PERMEABILITY AND SALT-FILTERING PROPERTIES
OF COMPACTED CLAY
by
I . H . MILNE, J . G. I~'~cKELVEY a n d R . P . T R U M P
Gulf R e s e a r c h a n d D e v e l o p m e n t C o m p a n y , P i t t s b u r g h , P e n n s y l v a n i a
EXTENDED
ABSTRACT
T h e ability of s o m e c o m p a c t e d clay m a t e r i a l s to restrict t h e p a s s a g e of s o d i u m chloride
relative to w a t e r w h e n s u b j e c t e d to flow w i t h s o d i u m chloride solutions h a s b e e n d e m o n s t r a t e d b y K e m p e r (1960) a n d b y M c K e l v e y a n d Milne (1962). C o n t i n u e d e x p e r i m e n t a l
w o r k o n t h i s p h e n o m e n o n , directed t o w a r d a n u n d e r s t a n d i n g of t h e effect of v a r i a b l e s
s u c h as t h e t y p e o f clay material, porosity, h y d r a u l i c p e r m e a b i l i t y a n d solution c o n c e n t r a t i o n h a s been s t i m u l a t e d b y t h e possibility t h a t salt filtering occurs in t h e s u b s u r f a c e
as a result of t h e m o v e m e n t o f brines in response to c o m p a c t i o n a n d o t h e r sources o f
h y d r a u l i c pressure.
T h e a p p a r a t u s in w h i c h e x p e r i m e n t s h a v e been carried o u t h a s been described in t h e
p a p e r b y M c K e l v e y a n d Milne. A clay s a m p l e , in t h e f o r m o f s d r y p o w d e r or w a t e r
slurry is c o m p a c t e d in a n y l o n - l i n e d steel cylinder b e t w e e n m o n e l p i s t o n s c a p p e d w i t h
p o r o u s m o n e l filter discs. T h e p i s t o n - c y l i n d e r a s s e m b l y is bolted b e t w e e n steel plates to
m a i n t a i n t h e c o m p a c t i o n pressure.
T h e i n p u t p i s t o n h a s two openings, one of w h i c h e n t e r s a circular groove n e a r t h e edge
o f t h e p i s t o n face while t h e o t h e r e n t e r s t h e c e n t e r of t h i s face. T h e second p i s t o n h a s a
single o p e n i n g a t t h e c e n t e r o f t h e face. T h e t w o o p e n i n g s in t h e i n p u t p i s t o n p e r m i t a
b y p a s s c i r c u l a t i o n to c a r r y a w a y t h e salt rejected b y t h e clay filter. F l o w t h r o u g h t h e
b y p a s s is controlled b y a suitable restriction in t h e b y p a s s line b e y o n d t h e i n p u t piston.
T h e salt filtration efficiency in a p a r t i c u l a r case is e x p r e s s e d as t h e p e r c e n t a g e of salt
r e m o v e d f r o m a solution in p a s s i n g t h r o u g h t h e clay filter. T h e c o n c e n t r a t i o n of t h e
solution a t t h e i n p u t face of t h e clay filter u n d e r flow c o n d i t i o n s m u s t necessarily rise
a b o v e t h a t o f t h e source solution. Since t h e clay filter m u s t be s u p p o r t e d b y a m e t a l
filter, t h i s c o n c e n t r a t i o n is d e p e n d e n t o n t h e efficiency of m i x i n g a n d flushing w i t h i n
t h e pore space o f t h e m e t a l filter. F i l t r a t i o n effieiencies w h i c h h a v e been, in practice,
c a l c u l a t e d f r o m t h e c o n c e n t r a t i o n s of s o l u t i o n s e m e r g i n g f r o m t h e b y p a s s a n d t h e t h r o u g h p u t lines are likely therefore to be less t h a n t h e t r u e values. C e r t a i n e x p e r i m e n t s i n v o l v i n g
m e a s u r e m e n t o f filtration efficiencies a t v a r i o u s p r e s s u r e s a n d o t h e r s in w h i c h salt h a s b e e n
a c c u m u l a t e d in t h e i n p u t m e t a l filter a n d finally flushed t h r o u g h t h e b y p a s s s y s t e m , h a v e
s u g g e s t e d t h a t t r u e filtration efficiencies are m u c h h i g h e r t h a n t h o s e w h i c h h a v e been
o b t a i n e d f r o m t h e c o n c e n t r a t i o n s of t h r o u g h p u t a n d b y p a s s solutions.
A s a m p l e of r e c e n t m u d f r o m t h e Mississippi River, c o m p a c t e d s u c h t h a t it c o n t a i n e d
14 per c e n t w a t e r , h a s s h o w n a filtration efficiency of 60 p e r cent w i t h a 0.5 n o r m a l
solution o f s o d i u m chloride a n d a decline to 30 per c e n t a t a c o n c e n t r a t i o n of 5 n o r m a l .
T h e s e filtration effects were o b t a i n e d u n d e r a h y d r a u l i c p r e s s u r e of 10,000 psi.
A m i x t u r e of b e n t o n i t e f r o m Clay S p u r W y o m i n g a n d a m o r p h o u s silica in e q u a l p a r t s
b y weight, c o m p a c t e d u n d e r 10,000 psi s h o w s a filtration efficiency of 30 per c e n t in t h e
0.1 n o r m a l c o n c e n t r a t i o n r a n g e a t relatively low h y d r a u l i c pressure (1000-2000 psi).
T h e h i g h w a t e r c o n t e n t p r o b a b l y e x p l a i n s t h e low efficiency. T h e efficiency is p r e s s u r e
d e p e n d e n t , declining to zero a t 8500 psi.
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PERMEABILITY AND SALT FILTERING PROPERTIES
251
A compacted m i x t u r e of bentonite a n d a l u n d u m p o w d e r in t h e ratio of 2 : 1 b y weight,
with a w a t e r content of 16 per cent gives filtration effieiencies u p to 70 per cent over a
range of hydraulic pressures from 1000 to 10,000 psi. M a x i m u m filtration occurs in t h e
vicinity of 2000 psi. The decline in filtration at lower pressure u n d o u b t e d l y involves t h e
limit imposed on salt filtration b y osmotic pressure. A pressure of a p p r o x i m a t e l y 700 psi
is required to m a i n t a i n a concentration difference of 1 n o r m a l i t y u n i t across a perfect
m e m b r a n e . T h u s with m e m b r a n e s which are imperfect, a decline in salt filtration would
a p p e a r u n d e r lower hydraulic pressures. The high pressure decline c a n n o t be explained
so easily. I t is possible t h a t the flushing s y s t e m at the i n p u t face of the clay filter b e c o m e s
less effective at higher pressures a n d flow rates, b u t other e x p l a n a t i o n s m u s t be considered.
The permeabilities of these clay filters range from 10 -5 to 10-7 m d w i t h the lower
permeabilities corresponding w i t h t h e b e s t filtration effects.
Geological interest in the salt filtration problem is justified b y the evidence t h a t t h e
process can occur in shale-forming materials b u t specific application in geological p r o b l e m s
requires a q u a n t i t a t i v e u n d e r s t a n d i n g of the effects of the m a n y variables which m u s t be
considered in the subsurface. Continued efforts in this direction can lead to an unders t a n d i n g of fluid flow in fine-grained rocks.
REFERENCES
K e m p e r , W. D. (1960) W a t e r a n d ion m o v e m e n t in t h i n films as influenced b y the electrostatic charge a n d diffuse layer of cations associated w i t h clay mineral surfaces:
Soil Sci. Soe. Amer. Proc., v.24, pp.10-16.
McKelvey, J. G., a n d Milne, I. i . (1962) The flow of salt solutions t h r o u g h compacted
d a y : Clays and Clay Minerals, 9th Conf., P e r g a m o n Press, pp.248-259.