Introduction Reservoir wettability is determined by com­plex interface boundary conditions acting within the pore space of sedimentary rocks. These conditions have a dominant effect on interface movement and associated oil dis­placement. Wettability is a significant issue in multiphase flow problems ranging from oil migration from source rocks to such en­hanced recovery processes as alkaline flood­ing or alternate injection of CO2 and water. In this paper, wettability will be discussed mainly in the context of recovery of light Oow-viscosity) oils by waterflooding.

Waterflooding has been widely applied for more than half a century; secondary recov­ery by waterflooding presently accounts for more than one-half of current U.S. oil pro­duction. Many research papers have ad­dressed the effect of wettability on waterflood recovery during this period. For much of the past 50 years, however, a large body of reservoir engineering practice has been based on the assumption that most reservoirs are very strongly water-wet (VSWW); i.e., the reservoir-rock surface al­ways maintains a strong affmity for water in the presence of oil.

The rationale for assuming VSWW con­ditions was that water originally occupied the reservoir trap; as oil accumulated, water was retained by capillary forces in the fmer pore spaces and as films on pore surfaces overlain by oil. Wettability behavior other than VSWW was observed for reservoir core samples, but was often ascribed to ar­tifacts related to core recovery and testing procedures. The majority of reservoir en­gineering measurements have been made on cleaned core with refined oil or air as the nonwetting phase to give results for, or equivalent to, VSWW conditions. Examples of such measurements are laboratory water­floods, determination of electrical resistivity vs. water saturation relationships, and capil­lary pressure measurements for determina­tion of reservoir connate water saturation. Mounting evidence on the effects of crude oil on wetting behavior l has now led to

Copyright 1990 Society of Petroleum Engineers

1476

Wettability and Its Effect on Oil Recovery Norman R. Morrow, SP~, New Mexico Petroleum Recovery Research Center, New MexIco Inst. of Mining & Technology

wide acceptance of the conclusion that most reservoirs are at wettability conditions other than VSWW. This conclusion has led to a resurgence of interest in satisfactory proce­dures for measuring reservoir wettability and determining its effect on oil recovery, especially with respect to waterflooding. De­termination of reservoir wettability and its effect on oil recovery by methods that in­volve core samples will be referred to as ad­vanced core analysis for wettability (ACAW).

Reservoir wettability is not a simply de­fined property. Classification of reservoirs as water-wet or oil-wet is a gross oversim­plification. Various procedures for meas­uring wettability have been proposed. Two methods of quantifying wettability based on rocklbrine/oil displacement behavior, the modified Amott test2 and the USBM test, 3

are in common use. Each method depends on water saturation measurements and re­lated capillary pressures or flow conditions to define a wettability scale. The tests show that reservoir wettability can cover a broad spectrum of wetting conditions that range from VSWW to very strongly oil-wet. With­in this range, complex mixed-wettability conditions given by combinations of preferentially water-wet and oil-wet surfaces have been identified. In this paper, the 'adopted scales of reservoir wettability and their relationships to interface boundary con­ditions are considered together with the dra­matic effects that wettability can have on oil recovery.

Contact Angles, Spreading, and Adhesion Contact Angle and Spreading. Contact an­gle is the most universal measure of the wet­tability of surfaces. Fig. 1 shows idealized examples of contact angles at smooth solid surfaces for oil and water of matched den­sity. Early studies of wetting phenomena showed that the wetting properties of a solid are dominated by the outermost layer of molecules. (Films that result from spreading and other thin adsorbed fIlms are not indi­cated in Fig. 1.) Large change in the wetta­bility of a surface, such as quartz, can be achieved by adsorption of a monolayer of polar molecules so that the outermost part

of the surface is composed of hydrocarbon chains. Extreme change in wettability (see Fig. 1), such as from a or b to e or f, or vice versa, is called wettability reversal. Ad­sorption of polar compounds from crude oil plays a critical role in determining the wet­ting properties of reservoir-rock surfaces.

Many early studies of wetting behavior, even for comparatively simple systems, were plagued by problems of reproduci­bility. Aside from surface contamination, other forms of heterogeneity in chemical composition, surface roughness, and static and dynamic interface properties contribute to the complexity of observed wetting phe­nomena. Large differences in contact angles, depending on whether an interface was ad­vanced or receded, called into question the validity of attempting to describe wettability by a single-valued equilibrium contact angle.

Successful systematic studies of closely reproducible equilibrium-contact-angle measurements have been summarized by Zisman. 4 By use of smooth (often poly­meric), solid surfaces and pure liquids, contact-angle hysteresis was limited to with­in 1 or 2°. In contrast, contact-angle hyster­esis is observed almost invariably for crude-oillbrine systems. Fig. 2 shows ex­amples of contact angles that exhibit small and large hysteresis. Receding angles are generally low ( < 30°) and seldom exceed 60° , whereas a wide range of advancing an­gles is observed. The shaded regions in Fig. 2 show the range of possible contact-angle values for a fixed position of the three-phase line of contact.

Contact-angle measurements on reservoir­crude-oillbrine systems provide one ap­proach to measuring reservoir wettability. For the most extensive set of data yet report­ed,5 contact angles for crude oil and simu­lated reservoir brine were measured at reservoir temperature and ambient pressure. Choice of mineral substrate, usually quartz or calcite, was based on what was judged from petrographic examination to be the predominant mineral at pore surfaces. (There are obvious limitations to represent­ing the rock surface by a single mineral.)

To determine contact angles, two parallel mineral plates are submerged in brine and

December 1990 • JPT

then a drop of oil is introduced between the plates. When the plates are moved relative to each other, advancing and receding con­ditions can be observed. Water-advancing contact angles, (J A' are reported as defining wettability because these are considered relevant to waterflooding. 5 Figs. 3a and 3b show the distribution of advancing con­tact angles observed for quartz and calcite, respectively.

Adhesion Behavior of Crude-Oil/Brine/ Solid Systems. Because of problems in making definitive contact-angle measure­ments, a simpler approach to characteriza­tion of the wetting behavior of crude oils has recently been adopted. 6-8 Smooth mineral surfaces are first pre-equilibrated for about 7 days with brine. A drop of oil is contacted with a surface overlain by brine for a stan­dardized time of 2 minutes. The oil drop maintains a low water-receding angle during enlargement. Upon withdrawal, one of two extremes of behavior are usually observed. Either the drop detaches from the surface (nonadhesion) or the interface boundary re­mains pinned at the three-phase line of con­tact (adhesion) (see Fig. 4).

For adhesion, the contact angle generally increases during withdrawal of oil until the liquid bridge ruptures because of capillary instability; after rupture, a drop of oil still remains on the solid surface. In some in­stances, an excess of surfactant, caused by a decrease in interface surface area after rup­ture, causes the oillbrine interface to become rigid. The interface eventually returns to its normal appearance with a few minutes (see Fig. 4). The area over which the oil drop remains attached to the solid is usually very strongly wetted by oil. Thus, adhesion cor­responds to large hysteresis of contact angle (Figs. 2 and 4). The angle at which the liquid-bridge rupture occurs is not necessari­ly the maximum advancing contact angle.

In testing the adhesion properties of 22 crude oils as a function of brine composition and pH, remarkable similarities in results were obtained for 20 oils, with pH being the dominant factor. 8 Two examples of pH vs. salinity mapping shown in Fig. 5 indicate the general form of these plots. The two oils that did not conform gave adhesion at both

JPT • December 1990

sPEDistinguished Author SERIES

(a) e = 0° (water spreading)

(b) e = 25° (c) e = 600

WATER

mJfIfl;)~l7nm!flm OIL. OIL

J11IJ1II1IIIIJJJlllllj'llllljjjj)lIj)llfllI

(d) e = 100° (e) e = 160° (f) e = 180° (oil spreading)

Fig. i-Idealized examples of contact angles and spreading.

(a) Small Hysteresis (b) Large Hysteresis

Fig. 2-Possible ranges of stable contact angle, 8, for small and large hysteresis of contact angle, with location of oil Indicated for 8 A'

low and high pH. In comparable tests on re­fined oil, adhesion was not observed at any of the tested pH levels.

The transition pH for nearly all the crude oils tested fell within the narrow pH band of about 6.5 ±2. For a given oil, only slight changes in pH at close-to-neutral conditions can cause a drastic change in adhesion be­havior. This may explain the problems often experienced in obtaining reproducible crude­oillbrine/mineral wetting behavior. A crude oil recovered and tested for adhesion be­havior under anaerobic conditions gave re­sults similar to those obtained after deliberate exposure to oxygen. Thus, the general form of the adhesion behavior is not an artifact of oxidation. 8

"Contact angle is the most universal measure of the wettability of surfaces."

1477

f! •

J '5 •

~ E

" z

Non-adhesion

0-20

16

Adhesion 14

f! 12 ·0 ~ Q) 10 II> Q)

a: '5 8

~ E 6

" Z

0-20 20-..0 ~ eo.eo 80-100 1~120 120-140 1<40-180 1eo-18O

Water Advancing Contact Angle

(a) Quartz Water Advancing Contact Angle

(b) Calcite

Fig. 3-Distribution of contact angles for crude-oil/brine systems on (a) quartz (29 measurements), and (b) calcite (30 measure­ments) (after Treiber et al. 5 ).

The similarity in effect of brine proper­ties or wetting behavior observed for all but two crude oils provides a case for the use of crude oil rather than a refined oil as a reasonably definitive and certainly, for reservoir engineering purposes, a more rele­vant model for the oleic phase. Note that the advancing contact angle values for crude oil/brine on quartz (see Fig. 3a) fall into two classes that seem to be consistent with ob­served adhesion tests conducted on glass or silica surfaces. In spite of the chemical com­plexity of crude oils, a distinct possibility exists that crude oils will give consistent re­sults with respect to other phenomena that depend on wetting properties.

Stability of Thin Films. The outcome of adhesion tests depends on the stability of the water film between the solid and the oil drop. Stabilization of the thin film by elec­trostatic repulsion requires that the charge at the brine/solid interface and the net charge of the brine/oil interface have the same sign. Film thickness, generally much less than 100 nm, is determined by a balance between van der Waals attractive forces and repUlsion by electrostatic and hydration forces. 9 The much-less-understood hydration forces come into play in the stabilization of very thin water films at high ionic strength. The high­contact-angle values shown in Fig. 3b are

NONADHESION

..

LOW RECEDING ANGLE «30°)

ADHESION j

=.----,

LOW ADVANCING ANGLE BEFORE DETACHMENT «ad' )

.. /

consistent with adhesion occurring more readily on calcite than quartz because, under comparable conditions, calcite has a more positive surface charge. It follows that the types and distribution of minerals such as carbonate cements or clay minerals at the surface of pore walls, even if they make up only a minor part of the total mineral con­stituents ofthe rock, also play an important role in how crude oil alters wetting proper­ties. With respect to the effect of pH in the reservoir, rock minerals will tend to buffer reservoir pH at some fixed characteristic value that is close to neutraL Because pH cutoff values are also close to neutral, adhe-

AFTER DETACHMENT

TEMPORARY ADHESION

• HIGH ADVANCING ANGLE (>9<1') TYPICAL DROP SHAPE

IF RIGID FILM FORMS UPON RUPTURE

AFTER RUPTURE OF OIL BRIDGE DROP SHAPE JUST BEFORE DETACHMENT

Fig. 4-lIIustrations of interface configurations for adhesion and nonadhesion of crude oil.

1478 December 1990 • JPT

14 14

Moutray Crude, 50· C NS-3,50·C 12 ........... , ., ...... ~. , ....... , ........... ':' .................... : .................... . 12 . ................... , ................................................................. .

~ ~. Noriad.hesion ~ ~. Nonkdhesion . ................... ! ..................... i·· .... ··x· .... ·Adhi,sion·· .......... . 10 .................... : ...................... , ......... ~ ..... Adtlpsloo ............ .

: . ----- ~H cutoff 10

i ~ ----- pH cutoff

I a. •

2

1·;::::;:1;::::::::[:: .................... ~ .................... .;. ..................... ; .................... .

r ~x ~ ····················1.······················~········· ............ :.' ..................... .

I a.

8

6

2

+·~·-::::r:::;;::F ::·:::.::.::·::::J::::::::::::::::::::r:::::::::::::::::r::::::::::::::::::

. . : : . . .:

o

10-3 10-2 10"1 1.,0 101 10-3 10-2 10-1 100 101

Molar Concentration of Na + (a)

Molar Concentration of Na + (b)

Fig. 5-Adheslon maps (pH vs. NaCI brine concentration) for two crude oils (after Buc.kley and MorrowS).

sion tests may not provide a definite predic­tion of the wetting condition that crude oil will establish witl1in a rock.

Just as the outcome of the adhesion test depends on the stability of thin water films, maintenance of water-wetness in a reservoir depends on the stability of thin water films at pore walls. If the water films are unsta­ble, crude oil has access to solid surface in the region of contact and adsorption of polar compounds can drastically, and often per­manently, alter the wetting properties of the solid. The adhesion test studies show ad­sorption to be specific to the area contacted by oil. This type of adsorption gives rise to .

Primary

what is known as mixed wettability because the surface wetting properties of the contact­ed area are distinctly different from the neighboring surface. Differences in adhe­sion and adsorption behavior at different mineral surfaces, together with the effects of rnicroporosity and surface roughness, will obviously contribute to the complexity of mixed-wettability systems.

Asphaltenes. Wettability alteration has often been ascribed to the adsorption of high­molecular-weight colloidal particles known as asphaltenes suspended in crude oil. 10

Asphaltenes are operationally defined as the

100

80

DRAINAGE /(FOrced) ii:"

~(+ve) Secondary

6 ~

DRAINAGE >- 60 (Forced) a:

w IMBIBITION >

(spontaneous) 0 0 w a:

0 ...J

I 6. Sws 6 40 w= --AS WI

IMBIBITION

I It.Sos (F=edi 0 = 4S

wt ~(-ve) /

IAH=lw·lo Secondary / 20 DRAINAGE (Spontaneous)

N w (USBM) = log A1

A2

0 S,,'" 100 0

0

precipitate resulting from adding a large volume of low-molecular-weight hydrocar­bon to the crude oil (typically 40 volumes of pentane). Their role in wettability alter­ation was further confirmed by the obser­vation that deasphalted crude oil no longer exhibited adhesion in the low-pH range. 8

Stabilization of water films by electrical double-layer repulsion in the Athabasca tar sands explains the strong water-wetness often observed for this sand. 11 When a sample of freshly mined Athabasca tar sand is kneaded under water, an abundance of clean sand grains fall from the sample. Thus, even though the tar sands have very high

4 1

8 3

9

2

CURVE IAH

1.0

2 0.9 3 0.7

4 0.5 5 0.3

0.1

·0.1 8 ·0.3

9 ·0.5

2 3 4 5

WATER INJECTED Vp

Fig. 6-Relatlonshlp of wettablllty measurement by Amott and USBM tests to Pc curves for a mlxed-wettability system.

Fig. 7-011 recovery vs. brine injected for COBR systems with Amott-Harvey wettability Index ranging from 1.0 to - 0.5 (after Jadhunandan and Morrow 15 ).

JPT • December 1990 1479

G z w G u:: U­W

I­Z W ::;; W

~ Cl. (Jl

is w >

~ w a:

o 0

2.0

1.5

Kyt8 at al. (1961)23

Donaldson at at (1969)'

SalalhulIl (1973)24

Wang (1986)21

Rathme!1 at al. (1973)n

COBR Systems (s8e Fig.7)

lauoONCR\J~D:"E --------~o;;-;-,

(.\~ ................. : r-:=-- : ~: ~c:::=~ .. -~ ... -... -.~ .. -... ? ~'().51

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\~<ssnEXAS \ -------------\, _---------LCONVENT10NAL CORE

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O~~~~~~I~~~I~~~~~~~ o 0.5 1.0 1.5 2.0 2.5 3.0

WATER INJECTED Vp

en Q) () C Q) .... .... :::J () ()

o '0 .... Q) .0 E :::J Z

16

14

12

10

8

6

4

2

o o 10 20 30 40 50

Residual Oil, percent

Fig. B-Effect of wettability on laboratory waterfloods relative to recovery at strongly water·wet conditions.

Fig. 9-ROS measured by the single-well tracer test for 112 reservoirs.

asphaltene contents (about 15%), the sand remains strongly water-wet over geologic time.

Perfect Wetting and the Effect of Capil­lary Pressure on Thin Films and Contact Angles. Under the early working assump­tion that reservoir rocks were VSWW, the rock surface was often discussed and illus­trated as if a drained rock surface were still overlain by a thick fIlm of water. Even when water spreads, however, retained water fIlms at drained solid surfaces that are in equilibrium with the neighboring bulk water will be extremely thin relative to typical pore dimensions.

Perfect wetting, or spreading, is the most extreme wettability condition that can be ex­pressed by contact angle-0° for water or 180° for oil (see Fig. I). As a general rule, liquids such as water or hydrocarbons will spread against air on the surface of a min­eral such as quartz or calcite. Systems ex­hibiting water spreading and oil spreading will be called perfectly water-wet, PWW, and perfectly oil-wet, POW, respectively. PWW systems are a special subset of VSWW systems because the latter can in­clude systems of low, but fmite, contact an­gle. Even though a PWW condition may be difficult to achieve for crude-oil/brine sys­tems, it provides a useful reference state. Contact angles cannot be measured direct­ly for porous media, and it may not always be possible to distinguish PWW (0° contact angle) and VSWW (low, finite contact an­gle) systems by their displacement behavior.

In mathematical treatments of capillarity , a common practice is to neglect the thick-

1480

ness of drained fIlms, even though their presence is of crucial importance to wetting behavior. If an adsorbed fIlm is at equilib­rium with the bulk liquid at its prevailing capillary pressure, the fIlm's thickness will depend on the combined effect of the capil­lary pressure of the bulk liquid and the cur­vature of the solid overlain by the film. A disjoining pressure, IT, which for a zero­curvature solid surface is equal to the prevailing capillary pressure, is identified with the fIlm. 9

If the adsorbed fIlm thickness changes with capillary pressure"then, in principle, the contact angles can also change. Then, scaling of displacement pressure will not be

. exactly linear with respect to pore size. While the theory of equilibrium relationships between capillary liquid, thin fIlms, and vapor have been the subject of numerous scientific pUblications, as yet no consistent body of experimental evidence shows that such effects produce measurable changes in the wetting properties of porous media. Thus, it can be assumed for most practical purposes that the boundary conditions that determine wettability are independent of the prevailing capillary pressure.

Measurement of WettabUlty of OIl/Brine/Rock Systems Capillary Pressure Behavior of Crude­Oil/Brine/Rock Systems. Understanding the relationships between wettability, capil­lary pressure, and the distribution of oil and water in pore spaces is a necessary step in the difficult problem of quantifying wetta­bility and its relation to oil recovery. Basic

relationships between capillary pressure, surface curvature, and interfacial tension (1FT) and the use of contact angle in describ­ing boundary conditions in cylindrical pores are well known. For crude-oil/brine/rock (COBR) systems, relationships between wet­tability and capillary displacement pressures are complicated by the complex pore struc­ture and mineralogy of reservoir rocks and the effects of adsorbed organic components from the crude oil.

Fig. 6 shows an augmented sketch 12 of relationships between primary drainage, im­bibition, and secondary drainage; a full data set for all the curves shown is not available for COBR systems. (The curves in Fig. 6 are not representative of all types of COBR behavior. For example, imbibition capillary pressures sometimes fall close to zero, but then the rock continues to imbibe water very slowly.) The curves shown are useful in comparing the three commonly used methods of measuring wettability for COBR systems: the Amott test,2 the USBM test,3 and imbibition rate measurements. 13 In dis­cussing capillary pressure curves and wet­tability, uncertainty can arise in what is meant by the terms drainage and imbibition. For the nomenclature used in Fig. 6, drainage is defined as a decrease in water saturation and imbibition as an increase. The forced drainage mechanism is characterized by larger pores tending to empty before smaller pores. The spontaneous imbibition mechanism is characterized by smaller pores filling before larger pores. For systems in which capillary pressure changes sign, as when spontaneous imbibition is followed by forced displacement, a further increase in

December 1990 • JPT

water saturation occurs by a drainage mech­anism. Similarly, a system can exhibit spon­taneous imbibition of oil, which by definition is a spontaneous drainage process (see Fig. 6).

Amott Test. In the Amott test,2 water is first displaced by oil by centrifuging or by use of a high flowing pressure gradient. Pressure levels and time taken to reach ini­tial water saturation, Swi, are somewhat ar­bitrary. The aim should be to begin at the same water saturation as in the reservoir.

The core at Swi is then immersed in water to allow spontaneous imbibition. Spontane­ous imbibition of water ceases at some change in water saturation, ~ws, when the oil/water surface curvature falls to zero. Further oil can usually be recovered by forced displacement to give an overall in­crease in water saturation, ~wt' by flow­ing water at a high pressure gradient or centrifuging. Fig. 6 shows the relationships of saturation changes to capillary-pressure­vs.-saturation curves. The wettability index to water, Iw, is given by

Iw=~wsf~wt . ................ (1)

An analogous index (see Fig. 6) can then be measured for oil such that

Io=~osf~wl . ................. (2)

The difference, Iw-Io ' is often used to characterize wettability by a single number, I AH , known as the Amott-Harvey index.

Several points should be noted. Cessation of spontaneous imbibition of either water or oil occurs when the relevant interfacial cur­vatures fall to zero. (The generally minor effect of buoyancy is neglected.) In practice, this exact condition can be difficult to iden­tify if imbibition rates become very low. An operational definition for the endpoints for spontaneous imbibition is then needed, such as allowing a period of 1 week or so. The use of CAT scanning14 to check on the uniformity of distributions obtained by spon­taneous imbibition could provide especial­ly valuable information on the character of spontaneous imbibition behavior in mixed­wettability systems. . The endpoints achieved for forced dis­placement sometimes change with each dis­placement cycle and may be somewhat arbitrary in any case. Waterflooding of mixed-wettability systems is often charac­terized by continued production of oil down to low residual oil saturations (ROS's). If forced displacement is performed at exces­sive capillary number, the residual oil ob­tained will be lower than that given by normal laboratory waterflooding procedure.

When the Amott tests are used, even though the wettability may be expressed as a single value, IAH =(/w-Io)' it is better to report all/w and 10 values (one of which is often zero) and associated saturations as part of the wettability result. Each time satura­tion data are converted to wettability indices and the difference between the indices is used to obtain a wettability number, valua­ble information is lost. In quoting indices,

JPT • December 1990

saturation values from which they were de­rived should also be provided together with operational details of how Swi' ~ws. ~wt. ~os, and ~ot were determined. A weakness of the Amott test is its failure to distinguish between important degrees of strong water-wetness, all of which will give an Iw of, or very close to, unity.

USBM Method. In the USBM method, 3 drainage and imbibition capillary pressures are measured, usually by centrifuging. As with the Amott test, the method was devel­oped from observation of COBR displace­ment behavior. The wettability number is defined (see Fig. 6) by

Nw=log(AI/A2)' ............... (3)

where A 1 =area under the secondary water­drainage curve (drainage from residual oil) and A2 =area under the imbibition curve falling below the zero-Pc axis. The chief advantages of this method are the speed and simplicity of the procedure and its adapta­tion to relative permeability measurements. Points to note, with respect to this interpre­tation, are that corrections must be applied to the average saturations measured by cen­trifuging, the claimed thermodynamic basis for the method that equates work of displace­ment to change in surface free energy does not recognize the effects of irreversibility in capillary pressure relationships, and sys­tems that imbibe to give positive A 2, for ex­ample VSWW systems, are not recognized in the proposed interpretation.

Imbibition Rates. The driving force for spontaneous imbibition rates is proportion­al to the imbibition capillary pressure. Meas­urements of spontaneous imbibition rate 13

provide an especially useful supplement to Amott indjces or USBM wettability num­bers. Whereas the Amott test depends main­lyon the saturation at which imbibition capillary pressure falls to zero, spontaneous imbibition rate depends on the magnitude of the imbibition capillary pressure. Measure­ments of imbibition rates are of special value as a sensitive measure of wetting in the range where the Amott index is at or close to uni­ty. Quantitative interpretation obviously will be aided by having reference results obtained at or very close to perfect wetting condi­tions. Improved interpretation of spontane­ous imbibition behavior for COBR systems is likely to provide an important advance in wettability characterization. Measurements of imbibition rates also provide information on dynamic IFf and wetting phenomena that may be important in the reservoir but are not reflected by Amott or USBM wettability tests. Advances in interpretation of spon­taneous imbibition results will depend on im­proved methods of scaling and comparison of results from one system to another.

Effects of WettabllHy on 011 Recovery-Laboratory Waterfloods The effects of wettability on oil recovery have been investigated through laboratory displacement tests. Wettability of COBR

systems is usually characterized by the Amott test or the USBM method. Results for VSWW conditions are commonly used as a reference state. Reported changes in recovery as systems become less water-wet range from being much lower to being much higher than those given by VSWW condi­tions. Important goals of wettability research are to provide rational and consistent expla­nations of these apparent inconsistencies and to identify optimum conditions for oil recovery.

In reviewing wettability effects, oil recov­ery will be described by displacement effi­ciency , ED, defined as

ED=[(Soi-.tT)1 Soi]100%, ....... (4)

where Soi is the initial oil saturation and SOT is the ROS of the core sample. When the ROS continues to decrease with Vp throughput, SOT and ED are functions of Vp:

.. ED(~)={[Soi-.tT(Jj,)]1 Soi} 100%.

..................... (5)

Plots of ED(~) vs. Vp of water injected provide a convenient way of comparing re­sults of laboratory waterflood tests.

Several early examples of laboratory waterfloods show oil recovery decreasing with decreasing water-wetness. This is con­sistent with the intuitive notion that strong wetting preference of the rock for water and associated strong capillary imbibition forces give the most efficient oil displacement: However, an increasing number of exam­ples of improved recovery with shift from strongly water-wet conditions are being reported for weakly water-wet or intermedi­ate wetting condition, particularly for COBR systems. (These results generally involve displacement of crude oils or refined oils from cores in which organic films have been deposited from crude oil.) Fig. 7 shows averaged results 15 for an extensive COBR data set. In preparation of these samples, it was found that an increase in aging temper­ature, a decrease in water saturation, and to a lesser extent, aging time, aU tended to make the cores less water-wet. Recovery is seen to pass through a maximum when I AH is close to zero.

If E Dsw is the displacement efficiency at strongly water-wet conditions and Eoo is the wettability at some other global wetting condition, e, then the effects of wettability on recovery curves can be conveniently compared by plotting the relative displace­ment efficiency,

Eoo(~)=(EooIEDsw)100%, ..•.. (6)

against the number of Vp injected, as shown in Fig. 8. The selected results3•15-19

show that a shift toward less water-wet con­ditions can range from being highly adverse to being highly beneficial to oil recovery. The validity of the E Dsw data is obviously an important aspect of this interpretation.

Reservoir Residual 011 The greatest practical significance of wet­tability is its effect on oil recoveries achieved in the reservoir by waterfloOding. The need

1481

for accurate ROS measurements was stimu­lated by the prospect of tertiary recovery. Many methods of measuring residual oil have been proposed. The single-well tracer test, which measures ROS for a radius of about 10 m around the wellbore, has been widely applied to reservoirs viewed as likely candidates for tertiary recovery. 20 Results for 114 reservoirs are plotted as a histogram in Fig. 9.· ROS's remaining after water­flooding range from as low as 4 % to more than 40%. The results show that low values of residual oil that can be achieved in the laboratory are also commonly observed in watered-out reservoirs. Obvious goals in wettability research are to explain why high waterflood recoveries are observed in some oil reservoirs and how they might be ob­tained in others.

Pore-Level Displacement Mechanisms The results presented in Fig. 7 show that departure from VSWW conditions can give distinctly reduced oil entrapment. Flowex­periments in 2D pore networks (micro­models) permit observation of pore-level displacement mechanisms that affect dis­placement efficiency. Under VSWW con­ditions in systems of high aspect ratio (high ratio of pore body to throat size) believed to be typical of reservoir rocks, residual oil is trapped as disconnected blobs in pore bod­ies. Numerous theoretical and experimental investigations2l have been reported on mechanisms of entrapment (snap-off) and mobilization of oil blobs at VSWW.

Simulated waterfloods of crude oil from micromodels have also been made at vari­ous wettability conditions achieved by changes in brine pH and salinity. While a great variety of distributions have been ob­served, no crude-oillbrine system showed the extensive trapping of oil in pore bodies that is characteristic of VSWW systems. * Overall, the most important observation with respect to crude oil displacement was that the process by which oil becomes discon­nected (snap-off) tended to be inhibited. Furthermore, the oil phase often maintained continuity through capillary structures with oil/water surface boundaries that were pinned at the solid surface. Local (pore­level) changes in saturation and capillary pressure were accommodated by changes in contact angle within the range permitted by hysteresis. Low advancing angles and high receding angles were seen to coexist, even for capillary structures within a single pore. The most important difference between re­fined oil and crude oil displacements in micromodels was that displacement of crude oil from at least some of the pore bodies al­ways made a significant contribution to overall recovery.

Experimental studies by Li and Wardlaw22 showed that snap-off in simple pore models of rectangular cross sections can be inhibited if the water-advancing con-

"These results were provided by Charles T. Carlisle, Chem­ical Tracers Inc., Houston, and Harry A. Deans, U. of Wyoming, Laramie.

1482

tact angle exceeds about 60°. Further im­portant advances in theoretical and ex­perimental accounts of snap-off mechanisms will continue to be made through study of capillary pressure and instability behavior in pores that have comers and are of nonu­niform cross section.

During the past lO years, there has been a notable revival in analysis of pore net­works for modeling displacement processes in porous media. The relevance of results given by such models depends on the valid­ity of the rules adopted in the analysis. The choice of rules to account for wettability ef­fects in network models is a problem that has only just begun to be explored.

Advanced Core Analysis for WeUablllty The decision to investigate the wettability of a reservoir and the level of effort expend­ed in ACA W work to match reservoir con­ditions should be based on the level of significance these results will have on making reservoir decisions. Determination of reservoir wettability and measurement of valid relative permeability curves are both very difficult problems. The time and ex­pense of making reservoir-condition tests re­sult in few being performed on anyone reservoir. Although there is no guaranteed prescription for measurement of reservoir wettability, its effect on reservoir perform­ance may be of sufficient importance that the effort to obtain the best possible data is worthwhile. The basic problem in ACA W is knowing whether the nature and distribu­tion of surface properties and fluid phases as they occur in the reservoir are maintained or restored.

Within the general topic of ACA W many areas can be identified: (1) recovery of cores; (2) core handling at the wellsite and core storage; (3) core cleaning; (4) restora­tion of reservoir wettability, including the correct connate water saturation; (5) meas­urement of wettability; (6) measurement of waterflood displacement behavior and as­sociated relative permeabilities; and (7) ef-

. fect of wettability on electrical resistivity. Each of these topics has an extensive litera­ture; useful bibliographies have been provid­ed by Anderson. 23-28 ACA W procedures are usually influenced by the character of the particular reservoir under study and ex­perience gained in working with that reser­voir. API core analysis procedures for wettability determination are now recog­nized to be largely outdated. Attention cur­rently is being turned to the greater standardization of ACA W within the in­dustry.

Recovery of Reservoir Cores. Alteration of the wettability of core samples from their reservoir state during cutting, surfacing, and subsequent handling can occur for a num­ber of reasons. These include (1) drilling­fluid contamination, (2) temperature and pressure reduction effects on crude oil com­position such as asphaltene precipitation or

wax deposition, (3) oxidation, (4) drying, and (5) flashing of connate water with pres­sure reduction.

When coring with water-based drilling fluids, emphasis has been placed on the use of s<xalled bland muds. Because particu­lates from the mud are retained mainly at the surface of the cut core, studies of wet­tability alteration caused by flushing are made with the mud filtrates. Early work concentrated mainly on identifying mud compositions that caused no alteration in the wettability of strongly water-wet cores. More recently, attention has been given to changes in mixed- and oil-wet cores.

Cores cut with reservoir crude, diesel, or other oil-based drilling muds of low water content as the drilling fluid have the distinct advantage (because of the importance of the amount and distribution of connate water to wetting behavior) that the connate water is left relatively undisturbed. Drilling condi­tions and other safety considerations may often preclude this approach. Increased use of oil-based (emulsion) muds has resulted in special attention being given to cleaning oil-based cores and restoring them to reser­voir wettability conditions. Wunderlich29

has given a detailed review of the various options and comparisons in recovering cores with unaltered wettability.

Core Handling and Storage. There may commonly be long time delays between core recovery and core testing during which cores must be stored. Cores are commonly pre­served in mud filtrate or brine. They are generally protected from oxygen, evapora­tion, light,. and extreme temperature changes. Unconsolidated cores are usually subjected to freezing as part of the recov­ery process. In ACA W work, preserved cores are generally called fresh cores, even though testing may continue for several years after the cores were taken.

Core CIeaoing. The usual objective of core cleaning by flow of organic solvents or ex­traction is to remove all organic compounds without altering the basic pore structure of the rock. 23,30 A wide range of combina­tions and/or sequences of solvents have been proposed. The type of crude oil and core li­thology are used for guidance in selection of solvents, but procedures are far from standardized. 1

Methods commonly used in judging that the cleaned cores give a completely water­wet condition may be inadequate with respect to the requirement that all adsorbed organic material be removed. For example, achievement of an Amott index of unity for refined oil, after cleaning, does not neces­sarily mean that all the adsorbed organic ma­terial has been removed or that a VSWW system has been achieved. More reliable checks on removal of adsorbed hydrocar­bons would be provided by measurement of spontaneous imbibition rates for displace­ment of refined oil.

In early core analysis work, the objective of core cleaning was to restore the core to a VSWW condition that would represent the

December 1990 • JPT

wettability believed to pertain in the reser­voir. Subsequent displacement tests, such as relative permeability measurement, were performed with brine and a refined oil. With growing recognition that VSWW conditions are not common, core cleaning in ACA W work is now a fIrst step in restoring the core sample to reservoir wetting conditions.

Restoration of Reservoir Wettability. In restoration procedures, the objective is to re-establish reservoir wettability conditions by contacting the core with reservoir crude oil. More widespread acceptance of the im­portance of reservoir wettability has resulted in special attention being given to adopting ACA W restoration procedures to old core samples that have been allowed to age and weather with little attention to the storage enviromnent.

An important step in wettability restora­tion is to establish an Swi that corresponds to the reservoir. This requires knowledge of the reservoir water saturation and a method of establishing this initial saturation in the core. Determination of reservoir con­nate water, aside from problems of wetta­biIity, is much more difficult than sometimes assumed. 31 If a satisfactory determination of Swi has been made, displacement proce­dures are designed to achieve this value in the laboratory core sample. This may in­volve flooding at high pressure, sometimes with a high-viscosity refIned oil, centrifug­ing, or removal of water by desaturation by means of a porous plate or partial drying. The core is flushed with crude oil and then aged at reservoir temperature for about 1 to 4 weeks. The objective of the aging proc­ess is to re-establish adsorption equilibrium between the rock surface and the crude oil that had been established in the reservoir over geologic time.

Various degrees of sophistication can be used.in the restoration procedure to achieve systems that are more closely representative of reservoir conditions. IdentifIcation of the key parameters that affect wettability in COBR systems will obviously lead to much greater confIdence in design of ACA W procedures.

Comparison of Fresh Core and Restored­State Results. A major concern in ACA W work is that there is no sure way to check the validity of results. Comparison of results for fresh cores to restored-state results for the same cores provides the closest ap­proach. 1 The best possible indication that the results are valid is that the two data sets agree. If they do not, the various possible causes of disagreement are reviewed in deciding which data set is likely to be more reliable.

Measurement of Relative Permeabilities. Having obtained a reservoir core sample andlor fluids, with careful attention being paid to maintenance or restoration of reservoir conditions, laboratory tests are conducted to obtain relative permeability

JPT • December 1990

measurements. 30•32 These measurements are used in reservoir simulation to obtain production forecasts. Design of surface fa­cilities is often tied to these forecasts.

The most commonly used and convenient method of measuring relative permeability is by the unsteady-state method. This method was originally developed and ap­plied with strongly wetted systems and high displacement rates. It is now widely recog­nized that such an approach cannot be ex­pected to simulate conditions of intermediate or mixed wettability. However, if tests are run at reservoir rates, the results may be in­fluenced by capillary end effects (mainly holdup of oil at the outflow face). Coreflood simulation can be used to correct for end ef­fects. There is growing evidence that end effects for waterfloods at reservoir rates in mixed-wettability systems can be quite small. In-situ measurements of saturation by X -ray adsorption or CAT scanning to check for end effects and general uniformity of fluid distributions provide for improved con­fIdence in procedures and results.

Reservoir Wettability and Electrical Resistivity. Electric logging is the most widely used method of identifying hydrocarbon-bearing intervals in a wellbore. Standard methods of relating oil saturation to electrical resistivity are based on meas­urements of brine in cleaned cores, usually with air as the nonwetting phase. These measurements will generally correspond to VSWW conditions. Most investigations of the effect of wettability on resistivity showed increases in resistivity with decreases in water-wetness that were often too large to be reasonable. However, most of these re­sults were for systems in which wettability change was induced by artifIcial means, such as silane treatment of cores. Relatively little information is available on saturation expo­nents for COBR systems. Swanson33 and Longeron et at. 34 showed that Archie satu­ration exponents were only moderately higher for COBR systems than for a clean core and refIned oil. Further systematic study of electrical resistivity of COBR sys­tems is needed, with special attention being given to wettability variation and obtaining data at low water saturations.

Remarks A practical solution commonly suggested for ACA W is to measure relative permeability and other parameters that enter reservoir simulation at conditions that duplicate the reservoir. However, the understanding of wettability gained by this approach and the overall value of data specifIc to a piece of core are very limited. Even more serious is the inability to check the validity of the data. With greater attention being paid to mixed­wettability systems and the use of CAT scan­ning to obtain direct measurements of fluid distribution, uncertainties in relative perme­ability measurement are being identifIed. In­terpretation of relative permeability curves in terms of interface behavior and fluid dis-

tributions is still fairly primitive. Rules of thumb commonly used in the interpretation of the effect of wettability on relative per­meability behavior should be treated with caution. Mechanistic accounts of the effect of wettability on capillary pressure, fluid distribution, and oil retention provide a more direct approach to understanding interactions between wettability and pore geometry and a logical starting point for developing a fuller understanding of relative permeability relationships.

Conclusions 1. Reservoir wettability can cover a wide

spectrum of conditions. Systems of inter­mediate or mixed wettability are quite com­mon, whereas VSWW systems may be a rarity.

2. Contact-angle behavior of crude-oill brine systems falls into two main classes dis­tinguished by either small or large hys­teresis.

3. Adhesion behavior of crude oils is strongly dependent on pH.

4. Methods in most general use for meas­uring the wettability of COBR systems (the Amott test, the USBM test, and imbibition rate measurements) are strongly dependent on capillary pressure. In all three tests, . decrease in interface curvature to zero is a key condition.

5. Laboratory waterfloods on COBR sys­tems show that oil recovery is optimum at neutral wettability.

6. Agreement in results for fresh core with those for restored core provides the best indication that reservoir wettability has been duplicated in the laboratory.

7. ROS values obtained in the laboratory for mixed-wettability systems and those determined by in-situ measurements show that it is possible for waterfloods to achieve very high displacement effIciencies.

Nomenclature A 1 = area under secondary drainage

curve A2 = area under imbibition curve

falling below zero-Pc axis ED = displacement effIciency

E Dsw = displacement effIciency at strongly water-wet conditions

EDe = displacement efficiency at some global wetting condition e

IAH = Amott-Harvey wettability index

10 = wettability index (for oil) Iw = wettability index (for water)

N w = wettability number by USBM centrifuge test

Pc = capillary pressure Soi = initial oil saturation of core

sample Sor = ROS of core sample Sw = water saturation

Swi = initial water saturation ~ = pore volume A = difference

1483

Author -- ...... .... heads the Petrophplce a ....... CIIien*b'l Group at the New IIuIco PeIr6Ieum Recovery All .. ICh Center end .. ell­)unct prot ••• or of peIraIeum ..... Ing at .... IIuIco

Tech. He .. euthor or coauthor of more ... ., ....... lIIIIInIyon CIIpII.tty ... IuId .......,In poroue medIL He hoIcIe • 88 ...... In chemIcIII ... _rIng .... PhD __ 1n .......... .,;.. from the U. of LeecIL He recetnd the 1110 New .... TeICh DIItInguIehed A •••• dI AWM'd; eerved on '**' ... c:ommItIMe for the 1178, 11", end 1tI7 AnnuIII _ •• Unge; end ...".. on the EdItorI8I ..... CommIttee.

(J = contact angle e = global wetting condition IT = disjoining pressure

Subscripts A = advancing i = initial 0= oil r = residual

R = receding s = spontaneous t = total (with saturation change AS')

w = water

References 1. Cuiec, L.E.: "Evaluation of Reservoir Wet­

tability and Its Effect on Oil Recovery," In­terfacial Phenomena in Oil Recovery, N.R. Morrow (ed.), Marcell Dekker, New York City (1990) 319-75.

2. Amott, E.: "Observations Relating to the Wettability of Porous Rock," Trans., AIME (1959) 216, 156--62.

3. Donaldson, E.C., Thomas, R.D., and Lorenz, P.B.: "Wettability Determination and Its Effect on Recovery Efficiency," SPFJ (March 1969) 13-20.

4. Zisman, W .A.: "Relation of the Equilibrium Contact Angle to Liquid and Solid Constitu­tion," Contact Angle, Wettability and Adhe­sion, Advances in Chemistry Series 43, American Chemical Soc., Washington, DC (1964) 1-51.

5. Treiber, L.E., Archer, D.L., and Owens, W.W.: "Laboratory Evaluation of the Wet­tability of Fifty Oil Producing Reservoirs, " SPFJ (Dec. 1972) 531-40.

6. Morrow, N.R., Lim, H.T., and Ward, I.S.: "Effect of Crude-Oil-Induced Wettability Changes on Oil Recovery," SPEFE (Feb. 1986) 89-103.

1484

7. Buckley, J.S., Takamura, K., and Morrow, N.R.: "Influence of Electrical Surface Charges on the Wetting Properties of Crude Oil," SPERE (Aug. 1989) 332-40 .

8. Buckley, J.S. and Morrow, N.R.: "Charac­terization of Crude Oil Wetting Behavior by Adhesion Tests, " paper SPE 20263 present­ed at the 1990 SPEIDOE Symposium on En­hanced Oil Recovery, Tulsa, April 22-25.

9. Hirasaki, G.l.: "Wettability: Fundamentals and Surface Forces," paper SPE 17367 presented at the 1988 SPEIDOE Symposium on Enhanced Oil Recovery, Tulsa, April 17-20.

10. Dubey, S.T. and Waxman, M.H.: "Asphal­tene Adsorption and Desorption From Min­eraI Surfaces," paper SPE 18462 presented at the 1989 SPE International Symposium on Oilfield Chemistry, Houston, Feb. 8-10.

11. Takamura, K. and Chow, R.S.: "A Mecha­nism for Initiation .of Bitumen Displacement from Oil Sand," J. Cdn. Pet. Tech. (Nov.­Dec. 1983) 22, 22-30.

12. Sharma, M.M. and Wunderlich, R.W.: "The Alteration of Rock Properties Due to Inter­actions with Drilling-Fluid Components," J. Pet. Science & Eng. (Dec. 1987) I, 127-43.

13. Denekas, M.O., Mattax, C.C., and Davis, G. T.: "Effect of Crude Oil Components on Rock Wettability," JPT(Nov. 1959) 330-33; Trans., AIME, 216.

14. Wellington, S.L. and Vinegar, H.J.: "X-Ray Computerized Tomography," JPT (Aug. 1987) 885-98.

15. Jadhunandan, P. and Morrow, N.R.: "Crude Oil Recovery In Laboratory Water Floods," paper presented at the 1990 IFP Research Conference on Exploration & Production, The Fundamentals of Fluid Transport in Porous Media, Aries, May 14-18.

16. Kyte, J.R., Naumann, V.O., and Mattax, C.C.: "Effect of Reservoir Environment on Water-Oil Displacements," JPT(luly 1961) 579-82; Trans., AIME, 222.

17. Salathiel, R.A.: "Oil Recovery by Surface Film Drainage in Mixed-Wettability Rocks," JPT (1973) 1216-24.

18. Rathrnell, 1.1., Braun, P.H., and Perkins, T.K.: "Reservoir Waterflood Residual Oil Saturation From Laboratory Tests," JPT (Feb. 1973) 175-85; Trans., AIME, 255.

19. Wang, F.H.L.: "Effect ofWettability Alter­ation on Water/Oil Relative Permeability, Dispersion, and Flowable Saturation in Porous Media," paper SPE 15019 present­ed at the 1986 SPE Permian Basin Oil and Gas Recovery Conference, Midland, March 13-14.

20. Deans, H.A.: "Single-Well Tracer Methods," Determination of Residual Oil Saturation, Interstate Oil Compact Commis­sion, Oklahoma City (lune 1978) 156-76.

21. Mohanty, K.K., Davis, H.T., and Scriven, L.E.: "Physics of Oil Entrapment in Water­Wet Rock," SPERE (Feb. 1987) 113-28.

22. Li, Y. and Wardlaw, N.C.: "The Influence of Wettability and Critical Pore-Throat Size Ratio on Snap-off," J. Colloid Interface Sci. (1986) 109, No.2, 461-72.

23. Anderson, W.G.: "Wettability Literature Survey-Part 1: RocklOillBrine Interactions and the Effects of Core Handling on Wetta­bility," JPT (Oct. 1986) 1125-44.

24. Anderson, W.G.: "Wettability Literature Survey-Part 2: Wettability Measurement," JPT (Nov. 1986) 1246--62.

25. Anderson, W.G.: "Wettability Literature Survey-Part 3: Effects ofWettability on the Electrical Properties of Porous Media," JPT (Dec. 1986) 1371-78.

26. Anderson, W.G.: "Wettability Literature Survey-Part 4: Effects of Wettability on Capillary Pressure," JPT (Oct. 1987) 1283-1300 .

27. Anderson, W.G.: "Wettability Literature Survey-Part 5: Effects of Wettability on Relative Permeability," JPT (Nov. 1987) 1453-68.

28. Anderson, W.G.: "Wettability Literature Survey-Part 6: Effects of Wettability on Waterflooding," JPT(Dec. 1987) 1605-22.

29. Wunderlich, R.W.: "Obtaining Samples with Preserved Wettability," Interfacial Phenom­ena in Oil Recovery, N.R. Morrow (ed.), Marcell Dekker, New York City (1990) 289-318.

30. Hirasaki, G.l., Rohan, J.A., and Dubey, S. T.: "Wettability Evaluation During Restored-State Core Analysis," paper SPE 20506 presented at the 1990 SPE Annual Technical Conference and Exhibition, New Orleans, Sept. 23-26.

31. Morrow, N.R. and Melrose, J.C.: "Appli­cations of Capillary Pressure Data to the De­termination of Connate Water Saturation," Interfacial Phenomena in Oil Recovery, N .R. Morrow (ed.), Marcell Dekker, New York City (1990) 257-87.

32. Heaviside, 1.: "Measurement of Relative Per­meability," Interfacial Phenomena in Oil Recovery, N.R. Morrow (ed.), Marcell Dek­ker, New York City (1990) 377-411.

33. Swanson, B.F.: "RationaIizingthelnfluence of Crude Wetting on Reservoir Fluid Flow With Electrical Resistivity Behavior," JPT (Aug. 1980) 1459-64.

34. Longeron, D.G., Argaud, M.l., and Feraud, J.P.: "Effects of Overburden Pressure, Na­ture, and Microscopic Distribution of the Fluids on Electrical Properties of Rock Sam­ples," paper SPE 15383 presented at the 1986 SPE Annual Technical Conference and Ex­hibition, New Orleans, Oct. 5-8.

SI Metric Conversion Factors A x 1.0* ft x 3,(148*

E-Ol = om E-Ol = m

'Conversion factor is exact.

This paper is SPE 21621. Distinguished AutMr SerIes ar­ticles are general, descriptive presentations that summar­ize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more defin~ive work and present specific details only to illustrate the technology. Purpose: To inform the general readership of recent advances in various areas of petroleum engineering. A softbound anthology, SPE Dislinguished Author SOOes: Dec. 1981-Dec. 1983, is available from SPE's Book Order Dept.

JPT

December 1990 • JPT