Re: EM signals

From: gresham (Gresham3_at_cox-internet.com)
Date: 11/14/04 

Date: Sat, 13 Nov 2004 22:20:58 -0600

in article VA.0000025a.0a9a77ab@mynameplus1.demon.co.uk.invalid, Aidan
Karley at aidan@mynameplus1.demon.co.uk.invalid wrote on 11/1/04 11:55 AM:

> In article <BDABBE9B.4BEC%Gresham3@cox-internet.com>, Gresham wrote:
>> The oil field may also be
>> related to the current since oil can be extracted from shale like
>> clay-oil-water plugs by electro osmosis.
>>
> References?
> I've never heard of this being done. But I've only been working
> in the oil business for 18 years now, so what would I know about it?
>
I found this in my files dated 1984.

IONIC TRANSPORT

J. G. McKelvey

The ultimate objective of this project is the development of a
comprehensive, coherent theory describing subsurface transport
phenomena. While this objective may appear somewhat esoteric, our
immediate objectives are a better understanding of the circumstances
surrounding the early migration of petroleum and how we may use this
information in better planned exploration. Although we are still some
distance from our ultimate objective, we have reached a point where some of
our understanding can be incorporated in current exploration.

Our present understanding of primary migration suggests strongly that
migration within and out of the formative shale is controlled by diagenetic
changes within the shale. It is suggested that these diagenetic changes are
brought about through ionic transport processes which involve both the shale
and the adjacent sands. It should be pointed out that the theories
expounded here diverge from current theories on primary migration at very
basic level, and hence may be expected to have ramifications on other older,
more established theories such as those concerned with fluid migration or
thermal maturity.

Our concept of these subsurface processes involves a synthesis of
ideas extracted from our knowledge of the mechanical, chemical, and
electrical properties of shales. During the early compaction of sediments,
simple water loss is the dominant process. This proceeds very rapidly to a
point where hydraulic permeability is reduced to a diffusive process.
Concurrent with this, the electrical conductivity has increased and
electrical transport becomes the dominant process. This, then, initiates
the diagenetic processes. Calcium ions, which were once the preferred clay
exchange ion because of their high charge, are now rejected because of their
low electrical mobility. This appears to be the key which controls the
genesis of oil field brines, shale diagenesis, and primary migration of
petroleum. Extension of this theory also offers insight into many other
subsurface phenomena, such as the formation of cap rock, marly zones,
fracture filling, and metasomatic processes in general.

The project title "Ionic Transport", as well as much of the theory
contained within it, is a direct outgrowth of a series of studies conducted
by the "Petrophysics" group at Gulf Research during the late 1940s and
early 1950s. This group, under M. R. J. Wyllie, realized that the
mechanical, chemical, and electrical properties of shales were not
independent properties but rather a manifestation of "coupled
phenomena". This realization was originally based on an understanding of
the SP (self potential log) as being due to a "coupling" between electrical
and chemical forces. By "coupling", we mean the spontaneous generation of a
flow or force through the application of a non-conjugated force. The SP is
thus due to the generation of an electrical force in response to an applied
chemical force. It was also realized that this coupling was due to the
ionic character of shales, and hence to "ionic transport". Although all
shale transport phenomena are now considered to be coupled phenomena, not
all can be considered as due to ionic transport. The title "ionic
transport" survives as a reference to this fundamental shale characteristic.

In 1808,, a pioneer colloid chemist in Russia named Rous studied the
conductance of electrical current by wet clay. To his amazement, he
observed that imposing an electrical potential difference across the clay
led not only to the expected flow of electricity but also to a pronounced
flow of water toward the cathode. Since, on a priori grounds, a flow of
water should be driven by a pressure head, Rous made the converse
experiment: he applied hydrostatic pressure to the piece of clay and
obtained a flow of electricity.

Rous's ingenious experiments were the first to demonstrate the
existence of coupled phenomena. They proved that a flow may not only be
driven by its directly conjugated force, but may also be coupled to other,
non-conjugated forces. Thus the flow of electricity is evidently caused by
an electromotive force, but it may be coupled to a hydrostatic pressure; and
conversely, the volume flow of water may be coupled to an electrical force.

The experiments of Rous are also important to us in another way; they
were performed using clay, the fundamental building block of shale, which is
considered to be the origin of petroleum. It is considered strange that so
little attention was paid these phenomena in the ensuing years.

The theoretical study of coupled phenomena was originally pioneered
by Lord Kelvin (Thomson) in 1854. Kelvin's treatment of thermoelectricity
was based on pseudo-themostatics, in which the process was split in two
parts, reversible and irreversible. Kelvin considered that heat
conductivity and electrical conductivity were both irreversible and thus
could not be treated using conventional thermostatics. As a result, Kelvin
considered only the reversible portion of thermoelectricity: the Thomson and
Peltier heats.

In his famous treatise on sound, Lord Rayleigh used a set of equations
that expressed, in an explicit manner, the linear dependence of all
mechanical flows on all mechanical forces operating in a system. Onsager,
in 1931, extended this concept to include all thermodynamic flows and
forces. This extension effectively combined all the Phenomenological Laws
(D'arcy's, Ohm's, Fick's, etc.) into one new law or set of equations.

Onsager's Law, however, suffers from several shortcomings. First, like
all phenomenological laws, it is a law of nature and hence independent of
any model; i.e., it cannot suggest a physical model for what we have seen in
nature or through experimentation. Second, the coefficients of the Onsager
equations, unlike the coefficients in the original laws, usually lack
physical significance and are not amenable to experimental determination.
The important feature of the Onsager equations is the fact that the matrix
of the coefficients is symmetrical. This has the effect of reducing the
number of independent experiments that are needed to fully characterize a
given system since a number of the coefficients are equal.

In the early 1950s, K.S. Spiegler, of the Gulf Petrophysics group,
developed a physical model for transport processes in shales. Spiegler' s
model effectively handled the coupling phenomena, did not disagree with
Onsager's Law, and could be cast in a form similar to the Onsager
equations i.e. in the form of a symmetrical matrix. Since this was a
physical model, Spiegler could make assumptions which had the effect of
further reducing the number of required experiments. Furthermore, he was
able to define and specify these experiments. Spiegler's model is based on
a simple Newtonian "frictional" model; i.e., the drive forces on a particle
are considered equal to the frictional forces on the same particle. For
example, the hydraulic pressure that forces water through a shale is exactly
balanced by the friction" between water and shale.

This rational approach to transport processes was widely acclaimed
and utilized by workers in many diverse fields. Unfortunately this model
was never applied to shales in their subsurface environment, possibly
because of the enormity of the task. Our understanding of subsurface
processes still relies on physical models derived and synthesized from a
variety of available experiments.

Research in biological membranes is one field that has supplied a new
understanding of shale transport processes. Kedem, in a study of
"ultrafiltration", concluded that ionic transport could occur in a direction
opposite to the applied pressure gradient. Kedem went on to postulate
that, in a heterogeneous media, containing hydraulic pathways shunted by
electrically conductive elements, circulating ionic current could exist. Her
model is based on the role of the "streaming potential" in ultrafiltration
This appears to be a fair description of shales, and an interpretation of
our own ultrafiltration data, collected using clay plugs, fully supports
this model.

One ramification of Kedem's model is the almost total rejection (by a
shale) of divalent cations (e.g., Ca) when in the presence of fast univalent
ions (e.g., H or Na). Experimental verification of this effect may be
found in the literature. This, then, explains the occurrence of calcium oil
field brines and the high bicarbonate content of certain Gulf Coast shales.

The circulating ionic current, induced by pressure gradients (or, more
probably, by salinity gradients), has acted to dissolve calcite and has
transported the calcium to adjacent sands. The only mechanism for the
transport of anions in such a system, is advective transport by migrating
water, and hence the bicarbonate collects in the shale.

This model for shale diagenesis is consistent with geologic observa-
tions. Most calcium type brines also contain radium which is out of
equilibrium with its parent. The short half life of radium precludes any
great migrational distance, thus it must have originated in the adjacent
shale. Calculations indicate that the migration must have been extremely
rapid, suggesting an electrical process. The chemical similarity between
calcium and radium also suggests that the same transport process operated in
both cases. Using reported radium and calcium concentrations for Gulf Coast
sands, we calculate that the process is sufficiently rapid to totally cement
most sands in about one million years.

This same type of electrical process is probably responsible for the
early migration of petroleum. One of the early studies at Gulf Research
was concerned with the recovery of oil from compacted clay plugs. This
was done in an attempt to duplicate the natural process. At that time,fluid
transport was considered to be responsible for primary migration, but we
were totally unable to recover even a small portion of the contained oil.
In desperation, we did what Rous did; we applied an electric current and
were amazed to find that the oil also migrated to the cathode. In these
experiments, we were able to recover a considerable portion of the oil,
frequently to 50%. These experiments also indicated that the velocity of
the oil was greater than that of the water within the plug. In a natural
environment, oil may also be moved by this electro-osmotic process.

We may speculate about the process that migrated the oil in our
experiments. Certainly the plugs were over pressured; this was evident. If
we recall that Gulf Coast sands show their greatest increase in calcium just
above the over pressured zone, and that this corresponds to the zone of
greatest oil production, then we may speculate further.

Sufficient for now, however, is the connection that may be drawn
concerning the mechanism that transports calcium and that which
migrated the oil. The calcium transport was considered to be due to the
"streaming potential", while the oil was transported by "electro-osmosis".
These two processes are known to be equivalent measures of the same
shale parameters Saxen, in experiments similar to those of Rous, showed that
there was correlation between the amount of water moved (per Faraday) in the
electro-osmotic experiments and the voltage developed (per unit pressure) in
the streaming potential experiments. Saxen reported his observations in
1892. Mazur and Overbeek later showed the relationship was of a very
general character, independent of any model, and a direct consequence of
Onsager's principle of reciprocity of irreversible phenomena. Thus there
may be more than accident in the occurrence of high calcium brines in
connection with oil deposits.

In summary then, we suggest that a high organic content is not the
only test that should be applied to a shale to determine its source
potential. There must also be sufficient calcium to accommodate the
diagenesis and, we suspect, sufficient salt to initiate the process. We do
not wish to imply that these are the only conditions which will result in
primary migration; only that these conditions appear to apply to the Gulf
Coast.
                     McKelvey, J. G.

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