Seismic Wave Attenuation and Velocity Dispersion in UAE Carbonates

  • Abdulwaheed Remi Ogunsami

Student thesis: Master's Thesis

Abstract

Interpreting the seismic property of fluids in hydrocarbon reservoirs at low frequency scale has been a cherished goal of petroleum geophysics research for decades. Lately, there has been tremendous interest in understanding attenuation as a result of fluid flow in porous media. Although interesting, the emerging experimental and theoretical information still remain ambiguous and are practically not utilized for reasons not too obscure. Attenuation is frequency dependent and hard to measure in the laboratory at low frequency. This thesis describes and reports the results of an experimental study of low frequency attenuation and velocity dispersion on a selected carbonate reservoir samples in the United Arab Emirates (UAE). For the low frequency measurements, stress-strain method was used to measure the moduli from which the velocity is derived. Attenuation was measured as the phase difference between the applied stress and the strain. For the ultrasonic component, the pulse propagation method was employed. To study the fluid effect especially at reservoir in situ conditions, the measurements were made dry and saturated with liquid butane and brine at differential pressures of up to 5000 psi with pore pressure held constant at 500 psi. Similarly to what has been documented in the literatures for sandstone, attenuation of the bulk compressibility mode dominates the losses in these dry and somewhat partially saturated carbonate samples with butane and brine. Overall, the observed attenuation cannot be simply said to be frequency dependent within this low seismic band. While attenuation seems to be practically constant in the low frequency band for sample 3H, such conclusion cannot be made for sample 7H. For the velocities, significant dispersion is observed and Gassmann generally fails to match the measured velocities. Only the squirt model fairly fits the velocities, but not at all pressures. Although the observed dispersion is larger than Biot's prediction, the fact that Squirt (Biot-Squirt and Gassmann-Squirt) over predicts the velocities at low pressure (e.g. Biot- Squirt from 500 psi for samples 7H, 8H and 10V) and under predicts at higher pressures (e.g. Gassman squirt beyond 1000 Psi to as high as 3500 psi for 10V and 7V) suggests that the Squirt model seems to contribute to the overall dispersion in these carbonate samples. In addition, although Gassmann fairly predicts some of the butane saturated velocity, it is not applicable for these carbonate samples when saturated with brine.
Date of AwardJan 2013
Original languageAmerican English
SupervisorMohammed Ali (Supervisor)

Keywords

  • Social sciences, Earth sciences, UAE carbonates, Carbonate reservoirs, Geology, Seismic prospecting, Seismic wave attenuation UAE carbonates, Velocity dispersion, Geophysics, Middle Eastern Studies, Petroleum Geology, 0583:Petroleum Geology, 0555:Middle Eastern Studies, 0373:Geophysics

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