1-100 Hz and 500 kHz) with different shale cores, we show that this assumption may not always be valid.
To quantify the stress sensitivity of shales in the field, systematic laboratory experiments are very useful. Here, field rocks are brought to the in-situ stress state and probed with different stress changes, whereupon the associated (typically ultrasonic) velocity changes are measured (Holt et al., 2018). The stress sensitivities are most often assumed to be independent of frequency and applicable for time-lapse seismic data interpretation. In our work, based on multi-frequency measurements (1-100 Hz and 500 kHz) with different shale cores, we show that this assumption may not always be valid.