Linking ultrasonic lab-measurements with well logs and seismic data is key for reservoir quality assessment and monitoring of human-induced changes of the subsurface. In this contribution we report on the acoustic properties of calcitic and dolomitic rock types characterized by widely varying, heterogeneous pore structures. Plugs with porosity ranging from 3% to 30% are subjected to varying stress states, including hydrostatic vs. deviatoric stress state and pore pressure inflation vs depletion (see Figure). Ultrasonic longitudinal (vp) and shear wave velocities (vs) are measured every 150-200 s, with mean values at key stress states quantified for internal comparison. The magnitudes of vp and vs are affected by axial stress rather than mean stress, emphasizing the importance of rock acoustics measurements under in-situ conditions for reservoir characterization, as opposed to (near-)isotropic stress conditions. Comparison of the sensitivity of vp to changing axial stress (dvp/dσax) or pore pressure (dvp/dPp) shows that pore type or structure plays an important role. We postulate that the stress-sensitivity of acoustic velocity in limestones can be used as a proxy for differentiating pore type, size, and shape.

Effects of stress state on the acoustic properties of carbonate rocks

ACS Spring 2022

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