Experimental Investigation of Rock Dynamic Geomechanical Properties at Seismic Frequencies

Time-lapse seismic reservoir monitoring has advanced quickly over the past decade to become a common technique for monitoring subsurface processes, especially for steam and CO2 injection. It has been widely used to determine and monitor changes occurring in a reservoir during hydrocarbon production or injection of water or gas into the reservoir. Petroleum engineers can predict the evolution of pressure and temperature and the saturation of oil, gas, and water during reservoir production with time-lapse seismic data. To understand the changes in seismic attributes during oil and gas production, researchers are challenged to combine rock physics models, geomechanics and laboratory measurements. Traditional laboratory measurement of reservoir rocks is conducted in the high kHz to MHz frequency range, which differs from seismic surveys (0.1-100Hz). Additionally, dynamic moduli determined from the elastic wave velocity and static moduli derived from the slope of a stress-strain curve from a deformational experiment are different for reservoir rocks due to the different strain measurement levels. Consequently, it is valuable to measure both dynamic and static elastic properties of rocks in the laboratory on core scale samples from seismic frequencies to ultrasonic frequencies. From this perspective, experimental research was undertaken to design reasonable and feasible experimental protocols that guide the infrastructure and process required to conduct seismic frequency dynamic property measurements under shear stress at elevated temperatures. A new experimental system within the Reservoir Geomechanical Research Group at the University of Alberta has the capacity to measure dynamic and static elastic properties at seismic and ultrasonic frequencies under deviatoric stress. This rock physics research is based on stress-strain method, and focus on the temperature, frequency (seismic – 0.01 Hz to 20 Hz) and most critically deformation (both isotropic compressibility and shear stress-induced) dependent properties of the poorly consolidated oil sands and highly overconsolidated (clay) shales associated with the McMurray and Clearwater Formations of NE Alberta. The results were shown that the frequencies within the reservoir are capable of significantly influencing reservoir properties. Acoustic impedance changes 59% at different effective pressures and up to 40% at different frequencies. An interpretation workflow based on the application of the newly gained understanding of frequency-dependent elastic properties was developed and validated by previous experimental studies. The newly acquired seismic frequency properties can then be applied in forwarding modelling using sequentially coupled reservoir geomechanical simulations to assess whether improvements in the a priori predictions of fluid saturations and steam chamber development are altered.

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