Open AccessUltrasonic compressional and shear velocities in dry clastic rocks as a function of porosity, clay content, and confining pressure
Author(s) -
Freund D.
Publication year - 1992
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.1992.tb00843.x
Subject(s) - clastic rock , porosity , overburden pressure , geology , mineralogy , volume (thermodynamics) , volume fraction , silicate , sedimentary rock , geotechnical engineering , composite material , materials science , chemistry , thermodynamics , geochemistry , physics , organic chemistry
SUMMARY For clastic silicate rocks sampled from a Rotliegendes well core the velocities v P and v s were obtained at 10 pressures up to 300 MPa using a pulse‐transmission technique. The porosities of all rocks (57 sandstones, 26 siltstones, five claystones) ranged from 0.01 and 0.15 by volume fraction, and the clay content varied from less than 0.01 to 0.88 by volume fraction. Both velocities increase with pressure. In the low‐pressure range the rate of increase is large, non‐linear and is greater for v P than for v s . Above 120 MPa both velocities increase linearly. Velocities, porosity, and clay content were fitted by least‐squares regression for pressures of 8, 24, 60, 120, 200, and 300 MPa. The fractional effect of porosity and clay on the velocities for dry clastic silicate rocks can be described (above 120 MPa) by v = A ‐ B ø‐ CClay , where v is the velocity of the P ‐wave or S ‐wave, φ is the volume fraction of pores, and Clay is the volume fraction of clay. From this it is possible to obtain pressure‐dependent velocity functions v = a + pb ‐ c exp ( ‐dp ), where a is the crack‐free velocity, linear in porosity and clay, b is the velocity slope under high pressure, a‐c is the zero‐pressure velocity, and d is related to closure of cracks.