Wettability and Its Effect on Oil Recovery
Authors: Morrow Norman R. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Introduction

Reservoir wettability is determined by complex interface boundary conditionsacting within the pore space of sedimentary rocks. These conditions have adominant effect on interface movement and associated oil displacement. Wettability is a significant issue in multiphase flow problems ranging from oilmigration from source rocks to such enhanced recovery processes as alkalineflooding or alternate injection of CO2 and water. In this paper, wettabilitywill be discussed mainly in the context of recovery of light (low-viscosity)oils by waterflooding. Waterflooding has been widely applied for more than halfa century; secondary recovery by waterflooding presently accounts for more thanone-half of current U.S. oil production. Many research papers have addressedthe effect of wettability on waterflood recovery during this period. For muchof the past 50 years, however, a large body of reservoir period. For much ofthe past 50 years, however, a large body of reservoir engineering practice hasbeen based on the assumption that most reservoirs are very strongly water-wet(VSWW); i.e., the reservoir-rock source always maintains a strong affinity forwater in the presence of oil. The rationale for assuming VSWW conditions wasthat water originally occupied the reservoir trap; as oil accumulated, waterwas retained by capillary forces in the finer pore spaces and as films on poresurfaces overlain by oil. Wettability behavior other than VSWW was observed forreservoir core samples, but was often ascribed to artifacts related to corerecovery and testing procedures. The majority of reservoir engineeringmeasurements have been made on cleaned core with refined oil or air as thenonwetting phase to give results for, or equivalent to, VSWW conditions. Examples of such measurements are laboratory waterfloods, determination ofelectrical resistivity vs. water saturation relationships, and capillarypressure measurements for determination of reservoir connate water saturation. Mounting evidence on the effects of crude oil on wetting behavior has now ledto wide acceptance of the conclusion that most reservoirs are at wettabilityconditions other than VSWW. This conclusion has led to a resurgence of interestin satisfactory procedures for measuring reservoir wettability and determiningits effect procedures for measuring reservoir wettability and determining itseffect on oil recovery, especially with respect to waterflooding. Determinationof reservoir wettability and its effect on oil recovery by methods that involvecore samples will be referred to as advanced core analysis for wettability(ACAW). Reservoir wettability is not a simply defined property. Classificationof reservoirs as water-wet or oil-wet is a gross oversimplification. Variousprocedures for measuring wettability have been proposed. Two methods ofquantifying wettability based on rock/brine/oil displacement behavior, themodified Amott test and the USBM test, are in common use. Each method dependson water saturation measurements and related capillary pressures or flowconditions to define a wettability scale. The tests show pressures or flowconditions to define a wettability scale. The tests show that reservoirwettability can cover a broad spectrum of wetting conditions that range fromVSWW to very strongly oil-wet. Within this range, complex mixed-wettabilityconditions given by combinations of preferentially water-wet and oil-wetsurfaces have been identified. In preferentially water-wet and oil-wet surfaceshave been identified. In this paper, the adopted scales of reservoirwettability and their relationships to interface boundary conditions areconsidered together with the dramatic effects that wettability can have on oilrecovery.

Contact Angles, Spreading and Adhesion

Contact Angle and Spreading. Contact angle is the most universal measure ofthe wettability of surfaces. Fig. 1 shows idealized examples of contact anglesat smooth solid surfaces for oil and water of matched density. Early studies ofwetting phenomena showed that the wetting properties of a solid are dominatedby the outermost layer of molecules. (Films that result from spreading andother thin adsorbed films are not indicated in Fig. 1.) Large change in thewettability of a surface, such as quartz, can be achieved by adsorption of amonolayer of polar molecules so that the outermost part of the surface iscomposed 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 indetermining the wetting properties of reservoir-rock surfaces. Many earlystudies of wetting behavior, even for comparatively simple systems, wereplagued by problems of reproducibility. Aside from surface contamination, otherforms of heterogeneity in chemical composition, surface roughness, and staticand dynamic interface properties contribute to the complexity of observedwetting phenomena. Large differences in contact angles, depending on whether aninterface was advanced or receded, called into question the validity ofattempting to describe wettability by a single-valued equilibrium contactangle. Successful systematic studies of closely reproducibleequilibrium-contact-angle measurements have been summarized by Zisman. By useof smooth (often polymeric), solid surfaces and pure liquids, contact-anglehysteresis was limited to within 1 or 20. In contrast, contact-angle hysteresisis observed almost invariably for crude-oil/brine systems. Fig. 2 showsexamples of contact angles that exhibit small and large hysteresis. Recedingangles are generally low (less than 30 degrees) and seldom exceed 60 degrees, whereas a wide range of advancing angles is observed. The shaded regions inFig. 2 show the range of possible contact-angle values for a fixed position ofthe three-phase line of contact. Contact-angle measurements onreservoir-crude-oil/brine systems provide one approach to measuring reservoirwettability. For the most extensive set of data yet reported, contact anglesfor crude oil and simulated reservoir brine were measured at reservoirtemperature and ambient pressure. Choice of mineral substrate, usually quartzor calcite, was pressure. Choice of mineral substrate, usually quartz orcalcite, was based on what was judged from petrographic examination to be thepredominant mineral at pore surfaces. (There are obvious limitations topredominant mineral at pore surfaces. (There are obvious limitations torepresenting the rock surface by a single mineral.)

JPT

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