Wettability Wettability can be defined as the ability of a fluid phase to preferentially wet a solid surface in the presence of a second immiscible phase Microscopic fluid saturation distribution in a water-wet and oil-wet rockWettability (1) water wet; contact angle q < 90 (2) neutral wettability, q = 90 (3) oil wet, q > 90 Fractional wettability – heterogeneous wetting; i.e., portions of the rock are strongly oil wet, whereas other portions are strongly water wet. Occurs due to variation in minerals with different surface chemical properties. Mixed wettability – refers to small pores occupied by water and are water-wet, while larger pores are oil-wet and continuous.Wettability The contact angle is a measure of the wettability of the rock-fluid system, and is related to the interfacial energies by Young’s equation, where: sos = interfacial energy between oil and solid, dyne/cm; sws = interfacial energy between water and solid, dyne/cm; sow = interfacial energy, or interfacial tension, between oil and water, dyne/cm; q = contact angle at oil-water-solid interface measured through the water phase, qssscosowwsosWettability Sessile drop method of measuring contact angles for water-oil systemsWettability USBM method of determining wettability for a water wet sample • Increasing positive values indicate a preference to water wet; i.e., A1 progressively becomes greater than A2. Negative values of the index indicate an oil-wet preference (A2 > A1) 21logAAwIWettability 1. Sample is 100% water saturated, 2. Oil displaces water to Swi, (drainage cycle) 3. spontaneous imbibition of brine, Vosp 4. Water displaces oil to Sor, (imbibition cycle) 5. spontaneous imbibition of oil, Vwsp 6. final displacement of water by oil (2nd drainage cycle). Combined USBM-Amott Wettability Test wowtVwspVotVospVwIAmott Index +1 Strongly water wet -1 Strongly oil wetCapillary Pressure force up = force down force up = 2pr s cos q force down = pr2 h Dg rcPrdownfor cerupfor cecPqsppcos22/2/ ghrairwghrcPrprrpD22Capillary Pressure In reservoirs, capillary pressure is the difference between the nonwetting phase pressure (Pnw) and the wetting-phase pressure (Pw). wPnwPcP Capillary Pressure Conversion of lab to reservoir conditions Example: Laboratory s (air-water) = 72 dyne/cm q(air-water) = 0 Reservoir s (oil-water) = 24 dyne/cm q (oil-water) = 30 rw = 65 lb /cu ft ro = 53 lb /cu ft.   labreslabcPrescPqsqscoscos)()()(289.00cos7230cos24)()( labcPlabcPrescP )(*0.125365144*rescPcPcPh DrCapillary Pressure Entry Pressure Drainage/ImbibitionCapillary Pressure • Entry pressure • Irreducible water saturation • Slope of transition zone curve • Grain size distribution • Grain and pore size Permeability effectCapillary Pressure Measurement Mercury injection Fig. 22 Mercury injection equipmentSampleMercuryPumpDisplacement readingPressuregaugeSchematic of mercury injection apparatus Example mercury-air capillary pressure curvesCapillary Pressure Measurement Schematic of a Ruska diaphragm pressure cell Porous Diaphragm Example of capillary pressure curve for a water-wet systemCapillary Pressure Measurement Centrifuge rotoraxisrotorsamplerotationwaterproduced incollection tubeCentrifugal force plays role of gravitational forceIncreasing rotation speed ---> increasing force ----> increasing PcGives increasing water production ---> SwFig. 23 CentrifugemeasurementCapillary pressure measurement by centrifuge Example capillary pressure curves from centrifugal data. Curves 2 and 4 are estimated because they typically cannot be determined by centrifugeCapillary Pressure Measurement LLerNxicPD22610096.1)(ricPdSdicPSiS)(*)(SCSBAicP1)( 21)(SCACBSdicPdCentrifuge Schematic illustrating the variation of pressure and water saturation as a function of core length. Hyperbolic least squares fitCapillary Pressure Measurement Fig. 24 Measuring Capillary PressureEquilibrium Air/Hg CentrifugeDuration 5 weeks 1day 3 days/runMax Height (m gas/oil) 30/60 7000/14500 80/160At stress? Yes Yes NoOn cuttings? No Yes NoSample damaged? No Yes Weak onlyUnconsolidated Yes Yes Yes?Equilibrium reached? Yes Yes NearlyClay correction No Yes NorequiredCosts Expensive Cheap MediumAdditional Imbibition Imbibition ImbibitionInformation RI WettabilityComparisonCapillary Pressure • Importance in defining: – the height of the transition zone – the initial distribution of reservoir fluids – the retention of the wetting phase in the reservoir • Assign based on: – rock types – flow processAveraging Capillary Pressure Data Methods to fit a laboratory measured capillary pressure curve for the purpose to produce a saturation-height function. 1. Averaging curve fit parameters (e.g. a, b, l vs. f) Of these the lambda-fit normally works best. It fits the wetting saturation Sw to the capillary pressure Pc using three fit constants, a, b and l, according to: Sw = a.Pc-l + b 2. Interpolation within data set 3. Leverett-J 4. Neural networks 5. Regression (linear, non-linear, multi-variate)Averaging Capillary Pressure Data calculate J(Sw) for each capillary pressure point using: plot J(Sw) versus Sw and draw a smooth curve through the points, fqskcPwSJcos)(    r eskowr eswSJhfrrqscos)(Leverett J Function calculate h for each Sw, for any set of k and f; plot h versus Sw.Averaging Capillary Pressure Data Set of capillary pressure curves for the D sands Resulting J-function curve for the D-sands J-function relates rock type (f and k) to Pc and normalizes data for application at different locations in a reservoirCapillary Pressure saturation-height as a function of rock type 24.1% 24.1% 30.4% 30.4%Capillary Pressure Causes of errors in capillary pressure curves are: • Alteration of wettability by invasion of drilling mud filtrate • Biased sampling • Sample integrity • Effect of core cleaning on wettability • Laboratory measurement and appropriate corrections for temperature, stress, and clays • Averaging Log vs core water saturation vs height for a Middle East carbonate Causes of errors in log calculations are: • tool calibrations and quality control • Invasion, thin bed and borehole effects • Application