Recent technological advances have led to dramatic improvements in seismic resolution which now enable seismic attenuation to be measured on a routine basis. The usefulness of seismic attenuation as an interpretation parameter is restricted at present by our incomplete knowledge of the relationships between attenuation and important reservoir parameters such as porosity, permeability and the nature of the pore fluid. In order to redress this imbalance, P-wave and S-wave velocities and attenuations were measured in the laboratory at a frequency of about 0.8 MHz and at an effective pressure of 60 MPa on a set of reservoir rocks comprising twenty-seven shaly sandstones and two sandy shales. It was discovered that both P-wave and Swave attenuation increase with increasing percentage of intrapore minerals (especially clay, but also micrite) from the composition of a clean sandstone to a value of 50%, and then they decrease with increasing clay content as the rock approaches the composition of a pure shale. Porosity plays a subsidiary role to pore fill. More work is needed to predict permeability from its complex relationships with porosity and pore filling minerals. Ultrasonic P-wave and S-wave velocities (Vp and Vs) were shown to be strongly dependent on the amount of pore fill: velocity is reduced by increasing amounts of clays and micrite, but it is increased by increasing amounts of sparry calcite cement. Contrary to popular understanding, the effect on velocity of the percentage of pore filling minerals is much stronger than that of porosity on velocity, although porosity is still an important parameter. There is a strong correlation between Vp and Vs, and between P-wave quality factor (Qp) and S-wave quality factor (Qs). Interesting relationships were also discovered between Qp and Vp, and especially between Qs and Vs. However, high frequency laboratory measurements may not be representative of the low frequencies used in seismic exploration. The frequency dependence of velocity and attenuation in reservoir sandstones was investigated by exploiting the inverse relationship between frequency and pore fluid viscosity predicted by the Biot theory. P-wave and S-wave velocities and attenuations were measured at a frequency of about 0.8 MHz and at an effective pressure of 50 MPa on shaly sandstones saturated with pore fluids of viscosities 0.3 cP to 1000 cP (equivalent frequency range 2.6 MHz to 780 Hz). The Biot theory accounts for the very high Q values encountered in clean sandstones, but not the very low Q values observed in clay-rich sandstones. The observed velocity dispersion in both clean and clay-rich sandstonesim plies a local fluid flow mechanism which predicts the opposite frequency-viscosity dependence of the Biot theory. Qp and Qs in clay-rich sandstones remain constant over nearly four decades in equivalent frequency. This implies a range of relaxation times which may be attributed to the wide distribution of pore sizes of clays and other pore filling minerals. Moreover, this constant Q behaviour and the magnitude of the laboratory Qp and Qs values tie in well with those obtained from field studies. This suggests that Qp and Qs are broadly independent of frequency from seismic frequencies to ultrasonic frequencies in clay-rich sandstones.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:331984 |
| Date | January 1992 |
| Creators | Best, Angus Ian |
| Publisher | University of Reading |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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