"the role of clay minerals in influencing porosity and permeability...

Clay Minerals (1982) ! 7, 41-54.

T H E R O L E OF C L A Y M I N E R A L S IN I N F L U E N C I N G P O R O S I T Y A N D P E R M E A B I L I T Y C H A R A C T E R I S T I C S IN T H E B R I D P O R T S A N D S

OF W Y T C H F A R M , D O R S E T

K . A . M O R R I S AND C. M. S H E P P E R D *

Exploration Department, Gas Council (Exploration) Ltd., 59 Bryanston Street, London WIA 2AZ, and * British Gas Corporation, Research and Development Division, London Research Station, Michael Road,

London SW6 2AD, U.K.

(Received 3 July 1981)

A B S T R A C T: The Bridport Sands is a widespread marine sandstone of Lower Jurassic age found in much of southern England. It is a very fine grained, moderately sorted quartz-arenite and is characterized by the alternation of friable and hard calcareous-cemented layers. The sands form the upper reservoir in the Wytch Farm Field, Dorset, which is currently producing at the rate of around 4000 barrels per day. Investigation of core material to assess the suitability of water injection for gas/oil recovery has shown that significant reductions of liquid permeability compared to air permeability occur. These reductions vary from 3096 or less in the best quality reservoir to more than 70% in low-permeability sandstones. Clay minerals in the Bridport Sands comprise mainly kaolinite and mixed-layer clays of both the illite-chlorite and illite-smectite types. Small amounts of vermiculite and chlorite also occur. The kaolinite is found as loosely-attached, discrete particles, whilst the mixed-layer clays form patchy pore linings. The permeability reductions may be explained by: (i) the adsorption of water and expansion of poorly-crystalline mixed-layer iUite-smectites causing blockage of pore space (this reduction is largely reversible) and (ii) the physical movement of authigenic kaolinite crystal aggregates blocking pore-throats (this reduction is largely non-reversible). The pore-size distribution, clay particle sizes, the distribution of the clays within the pore space, and the composition of the clays are all important factors in controlling porosity/permeability relationships and permeability reductions in the friable reservoir intervals in the Bridport Sands.

The Bridport Sands belong mainly to the uppermost zone of the Toarc ian Stage of the Lower Jurassic. A small port ion of the Upper Bridport Sands belongs to the lowest zone of the Bajocian Stage of the Middle Jurassic (Davies, 1969). The sands form the upper reservoir in the Wytch F a r m Field, Dorset (Fig. 1), a field operated by Gas Council (Exploration) Ltd. on behalf of the B P / G C ( E ) partnership (Colter & Havard , 1981). This reservoir contains 35.5 o API oil, which is currently being produced at the rate of approxi- mately 4000 barrels per day (Hinde, 1980).

As the gas/oil ratio of the Wytch Farm crude is very low and the reservoir pressure is also low (<1500 p.s.i.), a water-injection p rogramme has been developed to maintain downhole pressure and increase recovery. Initial tests on Bridport Sands core material showed a dramat ic decrease in liquid permeabilities compared to air permeabilities. To explain these reductions an extensive petrographic and petrophysical s tudy was undertaken and the more significant results of this s tudy are presented here.

�9 1982 The Mineralogical Society

WAREHAM 2<). :iiiiiii!::i=:)ii!i~Va r e l e

STOBOROUGH 1

JARNE 'G' 1

Poole~Harbour

*.'/x WYTCH FARM

KIMMERIDGE 1

o 1 2 I I i I o 1

I S L E O F

~'~ ~:: Corfe Castle

P U R B E C K

,~ ENCOMBE 1

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LOCAT ION MAP

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I, I, [,

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~/GAULr I LOWER GREeNSAN0 ~ I ~

IKELLAWAyS ~ D S ~ I ~ - - ,

I U~PER RAGS E O U I V * C E ~ I FULLE~'S EARTH

r

. IDOLE LtAS~ ~ LIMESTONE

~ ~ ANHYDR ITE i W H I T E L I A S L I M E S T O N E , I ~ , -

, ~ U M e S T O N E BANOS

~ - - ~ ~ B~CC~A

~ MET*MO~PHPCS

DSrONE ~ROUP

s~

GROOP

?

FIG. 1. Wytch Farm Oilfield, Dorset. Location map and generalized stratigraphic column (after Colter & Havard, 1981).

Clay minerals in the Bridport Sands, Wytch Farm 43

G E N E R A L P E T R O G R A P H Y A N D M I N E R A L O G Y

The Bridport Sands is a very fine-grained sandstone with an average mean grain size straddling the very fine sand/coarse silt boundary of the Wentworth size scale (Pettijohn et al., 1972, p. 71). At outcrop on the Dorset coast, and at Wytch Farm itself, the sandstones show a regular alternation of friable sand and hard calcareous-cemented sandstones (Colter & Havard, 1981). The sandstones are intensely bioturbated, only a few horizons showing any primary sedimentary structures.

Principal components

Angular quartz makes up the bulk (60-70%) of the friable reservoir sandstones. Optically-continuous authigenic overgrowths are developed on a small proportion of the quartz grains. Feldspars comprise 5-15% of the sandstone. Orthoclase and microcline are most common and also show some development of authigenic overgrowths. Occasional grains of albite and oligoclase are seen. Muscovite comprises 5-10% of the detrital particles, occurring as elongate flakes up to 500 #m in length. Biotite occurs in trace amounts. Other minerals which occur in minor amounts as detrital particles include glauconite, chert, garnet, pyrite and magnetite.

Cementing minerals

Ferroan calcite forms the bulk of the pore-filling cement and occurs in widely varying amounts ranging from 50% or more in the calcareous-cemented non-reservoir sandstones to only 10-15% in the friable reservoir sandstones. Minor amounts of dolomite also occur as cement.

Matrix materials

Grain-size analysis, performed by a combination of dry-sieving and sedimentation methods (Folk, 1965), shows that the Bridport Sands are strongly fine skewed with up to 30% of the sediment consisting of medium to fine Silt and clay. The matrix consists of silt-grade quartz, feldspar and mica with minor pyrite and magnetite. However, the bulk of the matrix is composed of clay minerals, many of which are larger than clay grade (>2 grn e.s.d.).

C L A Y M I N E R A L S

Composition

X-ray diffraction analysis shows that the clay mineral assemblages in the Bridport Sands are dominated by kaolinite, illite and mixed-layer clays; trace amounts of vermiculite and chlorite are also present. Kaolinite tends to be well-ordered with a basal spacing of around 7.15 A, although some samples show a distinct broadening of the 001 reflection suggesting the presence of some poorly-ordered kaolinite (Grim, 1968, p. 130). Illite occurs as a distinct 10 A phase and also as a component of the mixed-layer phases which are represented on the diffraction traces of the air-dried clays as a broad, diffuse

44 K. A. Morris and C. M. Shepperd

BRIDPORT SANDS WYTCH FARM < 2)Jm - - DRY

- - - - - - ETHYLENE GLYCOL

. . . . . . HEATED ~ 4 0 0 * C

I

,ii

f\ S

DEGREES 2 e C o - K ,=(.

FIG. 2. X-ray diffraction trace of <2pm fraction from the Bridport Sands, Wytch Farm, showing very little expanding clays.

shoulder to the 10 A illite peak (Fig. 2). These phases are particularly difficult to characterize but appear to consist of two types.

I. A non-expanding illite-chlorite mixed-layer phase which has some degree of ordering in the structure, as shown by a small but consistent 14 A peak. Heating to 400 and 600~ does not collapse the chlorite interlayers (Fig. 2).

2. A randomly-interstratified illite-smectite with differing proportions of expanding layers, similar to those described by Weaver (1956, p. 206). This interlayering is very poorly defined and only rarely produces a distinct 17 A peak on saturation with ethylene glycol (Fig. 3). Heating to 400~ collapses this material to 10 A (Fig. 3). Amounts of smectite layers cannot be estimated using e.g. the method of ~rod6n (1980) as all samples contain discrete illite. A comparison of the traces with computed theoretical XRD patterns of illite-smectite mixtures (Reynolds, 1980) suggests that the percentage of expanding layers does not exceed 40% and is commonly 20% or less.

Small amounts of discrete vermiculite occur in many samples. This mineral is unaffected by all treatments except heating at 600~ The very small 14 A peak remaining after heating was assumed to be due to chlorite, its presence being confirmed by subsequent acid treatment and thin-section analysis.


~ BRIDPORT SANDS WYTCH FARM <2~um DRY

ETHYLENE GLYCOL

. . . . . . HEATED tO 400~ d ' ~ f '

ii ~ /" ii j i i ~/r

- . ~

DEGREES 2 0 Co-K o<

FIG. 3. X-ray diffraction trace of < 2 #m fraction from the Bridport Sands, Wytch Farm showing expanding mixed-layer illite-smeetite clays.

Size distribution and quantification

Quantification of the X-ray diffraction patterns were made using the method of Brown as outlined in Weir et al. (1975), the kaolinite present being used as an internal standard. Consistent results were obtained throughout the reservoir, the largest variation being changes in abundance with changes in particle size. Fig. 4 shows the typical variation in the relative abundance of the different clay minerals in various size fractions. Percentages of kaolinite, chlorite and vermiculite remain fairly constant but there is a conspicuous increase in the amount of mixed-layer clays in the finer fractions.

Significance of the mixed-layer illite-smectite clays

The identification and interpretation of the mixed-layer clay is important as it forms the only expanding component in the reservoir. Outcrop material and shallow samples of the Bridport Sands show much higher percentages of expanding layers in the illite-smectite and often a distinct 17 A peak is obtained on glycolation. In some samples from outcrop, well-ordered smectite is seen (Morris, 1979, p. 117).

It would appear from this evidence that there is a trend of smectite to illite-smectite transformation with depth and also a trend of decreasing percentage of expanding layers in the illite-smectite with increasing burial depth. This trend is similar to that observed by Foscolos & Powell (1979) in sandstones from the Canadian Arctic and appears to be a gradual dehydration process with increasing burial depth.


A

E

n,- UJ I-- UJ ,r m a

o z . . I I-. I-- LIJ U)

U.I _> I - O I.IJ LI. IJ . LU

32-

20-

16-

8 -

4 -

110

i 210 310 40

KAOLINITE

1() 20 30 40 1() 20 30 40

ILLITE MIXED-LAYER CLAYS

fl 5 10

VERMICULITE & CHLORITE

FIG. 4. Variation in clay abundance in various size fractions, Bridport Sands, Wytch Farm.

FIG. 5. Scanning electron micrograph of discrete-particle kaolinite crystal aggregates attached to a pore wall. Crystal faces of authigenic quartz overgrowths are also visible. Scale bar = 4/~m.


Clay morphology

Neasham (1977) divided dispersed clay particles in sandstones into three general types: (i) discrete particles, (ii) pore-lining clays and (iii) pore-bridging clays. In the Bridport Sands, SEM examination shows the existence of both discrete particles and pore-lining clays. Pore-lining clays may occasionally extend into the pore-bridging category in areas where sufficient clay is present.

Kaolinite occurs largely as discrete particles (Fig. 5), mainly in the form of pseudo-hexagonal, sub-idiomorphic platy crystals loosely attached to pore walls, or as an intergranular pore fill. Individual crystals are often up to 12/tm across and form 'books' or aggregates up to 20/~m in size. These aggregates are loosely stacked on their basal planes and are generally not intergrown (Fig. 6). They are characteristically scattered throughout the pore system and do not form intergrown crystal frameworks. Authigenic kaolinite crystals are often associated with potassium-feldspars and micas. Such kaolinite crystals have the effect of reducing intergranular pore volume (Wilson & Pittman, 1977) but more importantly can act as migrating fines in the pore system (Neasham, 1977).

The iUite and mixed-layer clays on the other hand occur largely as pore-lining clays firmly attached to the pore walls and forming a relatively continuous clay mineral coating (Fig. 7). The crystals are up to 8/tm in size and are often oriented perpendicular to the pore wall. Crystals are commonly inter-grown in a 'cornflake-like' arrangement although the edges of the flakes are generally smooth and show none of the crenulations or lath-like

FIG. 6. Scanning electron micrograph of detail of kaolinite crystals showing loose basal plane stacking arrangement of pseudohexagonal crystals. Scale bar = 4 #m.


FIG. 7. Scanning electron micrograph of pore-lining network of mixed-layer clays showing intergrown cornflake-like arrangement. Scale bar = 4/zm.

projections seen by Wilson & Pittman (1977, p. 23) in mixed-layer illite-smectites. The cornflake-like arrangement produces extensive micropore space.

This entrapment of water results in 'static' permeability reduction (Todd & Tweddie, 1978) and is caused by clay expansion both within the structure and by increased separation of individual particles. This results in restriction and blocking of pore throats.

SEM analysis suggests that kaolinite crystal aggregates are largely responsible for the observed dynamic permeability reduction, as the crystals are of relatively large size compared to the pore size and also appear to be only loosely attached to detrital grains. The configuration of the illite and mixed-layer clays is such that it is unlikely that they contribute greatly to dynamic permeability reduction.

The static permeability reduction is more difficult to attribute. The water adsorption and expansion of smectite has been well-known for many years and has been accurately quantified in the laboratory (Mooney et al., 1952). However, the nature and amount of adsorption and expansion of mixed-layer iltite-smectites, although known from several reservoirs (Almon & Davies, 1977), is difficult to quantify given the uncertainty as to the actual amount of expanding layers present. It is likely, however, that even small expansions of illite-smectite would be sufficient to restrict or prevent water movement given the very small dimensions o f the pore throats. In addition, the cornflake-type occurrence of all the mixed-layer clays causes a very large increase in pore surface area, resulting in retention of water by surface tension forces. Such water-retention would also restrict water movement through pore system. Retention of water by non-expanding clays such as illite has been reported by Routson et al. (1972).


1000

100-

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O.1

BRIDPORT SANDS WYTCH FARM FIELD

\ " ' ; ~ \ . . . \ ~\ - . \ �9

\ �9 \ . X �9 �9 �9 � 9 1 4 9

\ - , \ �9 \ " " \ . ~\ . \ �9

~. - \ . .

KEROSENE FORMATION WATER

o lb 2'0 3'0 ,~ s'o do ,b do ~ PERMEABILITY REDUCTION

FIG. 8. Plot of % permeability reduction compared to air when using formation water or kerosene.

;o


THE E F F E C T S OF CLAYS ON P E T R O P H Y S I C A L P R O P E R T I E S

Porosity~air permeability

The Bridport Sands give low values of permeability for corresponding porosities compared to many other reservoirs, although this is directly related to the very fine grained nature of the sands and therefore the small size of individual pores. Theoretical considerations of spheres of similar grain size with tangential packing arrangements give a maximum possible pore size of 20 #m. The actual mean pore size measured by mercury injection varies from 10 to less than 0.1/~m. These figures are confirmed by direct SEM measurement.

A irpermeabiltty/liquidpermeability

As mentioned in the introduction, Kalr/KLiqutd reductions were found in the Bridport Sands. These reductions were found in fresh water, sea-water and formation water. Fig. 8 shows a typical range of permeability reductions for the range of reservoir permeabilities encountered.

When these experiments were carried out with kerosene only, a 20-30% reduction in air permeability was observed compared to the 30-80% seen when formation water was injected (Fig. 8). This suggests that the former reduction (using a non-polar solvent) is due to particle movement in the pore space and not due to clay adsorption and expansion. Drying of the samples and re-testing yielded only a partial recovery in permeability, also suggesting particle movement. This movement of particles results in 'dynamic permeability reduction' (Todd & Tweedie, 1978) and is caused by clay particles moving and becoming trapped at sub-critical pore throats. This dynamic permeability reduction is generally non-reversible.

The remaining observed reduction in permeability is achieved only with polar liquids, and subsequent drying of the samples largely restores the original permeability. This reduction is due to the adsorption and expansion of clay particles within the porespace.

Air~mercury capillary pressure curves

Discrete particle, pore-lining and pore-bridging clays show basic difference in capillary presure curves indicative of their pore network properties (Neasham, 1977). These curves differ distinctly both in shape and location (Fig. 9). Typical capillary pressure curves for the Bridport Sands show strong affinities to those of sandstones with pore-lining clays (Fig. 9). This indicates that the mixed-layer pore-lining clays in the Bridport Sands have the largest effect on pore network properties. However, the low initial mercury injection pressures shown by Bridport Sands samples are similar to values for sandstones with discrete particles, suggesting the influence of discrete kaolinite particles at low pore volumes.

C O N C L U S I O N S

Permeability reduction to water in the Bridport Sands can be explained by the type and distribution of the clay minerals within the pore system. Much of the observed permeability

1000

100 -

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>. gr

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I I ~, % I I I I I I

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\~ ~'., ,. ,, % , . �9 ............. ......~ '~~ \ \ \ "~ \ "~':Z" ...... " .............. , \'~ \ \ ~ , " \ ~ "-:~'~. ......... ",., \ \ \ '~ ~. \ , ' , , ~ ---.. . . .~" .......... ,.. ,..~ ~. \ \ \ ~ " \ \ "---, . ..,,

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Typical Curve for Bridport Sands

Pore Lining

......................... Pore Bridging

. . . . . . . . . . . . . Discrete Particle

l o o ' o~ ' 6 b ' 4 b ' 2'0 ' PV OCCUPIED BY Hg(~

FIG. 9. Typical air /mercury capillary pressure curve for Bridport Sands, Wytch Farm, compared to those of other sandstones with differing dispersed clay morphology (modified from

Neasham, 1977, fig. 7).


reduction to water can be attributed to the adsorption and expansion of pore-lining clays. Only small expansions of mixed-layer illite-smectites are sufficient to block or partly block the small pore throats in this very fine-grained reservoir. Other non-expanding pore-lining clays may contribute to this permeability reduction by retention of water by surface tension forces. The permeability reduction can be up to 50% of the original air permeability in the tighter (more compact) sandstones. This reduction is largely reversible. Air/mercury capillary pressure curves confirm the importance of pore-lining clays in controlling pore network properties.

The small pore throats in the Bridport Sands can also be easily restricted or blocked by the physical movement of kaolinite particles which occur as loosely attached or free aggregates in the pore space. This form of permeability reduction occurs with polar or non-polar liquids, is largely non-reversible and can account for up to 30% of the observed reduction.

Clay minerals in the Bridport Sands show size segregation. Mixed-layer clays dominate the finer fractions whilst many kaolinite crystals are significantly larger than clay grade.

Conventional whole rock and clay analysis are often insufficient to characterize fine-grained reservoirs. In this case the high proportion of large detrital muscovite mica obscures the mixed-layer clays in whole rock analysis whilst the small proportions of expanding layers in the mixed-layer clays can easily be overlooked in conventional clay X R D analysis.

In designing water injection facilities for the Bridport Sands, account has to be taken not only of the apparent water sensitivity of the clays but also of the very small pore throats. I f kaolinite particles of 10 #m or less can block pore throats, the injected water must contain virtually no particulate matter, otherwise non-reversible reservoir damage will occur.

ACKNOWLEDGMENTS

We thank the management of the British Gas Corporation and BP Petroleum Development Ltd. for permission to publish this paper and to Dr V. S. Colter and Miss W. A. Matthews for their critical reading of the manuscript. Mr R. B. Sinha provided SEM facilities. The illustrations were prepared by the Cartographic Section of the Drawing Office of British Gas.

REFERENCES

ALMON W.R. & DAVIES D.K. (1977) Understanding diagenetic zones vital. Oil gas J. June 6 209-216. COLTER V.S. & HAVARD D.J. (1981) The Wytch Farm Oil Field, Dorset. Pp. 495-503 in: Petroleum Geology

of the Continental Shelf of North-West Europe (L. V. Illing & G. D. Hobson, editors). Heyden, London. DAVIES D.K. (1969) Shelf sedimentation: an example from the Jurassic of Britain. J. Sedim. Petrol. 39,

1344-1370. FOLK R.L. (1965) Petrology of Sedimentary Rocks. Hemphill, Texas. FOSCOLOS A.E. & POWELL T.G. (1979) Catagenesis in shales and the occurrence of authigenic clays in

sandstones, North Sabine H-49 well, Canadian Arctic Islands. Can. J. Earth Sci. 16, 1309-1314. GRIM R.E. (1968) Clay Mineralogy. 2nd. ed. McGraw-Hill, New York. HINDE P. (1980) The development of the Wytch Farm Oilfield. Inst. Gas Engineers, 46th Ann. meeting, paper

1133. MOONEY R.W,, KEENAN A.G. & WOOD L.A. (1952) Adsorption of water vapor by montmorillonite, Part II,

effect of exchangeable ions and lattice swelling as measured by X-ray diffraction. J. Am. Chem. Soc. 74, 1371-1374.

MORRXS, K.A. (1979) An integrated facies analysis of Toarcian organic-rich shales and contiguous deposits in Great Britain. PhD Thesis, Univ. Reading.

Clay minerals in the Bridport Sands, Wyteh Farm 53

NEASHAM J.W. (1977) The morphology of dispersed clay in sandstone reservoirs and its effect on sandstone shallness, pore space and fluid flow properties. Soc. Petroleum Engineers of AIME, Denver, SPE 6858, 7 pp.

PEYrUOHN FJ., POTTER P.E. & SIEVER R. (1973) Sand and Sandstone, Springer-Verlag, Berlin. REYNOLDS R.C. (1980) Interstratified clay minerals. Pp. 249-303 in: Crystal Structures of Clay Minerals

and Their X-ray Identification ((3. W. Brindley & G. Brown, editors). Mineralogical Society, London. RotrrsoN R.C., KrrrYmcx J.A. & HOPE E.H. (1972) Interlayer hydration and broadening of the 10 ,/~ X-ray

peak in illite. Soil Sci. 113, 167-174. SRooofi J. 0980) Precise identification of illite/smectite interstratifications by X-ray powder diffraction. Clays

Clay Miner. 28, 401-411. TODD A.C. & TWEEDIE J. (1978) Total rock characterisation of North Sea sandstones with particular reference

to interstitial clays. Europec, London, paper 93. WEAVER C.E. (1956) The distribution and identification of mixed-layer clays in sedimentary rocks. Am. Miner.

41,202-221. WIER A.H., ORMEROD E.C. & EL MANSEY I.M.I. (1975) Clay mineralogy of sediments of the western Nile

Delta. Clay Miner. 10, 369-386. WILSON M.D. & PITrMAN E.D. (1977) Authigenic clays in sandstones: recognition and influence on reservoir

properties and paleoenvironmental analysis. J. Sedim. Petrol. 47, 3-31.

R E S U M E: Les Sables de Bridport constituent une formation marine d'fige jurassique inf6rieur tr6s r6pandue en Angleterre du Sud. Il s'agit d'une ar6nite quartzeuse de granulom&rie tr/~s fine, caract6ris6e par l'alternance de couches friables et endurcies par un ciment calcaire. Ces sables forment le r6servoir sup6rieur de la r6gion de Wytch Farm, Dorset, qui a une production courante de 4000 barils par jour. Des recherches sur les mat6riaux de cette formation, en vue d'&udier la possibilit~ d'injecter de reau pour r~cup~rer l'huiie ou le gaz, on montrb des r&iuctions importantes de la perm6abilit6 aux liquides par rapport ~, la perm6abilit~ h rair. Ces r~.ductions varient de 30% au moins darts le r~servoir de meilleure qualit6 h plus de 70% dans les gr6s peu perm6ables. Les min6raux argileux des sables de Bridport comprennent essentieUement la kaolinite et des interstratifi6s illite-chlorite et illite-smectite. On y trouve aussi des faibles quantit6s de vermiculite et de chlorite. La kaolinite se trouve sous forme de particules discr&es, peu li6es, alors que les interstratifi6s forment par chevauchement les rev&ements des pores. Les r6ductions de perm~abilit6 peuvent s'expliquer par (i) l'adsorption de l'eau et le gonflement des interstratiii6s mal cristallis6s du type illite-smectite, provoquant le blocage de respace des pores (cette r6duction est essentiellement r6versible), (ii) le d6placement des agr6gats de kaolinite authig6ne encombrant les orifices des pores (r6duction pratiquement irr6versible). La distribution des tailles de pore, les tallies des particales d'argile, la distribution des argiles dans l'espace poreux et la composition des argiles sont tous des facteurs importants dans le contr61e des relations porosit6-perm~abilit6 et des r6ductions de perm6abilit6 dans les intervalles friables du r6servoir des Sables de Bridport.

K U R Z R E F E R A T: Die Bridport Sands sind ein weitverbreiteter marine-Sandstein des unteren Jura in Siidengland. Er setzt sich aus sehr fein gek6rntem, gelegentlich sortiertem Quarz-Arenit zusammen und ist dutch den Wechsel yon spr6den und batten, kalkverkrusteten Schichten gekennzeichnet. Die Sandsteine bilden die obere Speicherschicht im Wytch Farm Fieldl Dorset, welches gegenwiirtig eine Produktionsleistung yon 4000 Barrel pro Tag besitzt. Die Erforschung des Kernbereiches zur Absch~itzung der M6glichkeit einer Wasserinjektion f'tir die Gas/OI-Gewinnung zeigte, dab signifikante Abnahmen tier Fliissigkeitspermeabilit~it im Vergieich zur Luftpermeabifit~it auftreten. Dieser Verlust vafiiert yon 30% oder weniger im Speicher mit bester Qualit~it, his zu 70% und dariiber in gering permeablen Sandsteinen. Die Tonminerale der Bridport Sands bestehen hanptsiichlich aus Kaolinit und Illit-Chlorit sowie Illit-Smectit WechseUagerungsmineralen. Nebenbei sind noch geringe Mengen Chlorit und Vermiculit enthalten. Kaolinit kommt in Form von locker gebundenen Einzelteilchen vor, wechselgelagerte Tonminerale dagegen bilden unregelm~ii3ige Porenbel~ige aus. Die Permeabilitlltsabnahmen k6nnen erkl~.rt werden durch: (i) Wasseradsorption und Expansion von schlecht kristallisierten IUit-Smectit Wechsellagerungen, wodurch eine Blockade


des Porenraumes entsteht (weitgehend reversibler Vorgang) und (ii) die physikalische Bewegung authigener Kaolinitkristallaggregate, wodurch Poreng~inge blockiert werden (dies ist ein gr6Btenteils irreversibler Vorgang). Die Porengr6Benverteilung, die Teilchengr6Be des Tones, die Verteilung des Tomes innerhalb der Porenvolumina und dessen Zusammensetzung, sind alles wichtige Faktoren dfir die Kontrolle yon Porosit/its/Permeabilit~its-Beziehungen und Permeabilit~itsverminderungen der spr6den Abschnitte in den Speicherschichten der Bridport Sands.

R E S U M E N : Las arenas Bridport son unas areniscas marinas del jur/tsico inferior que se encuentran ampliamente difundidas en el sur de Inglaterra. Son unas areniscas cuarciferas de tamafio muy fino, con una moderada heterometria y que se caracterizan por una alternancia de niveles, con cemento calcareo, deleznables y compactos. Las arenas forman la parte superior del campo petrolifero de Wytch Farm, que produce unos 4000 barriles diarios. La investigaci6n de materiales procedentes de los testigos de sondeo, para estudiar la conveniencia de utilizar inyecciones de agua para la recuperaci6n de gas y petroleo, ha mostrado qae se producen reducciones significativas en la permeabilidad a los liquidos cuando se las compara frente a la permeabilidad al aire. Estas reducciones varian desde el 30% o menos, en el yacimiento de mejor calidad, hasta mas del 70% en las areniscas de mas baja permeabilidad. Los minerales de la arcilla presentes en las areniscas Bridport son, esencialmente, caolinita e interestratificados ilita-clorita e ilita-esmectita. Tambien se encuentran pequefias cantidades de vermiculita y clorita. La caolinita se encuentra como particulas discretas apiladas libremente, mientras que los interestratificados forman agregados que recubren los poros. Las reducciones en la permeabilidad se pueden explicar por: (1) la adsorcion de agua y expansi6n de los interestratificados ilita-esmectita, pobremente cristalinos, que bloquean los espacios porosos (esta reducci6n es claramente reversible), y (2) el movimiento de agregados de cristales de caolinita autig6nica que bloquean los orificios de entrada de los poros (esta reducci6n no es reversible). La distribuci6n del tamafio de poros, los tamafios de particula de las arcillas, la distribuci6n de las arcillas dentro del espacio poroso y la composicion de las arcillas son los factores mas importantes en el control de las relaciones porosidad/permeabilidad yde las reducciones de permeabilidad en los niveles friables de los yacimientos petroliferos en las arenas Bridport.

"the role of clay minerals in influencing porosity and permeability...

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