Clay Minerals and Permeability of Subsurface Reservoirs

By Darrel Dunn, Ph.D., PG, Hydrogeologist-Geologist

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This technical webpage describes the effect of clay minerals on the permeability of subsurface reservoirs.   It deals primarily with the interrelationship between clay minerals, pore fluid composition, and permeability.

Types of Clay Minerals in Reservoir Rocks

The clay minerals commonly found in reservoir rocks are illite, kaolinite, montmorillonite,chlorite, and mixed-layer types (Foster, 1955).  Glauconite, which is often conspicuous by virtue of its color, may be either an illite or a mixed-layer assemblage of illite and montmorillonite.  Paleozoic glauconite is largely illite.  Petroleum reservoirs of California and the U. S. Gulf Coast often contain significant amounts of montmorillonite; whereas, in the rest of the United States, it is much less likely to be abundant.  It is generally absent in Paleozoic sediments.  This is probably due to alteration to micas (Grim, 1953).  Ferromagnesian minerals, calcic feldspars and volcanic glasses commonly alter to montmorillonite (Ross and Hendricks, 1945).  Mesozoic and Cenozoic sediments containing volcanic detritus are likely to contain significant amounts of the mineral.  Kaolinite is a common constituent of petroleum reservoirs.  This may be partly due to the tendency for the relative abundance of kaolinite in contemporaneous rocks to increase toward the shoreline (Smoot, 1960) combined with an abundance of petroleum accumulations found in sandstone of near-shore origin.  Kaolinite is common in arkosic sediments, and occurs in about a 50-50 ratio with illite in many graywackes (Griffiths and others, 1956).  It is also often found in  sandstone due to decomposition of feldspar grains.  Alkali feldspar and mica tend to alter to kaolinite (Ross and Hendricks, 1945) and it can also be precipitated from alumina-rich solutions (Carozzi, 1960).

Smoot (1960) studied the clay mineralogy of the pre-Pennsylvanian reservoirs in the Illinois Basin.  He found the sandstone clay mineral suites to be composed of kaolinite, illite, chlorite, and mixed layer material, with minor amounts of montmorillonite.  The mixed layer material is partially  expandable and probably contains some montmorillonite.    He indicates that the composition of the clay mineral suites is related to the permeability of the rocks because of interaction between clay minerals and interstitial solutions.  He states that illite originally deposited in the permeable rocks is altered to illite plus mixed layer material, and possibly to montmorillonite.  Since chlorite is rare in the permeable sandstone and illite is a minor constituent, he goes on to suggest that chlorite is more susceptible to degradation than illite and that chlorite alters to chlorite and mixed layer material which is partially expandable and probably contains some montmorillonite.  Chloritic material may be  slightly expandable; this may be due to degradation along the edges of the plates.  Clay-mineral assemblages in the outcrops of permeable beds are different from those in the same bed in the subsurface.  Montmorillonite or montmorillonite-illite mixed layer minerals are major constituents of the outcrop sandstone but are minor in subsurface samples.  This suggests that one should consider the clay minerals when trying to predict subsurface permeability on the bases of surface samples.

Mode of Occurrence of Clay Minerals in Subsurface Reservoirs

The clay minerals may be present in the porous rocks as filler in the pores, as coatings on the grains, or as a combination of the two modes of occurrence. (Grim, 1947).  When they are present as coatings, they may be difficult to separate from the grains for analysis and might also escape detection under the microscope.  Coatings of montmorillonite are generally smooth, while kaolinite and illite coatings are uneven (Grim and Cuthbert, 1945).

Theory of Water Adsorption of Clay Minerals

The ability to adsorb water and increase their volume is the principal phenomenon which makes the clay minerals important in the study of reservoir rock permeability.  This adsorption is accomplished by two different mechanisms - by oriented water, and by "osmotic" effects due to the negative charge of the lattice (Wilson,M. J. and others, 2012).

Montmorillonite absorbs oriented water between the silicate sheets.  The position of this water within  about 10 angstroms of the silicate sheet surface is thought to have about the same crystalline structure as ice (Norrish, 1954).  This thickness is related to the nature of the adsorbed cations partly because their ionic radii vary, and some fit less perfectly than others into the holes in the crystalline structure of the water.  The oriented water, therefore, is distorted and weakened in various degrees, causing corresponding decreases in the thickness.  The ability of the water to hold the sheets apart against the pull of Van der Waals force is dependent on the strength of the tendency to build oriented water layers, so the interlayer spacing is controlled by the balance between the tendency of crystalline water to fit on the surface and the force of attraction between the silicate sheets.   In addition to distorting the water structure, the cations also affect the amount of attractive forces between the sheets, because divalent ions have a much larger sphere of dielectric saturation than monovalent ones, and the electrostatic attractive forces between sheets do not diminish as rapidly as they do for monovalent ions (Norrish, 1954).  

Experiments by Norrish indicate that the interlayer water of montmorillonite associated with divalent magnesium, calcium and barium cation as well as trivalent aluminum cations expands to a maximum spacing of 19 angstroms with decreasing concentration of the solution, and montmorillonite  saturated with monovalent potassium, ammonia, and cesium  cations do not expand beyond 15 angstroms.  Norrish's work showed that sodium montmorillinite develops oriented water in a stepwise fashion to a spacing of 19 angstroms and then the spacing jumps to 40 angstroms..  Above 40 angstroms the clay proceeds to swell asymptotically to 140 angstroms where the diffraction pattern becomes too diffuse for further observations.

Osmotic swelling is considerably different from that caused by oriented water.  The osmotic effect is a result of the overall negative charge  of the clay particle.  The negative charge attracts cations from the solution which form a cloud around the particle so that the charge on the particle is balanced  However, this cloud must also be in osmotic  equilibrium with the solution so that the product of the activities of the ions within the cloud is equal to the product of the activities of the external ions.

The boundary between the external and internal volumes is actually a transitional one and anions and cations are constantly moving across it.  Since the cation-anion ratio is greater inside the cloud, the water of the cloud is fixed in the sense that it cannot move relative to the surface of the clay particle without a compensating movement of the associated ions.

In sodium-montmorillonite there appears to be a gradation from well oriented water close to the particles containing part of the balancing sodium ion to an osmotic cloud containing less well organized to non-organized water.  The clouds of positively charged ions tend to repel each other and hold the adjacent silicate layers apart against the forces of attraction.  Montmorillonite expands with decreasing concentration of sodium chloride.

If the clouds become deep and diffuse enough, they will cause the clay to become separated into its finest discrete particles.  Such a system is said to be dispersed.  If the particles are less than about 0.5 microns in diameter, the suspension will have colloidal properties.  When the system is allowed to stand without stirring, more oriented water may form causing it to gel and develop a certain amount of strength and elasticity which is immediately destroyed upon stirring.  This is called thixotropy.  If the suspension is in relatively fresh water and highly saline water is introduced, the clay may flocculate and potentially plug pore restrictions (Wilson and others, 2014).

Kaolinite adsorbs water in a manner similar to montmorillonite.  The thickness of the adsorbed oriented water is likely to be no greater than that of montmorillonite (Grim, 1953) and is probably of the order of 10 angstroms (Street, 1961).  Judging from the properties of kaolinitic sandstones, this adsorption must be on the surfaces of multilayered flakes and does not penetrate between individual silicate layers.  Since there is no significant replacement within the kaolinite lattice, a net negative charge on the silica sheet must be developed at the broken edges where multivalent anions may replace exchangeable anions with less charge.  Only part of the negative charge associated with the multialent anion is used by attraction to the exposed cation of the lattice and the net negative charge of the particle is therefore increased.  This  negative charge controls the ability of the particles to develop an ionic cloud.  Changes in pH may change the charge distribution on kaolinite particles and cause disaggregation and mobilization (Wilson and others, 2014).

The water adsorption and swelling properties of illite appear to be intermediate between montmorillonite and kaolinite.

The degree of dispersion in a clay-water-ionized salt system is generally determined by the diffuseness of the cloud of cations about the clay particles.  Several  factors affect the diffuseness:

There is not much evidence that chlorite causes formation damage.  It is a relatively non-reactive clay mineral. (Wilson and others, 2014).

The behavior of clay minerals described above is based on observation at ambient temperatures and pressures in laboratories not in deep reservoirs where the temperatures and pressures are much greater.

Exchange of Adsorbed Ions of Clay Minerals

Base exchange is a stoichiometric exchange of ions in solution for exchangeable ions in the clay mineral lattice.  The order of replacement is not the same for each clay mineral but varies according to such things as the size of the holes in the lattice, the seat of the lattice charge, and epitaxial fit.  The order of replacement  given by Griffiths is

Br > Ba > Ca > Mg > K > Na  > Li .  Thus at equivalent concentrations calcium well replace sodium, etc (Griffiths,1946).  This order is not universally true.  The base exchange capacity of minerals important in reservoir technology is:

Thus it is possible to alter the amount of adsorbed water and the degree of dispersion of the clay minerals by chemical treatment.

Effect of Clay Minerals on the Permeability of Reservoir Rocks

Clay minerals are capable of changing the permeability of rocks by swelling and, in effect, reducing the cross-sectional area of the pore space available for flow.  They are also capable of being transported to the pore connections which they may plug.  The plugging action of clay minerals is aided because the ability to swell and hydrate results in loosening the flakes that are plastered on sand grains.  This loosening is likely to occur when the composition or concentration of the interstitial solution is changed.

Another possible mechanism whereby clay minerals reduce the apparent permeability of a rock has to do with electroviscosity.  The electroviscous effect results whenever a charged mineral surface is in contact with a moving aqueous solution in a capillary medium.  When such a surface and liquid are in contact, there is an equal and opposite charge in the liquid which exactly neutralizes the mineral surface charge.  Such a phenomenon was discussed in a previous section in connection with osmotic phenomena.  As the liquid is forced past the charged surface a potential difference is set up in the direction of pressure drop.  The impression of a potential difference causes a back electroosmotic movement in the liquid.  The result is that less fluid flow through the tube than would be expected, and this is the electroviscous effect.

Other things being equal, when the surface charge density is large the  electroviscosity is larger.  Clay minerals,with a high surface charge density, should result in a relatively large electroviscosity.  The magnitude of the effect of clay minerals on  permeability has been determined in numerous experiments and observations.  Since many of the papers on this subject are somewhat repetitious, only a few representative or especially interesting ones will be discussed here.

Johnson and Beeson (1944) studied the permeability of hundreds of reservoir sand samples from 23 oil fields, mostly in California.  They present tables which compare permeabilities of cores to salt water and fresh water.  This data shows that the fresh water permeabilities are significantly lower than the salt water permeabilities.  The core with the lowest permeability ratio had a fresh water permeability of zero and a saltwater permeability of 3050 millidarcys (md).  The highest permeability to salt water for any core was 23,600 md and the same core's permeability to fresh water was  9.9 md.  Such great effects are probably due to a high sodium montmorillonite content.  Although it does not  make the results any less convincing, Johnson and Beeson (1944) point out that it is not possible to measure the water permeability of an oil bearing sandstone without first flushing the core with a solvent and removing the solvent.  This operation undoubtedly has some effect on the interstitial clay.

Kerston (1946) showed there was a 2 to 1 improvement in initial productivity index of certain wells completed with oil-base drilling mud as compared with fresh-water clay-base mud.  He stated that the higher index for oil-base completions was apparently maintained through much of the well history.  Productivity index is the barrels of liquid produced per day per pound per square inch mean pressure differential between reservoir and well bore.  An oil-base mud is one which consists entirely of oil in the place of water and therefore eliminates the possibility of contaminating the reservoir with fresh water derived from the mud.  In preparing oil-base drilling mud it is possible to utilize the swelling properties in oil of reaction products of long-chain amines and bentonite.  Such oleophilic bentonites impart both gel strength and low fluid loss to oil-base muds, and also enable the emulsification of any water encountered during use into the mud as a stable water in oil emulsion.

Wade (1947) agreed with Kerston's results and also concluded that for wells drilled with water-based drilling mud the productivity index is higher the shorter the time the mud stood against the reservoir formation.  This deterioration of  permeability with time may be due to the action of several different phenomena:  

The clay minerals react very quickly to changes in the electrolytes (Nahin, 1955) so that their speed o reaction probably does not contribute significantly to the increase in damage with time.

This relationship of formation damage with time is important if wells are drilled through shallow potential producing sands and deepened to deeper horizons before doing any productivity testing.  When such a practice is followed the mud may stand against the formation for a long period of time .  It is possible that under such conditions of reduced permeability a good petroleum accumulation would appear non-commercial on the basis of drill stem testing.  

Dodd and Barnes (1955), working with reservoir samples from fields all over the United States, found that water sensitivity could be predicted fairly accurately by measuring the intensity of the x-ray diffraction peak of the glycerol-expanded basal spacing.  He found that large concentrations of non-expandable kaolin, chlorite, and mica clay minerals did not produce serious water sensitivity effects in the absence of expandable minerals, and he postulates that the expandable minerals are the most general cause of significant water sensitivity.  Samples with large concentrations of kaolinite, chlorite and mica clay minerals did not appear to be sensitive in the absence of expandables.

Dodd also points out that higher permeabilities can tolerate higher concentrations of expandable clay minerals before exhibiting serious water sensitive behavior.  Additionally, he found that reduced permeabilities caused by flowing fresh water can usually be reversibly increased by flowing brines and the cycle can be repeated.  This indicates that swelling rather than plugging is the most gernaral cause of sensitivity.  Also where plugging is involved, the non-expandable minerals lining pore walls are less easily separated from the walls than the expandables.

Nowak and Krueger (1951) experimented with consolidated sand cores obtained with oil-base mud from the Stevens zone of the Paloma Field, California.  The cores contained montmorillonite .  Novak restored the cores to their original reservoir state and measured a restored state permeability.  Then he flowed water with various salt concentrations through the cores after which he reintroduced oil.  He found, for example, that after saturation with distilled water only 26% of the initial permeability to oil was recovered.  In addition, he found that the core restored to initial reservoir conditions had 32.5% water saturation but after flushing with fresh water and reintroducing oil the water saturation was 45.8%.  This greater water saturation was thought to be due to water adsorbed by clay minerals plus the effect of the reduction of pore space.  Nowak and Krueger also observed that, while flowing fresh water at a 50 psi pressure differential, montmorillonite was produced from the core, showing definitely that montmorillonite can be transported within the reservoir.

Effect of Drilling Mud and Filtrate Invasion

Nowak and Krueger also performed some experiments on drilling mud and filtrate invasion under simulated bottom-hole drilling and circulating conditions involving jetting and scraping (to simulate drilling conditions) and at 1000 psi pressure gradient across "cores" of inert aluminum hydroxide. They observed that clay particles were included in the filtrate emerging from the cores when clay-water base and emulsion mud was used, and that more clay emerged from high permeability cores than from low permeability cores.  These emerging particles varied in size from a small fraction of a micron to over one micron in diameter.  The average size of the pore spaces were o fthe magnitude of 20 microns.  For clay-water and emulsion muds, the filtrate losses are about ten times greater during jetting-scraping tests than during circulating tests with no jetting and scraping; whereas for oil base mud the difference is small.  Clay-water base and emulsion drilling muds have very similar and high filtrate losses while oil-base filtrate losses are smaller by a factor of approximately ten.  Inert cores of low permeability exhibit higher filtrate loss than cores of high permeability in both jetting-scraping and circulating tests.  This may be due to greater invasion of clay and the resultant plugging action.  When inert cores of 14 to 27 md permeability were subjected to circulation of oil base, clay-water base, and emulsion mud mud under simulated bottom-hole conditions of circulation without jetting and scraping, nearly all the original permeability could be recovered by backflowing 5000 pore volumes of oil.  When jetting and scraping was added, cores with permeabilities of  14 to 35 md recovered ofer 85% of their origianl oil permeability on backflow tests, but cores with 243 to248 md recovered only 80% or less.  In general, Nowak and Krreger's data indicate that greater permeability damage around a well bore results from interaction  of filtrates with clay in the formation than from mud-particle intrusion.

Bertness (1853) performed experiments on cores from the same formation as Nowak and Krueger.  He concluded, however, that nearly all the permeability to coil could be restored after water damage,  Miller and others (1946) did work on the same formation which agred with Nowak and Krueger's results.

Manipulation of the Effects of Clay Minerals on Permeability

Bertness also performed an experiment in which he investigated the permeabilities  of the montmorillonite bearing cores before and after conversion of the clay to bentonite by treatment with quarternary ammonium compounds.  The oil permeabilities of the cores were severely reduced by conversion of the clay minerals to bentonite but the water permeabilities were greatly increased.  Such a reaction might have some applications related to fresh water  and to water injection in secondary recovery operations.  

The above results show that the permeability of reservoirs containing swelling clay minerals can be manipulated by replacing unfavorable ions by favorable ones.  In some cases (perhaps most) such manipulation can be accomplished without permanent damage to permeability by plugging.  Proper experimental work should help predict the behavior of a reservoir rock.  Reservoirs containing non-swelling clay minerals are probably not very water sensitive and their effect on any specific reservoir would  have to be determined by experimentation.  These minerals may have a greater tendency to plug pore restrictions than montmorillonite because of their generally large grain size.  Changes in brine concentration may aid in loosening the non-swelling minerals from the walls of the pores.

The clay minerals in a formation can contribute greatly to the permeability damage around a borehole, and care must be taken to use drilling mud which will not cause economically important damage and to use completion techniques which correct or reduce the damage.  Such techniques must be tailored to the particular formations concerned.  No generalizations can be made which would have universal application.


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