VELOCITY DISPERSION AND WAVE ATTENUATION IN RESERVOIR ROCKS

Abstract

My dissertation focuses on establishing a link between the rock heterogeneities and the fluid flow-related velocity dispersion and wave attenuation in sedimentary rocks, through both theoretical and experimental studies. From theoretical studies, I found that the frequency dependency of velocity or modulus is ultimately determined by the amount of additional support the porous frame gets from the fluid, if the fluid is allowed to flow in or out of the pore. An incoming fluid flow stiffens the frame and an outgoing flow softens the frame. A frequency-dependent “Dynamic Fluid Modulus” (DFM) can effectively describe this effect, and model the fluid flow-related velocity dispersion and wave attenuation. Using DFM, the application of Gassmann’s equation is extended from homogeneous to heterogeneous rocks. More importantly, the DFM can be easily and deterministically inverted from measured data. The inverted result is a good indicator of the degree and distribution of heterogeneities in the rock. For experimental studies, a low frequency measurement system was developed at The University of Houston Rock Physics Lab. This system can measure the velocities and attenuation in rock samples at seismic frequency range (2-500 Hz). As an essential part of the system, a virtual lock-in amplifier was developed to improve the quality and efficiency of the data acquisition and processing. Its performance was carefully evaluated with synthetic and real data. The random errors in our measurements are controlled to be equal or better than published data. The systematic errors on attenuation (phase) are calibrated to remove the electronic circuit effects. The systematic errors on velocities (amplitude) have to be calibrated with velocities measured at ultrasonic frequency for dry samples. Tests on aluminum, Lucite, and sandstone samples demonstrated that this system can successfully capture the velocity dispersion and attenuation coming from the various fluid flows in sedimentary rocks.