Acoustic properties of shale and sandstone

Abstract

Acoustical properties employ sonic wave velocities as spring board to mirror the sediments. The albatross associated with analyzing constituent sediment warrants that grain structures and mineral composition are well highlighted for meaningful results. The combination of whole rock mineralogy and well logs give a detailed road map to unraveling the rock physics trend of sediments through mechanical properties and petrophysics generation. The generations of velocity ratio (Vp/Vs), acoustic impedance (AI), Poisson ratio (PR) in this work were in no small means helpful. These accomplishments held without losing focus of the dominance of either smectite or kaolinite and even their combination by pore filling smectite of kaolinite in the case of clay volume in shale; quartz cementation by pore filling smectite in the case of sandstone to pure sandstone (zero clay volume) without down-playing the all important K-feldspar. In well 1, quartz has a range of 19-27% with some points showing high Vp/Vs, PR and high AI at close percentage of S-I and KDH; other points yielded low Vp/Vs, PR and high AI; points of high percentage pore filling smectite showed low Vp/Vs, PR but high AI. In well 2, points of high Vp/Vs, PR but abnormally low AI are present; also are points of low Vp/Vs, PR but high and low AI. High velocity ratio may be due to poorly compacted situation of possibly high overpressure and low effective stress. Porosity and permeability diminish with reducing Vp/Vs and PR. Contrast in permeability of shale minerals (smectite and kaolinite) for which the latter rates higher is a requisite in the assessment of shale for the purpose of seal and reservoir alternation.

Keywords: Velocity ratio, Poisson ratio, acoustic impedance, quartz cementation and dilation factor.