Boundary Tension and Wettability

Immiscible Phases Earlier discussions have considered only a single fluid in the pores –porosity –permeability Saturation: fraction of pore space occupied by a particular fluid (immiscible phases) – S w +S o +S g =1 When more than a single phase is present, the fluids interact with the rock, and with each other

DEFINITION OF INTERFACIAL TENSION Interfacial (boundary) tension is the energy per unit area (force per unit distance) at the surface between phases Commonly expressed in milli- Newtons/meter (also, dynes/cm)

BOUNDARY (INTERFACIAL) TENSION Modified from PETE 311 Notes Imbalanced molecular forces at phase boundaries Boundary contracts to minimize size Cohesive vs. adhesion forces LIQUID (dense phase) Molecular Interface (imbalance of forces) GAS SOLID LIQUID GAS  SOLID Cohesive force Adhesion force

DEFINITION OF WETTABILITY Wettability is the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids. Wettability refers to interaction between fluid and solid phases. Reservoir rocks (sandstone, limestone, dolomite, etc.) are the solid surfaces Oil, water, and/or gas are the fluids

WHY STUDY WETTABILITY? Understand physical and chemical interactions between Individual fluids and reservoir rocks Different fluids with in a reservoir Individual fluids and reservoir rocks when multiple fluids are present Petroleum reservoirs commonly have 2 – 3 fluids (multiphase systems) When 2 or more fluids are present, there are at least 3 sets of forces acting on the fluids and affecting HC recovery

DEFINITION OF ADHESION TENSION Adhesion tension is expressed as the difference between two solid-fluid interfacial tensions. A negative adhesion tension indicates that the denser phase (water) preferentially wets the solid surface (and vice versa). An adhesion tension of “0” indicates that both phases have equal affinity for the solid surface

CONTACT ANGLE The contact angle, , measured through the denser liquid phase, defines which fluid wets the solid surface. A T = adhesion tension, milli-Newtons/m or dynes/cm)  = contact angle between the oil/water/solid interface measured through the water, degrees  os = interfacial energy between the oil and solid, milli-Newtons/m or dynes/cm  ws = interfacial energy between the water and solid, milli-Newtons/m or dynes/cm  ow = interfacial energy (interfacial tension) between the oil and water, milli-Newtons/m or dynes/cm Solid Water Oil  os  ws  ow   os

Wetting phase fluid preferentially wets the solid rock surface. Attractive forces between rock and fluid draw the wetting phase into small pores. Wetting phase fluid often has low mobile. Attractive forces limit reduction in wetting phase saturation to an irreducible value (irreducible wetting phase saturation). Many hydrocarbon reservoirs are either totally or partially water-wet. WETTING PHASE FLUID

Nonwetting phase does not preferentially wet the solid rock surface Repulsive forces between rock and fluid cause nonwetting phase to occupy largest pores Nonwetting phase fluid is often the most mobile fluid, especially at large nonwetting phase saturations Natural gas is never the wetting phase in hydrocarbon reservoirs NONWETTING PHASE FLUID

WATER-WET RESERVOIR ROCK Reservoir rock is water - wet if water preferentially wets the rock surfaces The rock is water- wet under the following conditions:  ws >  os A T < 0 (i.e., the adhesion tension is negative) 0  <  < 90  If  is close to 0 , the rock is considered to be “strongly water-wet”

WATER-WET ROCK Adhesive tension between water and the rock surface exceeds that between oil and the rock surface. 0  <  < 90  Solid Water Oil  os  ws  ow   os

OIL-WET RESERVOIR ROCK Reservoir rock is oil-wet if oil preferentially wets the rock surfaces. The rock is oil-wet under the following conditions:  os >  ws A T > 0 (i.e., the adhesion tension is positive) 90  <  < 180  If  is close to 180 , the rock is considered to be “strongly oil-wet”

OIL-WET ROCK 90  <  < 180  The adhesion tension between water and the rock surface is less than that between oil and the rock surface. Solid Water Oil  os  ws  ow   os

From Amyx Bass and Whiting, 1960; modified from Benner and Bartel, 1941 INTERFACIAL CONTACT ANGLES, VARIOUS ORGANIC LIQUID IN CONTACT WITH SILICA AND CALCITE ORGANIC LIQUIDS SILICA SURFACE CALCITE SURFACE WATER

GENERALLY, Silicate minerals have acidic surfaces Repel acidic fluids such as major polar organic compounds present in some crude oils Attract basic compounds Neutral to oil-wet surfaces Carbonate minerals have basic surfaces Attract acidic compounds of crude oils Neutral to oil-wet surfaces Tiab and Donaldson, 1996 Caution: these are very general statements and relations that are debated and disputed by petrophysicists.

WATER-WETOIL-WET Ayers, 2001 FREE WATER GRAIN SOLID (ROCK) WATER OIL  SOLID (ROCK) WATER OIL  GRAIN BOUND WATER FREE WATER OIL RIM  < 90   > 90  WATER   Oil Air WATER

OIL-WETWATER-WET WATER   Air Oil

From Levorsen, 1967

Brown, G.E., 2001, Science, v. 294, p

From Tiab and Donaldson, 1996 n = 161 ls., dol. CONTACT ANGLE: Triber et al. -Water-wet = 0 – 75 degrees -Intermediate-wet = 75 – 105 degrees -Oil-wet = 105 – 180 degrees n = 30 silicate and 25 carbonates CONTACT ANGLE: -Water-wet = 0 – 80 degrees -Intermediate-wet = 80 – 100 degrees -Oil-wet = 100 – 180 degrees

WETTABILITY IS AFFECTED BY: Composition of pore-lining minerals Composition of the fluids Saturation history

WETTABILITY CLASSIFICATION Strongly oil- or water-wetting Neutral wettability – no preferential wettability to either water or oil in the pores Fractional wettability – reservoir that has local areas that are strongly oil-wet, whereas most of the reservoir is strongly water-wet - Occurs where reservoir rock have variable mineral composition and surface chemistry Mixed wettability – smaller pores area water-wet are filled with water, whereas larger pores are oil-wet and filled with oil - Residual oil saturation is low - Occurs where oil with polar organic compounds invades a water-wet rock saturated with brine

IMBIBITION Imbibition is a fluid flow process in which the saturation of the wetting phase increases and the nonwetting phase saturation decreases. (e.g., w aterflood of an oil reservoir that is water-wet). Mobility of wetting phase increases as wetting phase saturation increases –mobility is the fraction of total flow capacity for a particular phase

WATER-WET RESERVOIR, IMBIBITION Water will occupy the smallest pores Water will wet the circumference of most larger pores In pores having high oil saturation, oil rests on a water film Imbibition - If a water-wet rock saturated with oil is placed in water, it will imbibe water into the smallest pores, displacing oil

OIL-WET RESERVOIR, IMBIBITION Oil will occupy the smallest pores Oil will wet the circumference of most larger pores In pores having high water saturation, water rests on an oil film Imbibition - If an oil-wet rock saturated with water is placed in oil, it will imbibe oil into the smallest pores, displacing water e.g., Oil-wet reservoir – accumulation of oil in trap

DRAINAGE Fluid flow process in which the saturation of the nonwetting phase increases Mobility of nonwetting fluid phase increases as nonwetting phase saturation increases –e.g., waterflood of an oil reservoir that is oil-wet – Gas injection in an oil- or water-wet reservoir – Pressure maintenance or gas cycling by gas injection in a retrograde condensate reservoir –Water-wet reservoir – accumulation of oil or gas in trap

IMPLICATIONS OF WETTABILITY Primary oil recovery is affected by the wettability of the system. –A water-wet system will exhibit greater primary oil recovery.

WATER-WETOIL-WET Ayers, 2001 FREE WATER GRAIN SOLID (ROCK) WATER OIL  SOLID (ROCK) WATER OIL  GRAIN BOUND WATER FREE WATER OIL RIM  < 90   > 90  WATER   Oil Air WATER

IMPLICATIONS OF WETTABILITY Oil recovery under waterflooding is affected by the wettability of the system. –A water-wet system will exhibit greater oil recovery under waterflooding.

From Levorsen, 1967 Effect on waterflood of an oil reservoir? Water-Wet System Oil-Wet System

IMPLICATIONS OF WETTABILITY Wettability affects the shape of the relative permeability curves. –Oil moves easier in water-wet rocks than oil-wet rocks.

IMPLICATIONS OF WETTABILITY Core no Percent silicone Wettability Curves cut off at F wd Water injected, pore volumes Recovery efficiency, percent, S oi Modified from Tiab and Donaldson, 1996 ? p. 274

IMPLICATIONS OF WETTABILITY Water injection, pore volumes Squirrel oil N NaCl - Torpedo core (  33 O W 663, K 0945, S wi 21.20%) Squirrel oil N NaCl Torpedo Sandstone core, after remaining in oil for 84 days (  33.0 W 663, K 0.925, S wi 23.28%) Recovery efficiency, percent S pi Modified from NExT, 1999

WETTABILITY AFFECTS: Capillary Pressure Irreducible water saturation Residual oil and water saturations Relative permeability Electrical properties

LABORATORY MEASUREMENT OF WETTABILITY Most common measurement techniques –Contact angle measurement method –Amott method –United States Bureau of Mines (USBM) Method

NOMENCLATURE A T = adhesion tension, milli-Newtons/m or dynes/cm)  = contact angle between the oil/water/solid interface measured through the water (more dense phase), degrees  os = interfacial tension between the oil and solid, milli-Newtons/m or dynes/cm  ws = interfacial tension between the water and solid, milli-Newtons/m or dynes/cm  ow = interfacial tension between the oil and water, milli-Newtons/m or dynes/cm

References 1. Amyx, J.W., Bass, D.M., and Whiting, R.L.: Petroleum Reservoir Engineering, McGrow-Hill Book Company New York, Tiab, D. and Donaldson, E.C.: Petrophysics, Gulf Publishing Company, Houston, TX Core Laboratories, Inc. “A course in the fundamentals of Core analysis, Donaldson, E.C., Thomas, R.D., and Lorenz, P.B.: “Wettability Determination and Its Effect on Recovery Efficiency,” SPEJ (March 1969)