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SUMMARY:Streaming potentials generated by a two-phase flow in capillary - 
 Etienne Lac
DTSTART:20080117T113000Z
DTEND:20080117T123000Z
UID:TALK9907@talks.cam.ac.uk
CONTACT:Anne Alexander
DESCRIPTION:Many rock surfaces are (weakly) charged\, due to the surface c
 hemistry of the rock. Counterions\, which neutralize this charge\, form a 
 charge cloud in the fluid immediately adjacent to the rock surface. When t
 he fluid flows\, these ions are convected\, and electric fields are set up
  to ensure that electrical neutrality is maintained\; the generated potent
 ial is referred to as streaming potential.\nWe study numerically the strea
 ming potential generated by flow along a small cylindrical capillary\, mea
 nt to represent a porous medium at pore scale. In the case of single-phase
  flow\, it is well known that the potential difference ΔΦ between the tw
 o ends of the capillary is proportional to the pressure drop Δp in the fl
 uid. This relationship holds for streaming potentials in complex porous me
 dia such as rocks. The goal of this work is to investigate the effect of a
  second phase on the relation between ΔΦ and Δp by considering the pres
 ence of a drop in the capillary.\nThe streaming potential is modified by t
 he change of resistivity of the pore\, as well as by the change in the con
 vective electric current along the capillary wall.\n\nIf gravity effects a
 re neglected\, the drop motion (hydrodynamics) is determined by three inde
 pendent parameters: the size <em>a</em> of the undeformed drop relative to
  that of the capillary\, the viscosity ratio λ  between the drop phase an
 d the wetting phase\, and the capillary number <em>Ca</em>\, which measure
 s the relative importance of viscous and capillary forces. Assuming that t
 he drop phase is a perfect insulator\, and that the drop surface is unchar
 ged\, the electrokinetic problem\, which yields the streaming potential\, 
 is solely determined by the hydrodynamics. The results indicate that the d
 ifference ΔΦ-Δp (in dimensionless variables) is always positive. Since 
 this quantity is independent of the pore length (unlike ΔΦ and Δp)\, it
  is an interesting quantity to measure experimentally to characterize the 
 impact of the second phase on the streaming potential. Novel asymptotic an
 alysis provides theoretical predictions for the streaming potential in the
  case of a vanishingly small spherical droplet\, as well as for large drop
 s with vanishingly small surface tension (large capillary number).
LOCATION:Open Plan Area\, BP Institute\, Madingley Rise CB3 0EZ
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