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SUMMARY:The effect of pore structure and pore fluids on the elastic moduli
  and wavespeeds in porous rocks - Prof. Robert Zimmerman. Imperial College
  London
DTSTART:20190517T130000Z
DTEND:20190517T140000Z
UID:TALK120556@talks.cam.ac.uk
CONTACT:46601
DESCRIPTION:Abstract: The speed at which elastic waves travel through a fl
 uid-saturated porous rock depend on the mineral composition\, the pore geo
 metry\, and the pore fluid properties. Since the pore structure of a rock 
 will vary with the external stress\, the wave speeds will generally vary w
 ith stress. Since the ability of the pore fluid to contribute to the elast
 ic stiffness of the rock-fluid system depends on the ease with which the f
 luid can move into or out of a pore as it is compressed by a passing wave\
 , the wave speeds will also be functions of the pore fluid viscosity and w
 ave frequency. \nVarious effective medium theories\, such as the Mori-Tana
 ka model or the differential effective medium approach\, can be used to re
 late the elastic moduli of the rock to its pore structure. The effect of s
 tress on the elastic moduli can be accounted for by combining an effective
  medium prediction for cracked rocks with an equation that describes crack
  closure under stress. The effect of pore fluids\, however\, depends on th
 e frequency regime. At “high” frequencies\, the pore fluid will not ha
 ve sufficient time to travel between adjacent pores during the period of t
 he wave\, and can be considered to be “trapped” in each individual por
 e. In this regime\, the effective moduli of the fluid-saturated rock can b
 e calculated from an effective medium theory\, with the fluid-filled pores
  considered as isolated inclusions in a rock matrix. At very low frequenci
 es\, the fluid has sufficient time to drain out of any pore that is compre
 ssed by the wave\, and the effective elastic moduli of the rock-fluid syst
 em will correspond to the drained (i.e.\, dry) limit. It is generally assu
 med that there is an intermediate range of frequencies in which the fluid 
 is able to travel between adjacent pores\, so as to locally equilibrate th
 e fluid pressure\, but cannot fully drain out of the region of rock that i
 s compressed by the passing wave. In this case\, Gassmann’s equation is 
 used to predict the stiffening effect that the fluid has on the rock-fluid
  system. \nI will present some of the models that have been developed to r
 elate the elastic moduli and wavespeeds to pore structure\, and to account
  for the effect of stress. The model predictions will be tested against ex
 perimental data on sandstones. Finally\, the applicability of Gassmann’s
  equation will be discussed in light of available experimental results. \n
LOCATION:Oatley Seminar Room\, Department of Engineering
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