Next: INDUCED MASS EFFECT
Up: Berryman: Dynamic permeability
Previous: INTRODUCTION
For longwavelength disturbances (, where h is a
typical pore size) propagating through such a porous medium, we define
average values of the (local) displacements in the solid and also in
the saturating fluid. The average displacement vector for the solid
frame is while that for the pore fluid is . The
average displacement of the fluid relative to the frame is
, where is the porosity.
For small strains, the frame dilatation is
 
(1) 
where e_{x},e_{y},e_{z} are the Cartesian strain components.
Similarly, the average fluid dilatation is
 
(2) 
(e_{f} also includes flow terms as well as dilatation) and the
increment of fluid content is defined by
 
(3) 
With these definitions, Biot (1962) obtains the stressstrain relations in the form
 
(4) 
and similarly for ,,
 
(5) 
and again similarly for ,, and finally
 
(6) 
The are deviations from equilibrium of
average Cartesian stresses in the saturated porous material
and is similarly the isotropic pressure deviation
in the pore fluid.
With time dependence of the form , the coupled wave
equations that incorporate
(4)(6)
are of the form
 
(7) 
 (8) 
where , with being the
fluid density, being the solid density, and being the
overall bulkdensity of the material,
while
 
(9) 
is the frequency dependent effective density of the fluid in relative motion.
This expression is also the one that we study later in order to
introduce the concepts of dynamic permeability and tortuosity
(Johnson et al., 1987). So we want to emphasize now how
important this equation for will be to our later analyses.
The kinematic viscosity of the liquid is ;the low frequency permeability of the porous frame is ;the 3D dynamic viscosity factor [as it was first called by Biot (1956b)]
is given, for our choice of sign for the frequency dependence,
by ,where and .The functions and are the real and
imaginary parts of the Kelvin function. The dynamic parameter h
is a characteristic length generally associated with and comparable in
magnitude to the steadyflow hydraulic radius. (For a model
calculation we discuss later, h = a, the radius of a cylindrical
pore.) The tortuosity
is a pure number related to the frame inertia which has
been measured by Johnson et al. (1982) and has also been estimated theoretically by Berryman (1980).
We discuss and its various interpretations
in greater detail later in this review.
The coefficients H, C, and M are given
(Gassmann, 1951; Biot and Willis, 1957; Biot, 1962) by
 
(10) 
<I>CI> = <I>KI>_{<I>sI>}(<I>KI>_{<I>sI>}<I>KI>)/(<I>DI><I>KI>), 


(11) 
and
<I>MI> = <I>KI>_{<I>sI>}^{2}/(<I>DI><I>KI>), 


(12) 
where
 
(13) 
with K_{f} being the fluid bulk modulus and K_{s} being the solid bulk
modulus. The frame (porous solid without liquid in the pores)
constants are K for bulk and for shear. Equations
(10)(13) are correct as long as the porous material
may be considered homogeneous on the microscopic scale as well
as the macroscopic scale.
Eq. (7) is essentially the equation of elastodynamics of the
solid frame with coupling terms (involving and )to the fluid motion. Eq. (8) reduces exactly to Darcy's
equation when the solid displacement and frame strain e are zero,
since the right hand side of the equation is just .
To decouple (and subsequently solve)
the wave equations in (7) and (8)
into Helmholtz equations for the three modes of propagation,
note that the displacements and can be decomposed as
 
(14) 
where , are scalar potentials and
, are vector potentials. Substituting
(14) into (7) and (8), the two equations
are solved if two pairs of equations are satisfied:
 
(15) 
and
 
(16) 
The wavenumbers in (15) and (16) are defined by
 
(17) 
and
 
(18) 
 

 (19) 
with .The linear combination of scalar potentials has been chosen to be
,where
 
(20) 
With the identification (20), the decoupling is complete.
Equations (15) and (16) are valid for any choice of coordinate
system, not just Cartesian coordinates, and they are therefore very
useful in all applications of the theory.
Next: INDUCED MASS EFFECT
Up: Berryman: Dynamic permeability
Previous: INTRODUCTION
Stanford Exploration Project
7/8/2003