Laboratory Measurement of Capillary Pressure

LABORATORY METHODS FOR MEASURING CAPILLARY PRESSURE Determination of Pc(Sw) function Porous diaphragm method Mercury injection method Centrifuge method Dynamic method

POROUS DIAPHRAGM METHOD FOR CAPILLARY PRESSURE Scale of Brine Ultra-fine fritted glass disk Seal of red oil squared paper Saran tube Crude oil Nitrogen pressure Neoprene stopper Nickel- plated spring Kleenex Core Porous diaphragm method of measuring capillary pressure Laboratory procedure Begin with core saturated with wetting fluid. In addition, saturate the porous disk with same wetting fluid Use pressure to force nonwetting fluid (usually air) into core, thus displacing wetting fluid through the porous disk The pressure difference between the pressure in the nonwetting fluid and the pressure in the wetting fluid equals the capillary pressure Repeat at successively higher pressures until equilibrium is reached (i.e., no more wetting fluid is displaced) Measure wetting phase saturation in core at each pressure increment Modified from Welge and Bruce, 1947

CAPILLARY PRESSURE USING POROUS DIAPHRAGM METHOD Measured data points Capillary pressure, psia Irreducible Wetting Phase Saturation Displacement pressure Wetting phase saturation, % 100

COMMENTS ON POROUS DIAPHRAGM METHOD Capillary Pressure 5 COMMENTS ON POROUS DIAPHRAGM METHOD Advantages Very accurate Can use reservoir fluids Disadvantages Very slow (days, weeks, months) Range of capillary pressure is limited by “displacement pressure” of porous disk Wetting phase of disk should be same as core sample Holes in porous disk act as capillaries, allowing only wetting to flow out until displacement pressure is exceeded

MERCURY INJECTION METHOD FOR CAPILLARY PRESSURE 0-200 psi pressure gauge 0-2,000 psi pressure gauge Regulating valve To Lucite window atmosphere Cylinder U-tube monometer Lucite window Laboratory procedure Uses mercury as the nonwetting fluid and air (usually under vacuum) as the wetting phase Force mercury into core Measure pressure required to force additional mercury volume into core Calculate mercury saturation (i.e., nonwetting phase saturation) from known injection volumes and known pore volume of core sample From Purcell, 1949

COMMENTS ON MERCURY INJECTION METHOD Capillary Pressure 7 COMMENTS ON MERCURY INJECTION METHOD Advantages Results obtained quickly (minutes,hours) Method is reasonably accurate Very high range of capillary pressures Disadvantages Ruins core / mercury disposal Hazardous testing material (mercury) Conversion required between mercury/air capillary data to reservoir fluid systems

CENTRIFUGE METHOD FOR DETERMINING CAPILLARY PRESSURE Seal Cap Core holder body Window Tube body Support disk O - Ring Laboratory procedure Similar to porous disk method except centrifugal force (rather than pressure) is applied to fluids in core Pressure (force per unit area) is computed from centrifugal force (which is related to rotational speed of centrifuge) Saturation is computed from fluid removed From Slobod and others, 1951

COMMENTS ON CENTRIFUGE METHOD Capillary Pressure 9 COMMENTS ON CENTRIFUGE METHOD Advantages Results can be obtained fairly quickly (hours, days, weeks) Reasonably accurate Can use reservoir fluids Disadvantages Complex analysis required can lead to calculation errors

DYNAMIC METHOD OF MEASURING CAPILLARY PRESSURE To atmosphere Oil burette Porcelain plate Oil inlet Po Pg Pc Gas inlet outlet Core Laboratory procedure Establish simultaneous steady-state flow of two fluids through core Measure pressures of the two fluids in core using special wetted disks (one porous diaphram wet by core’s wetting phase, and one porous diaphram wet by core’s non-wetting phase). Difference between the pressure between phases is capillary pressure Saturation is varied by regulating quantity of each fluid entering core From Brown, 1951

COMMENTS ON DYNAMIC METHOD Capillary Pressure 11 COMMENTS ON DYNAMIC METHOD Advantages Simulates reservoir flow conditions Can use reservoir fluids Disadvantages Very tedious to perform (weeks, months) High cost

AVERAGING CAPILLARY PRESSURE DATA USING THE LEVERETT J-FUNCTION The Leverett J-function was originally an attempt to convert all capillary pressure data to a universal curve A universal capillary pressure curve does not exist because the rock properties affecting capillary pressures in the reservoir have extreme variation with lithology (rock type) BUT, Leverett’s J-function has proven valuable for correlating capillary pressure data within a lithology (see ABW Fig 3-23).

EXAMPLE J-FUNCTION FOR WEST TEXAS CARBONATE Capillary Pressure 13 Black data are from an original capillary pressure experiments. The blue, red, and green data were for three new experiments, performed to determine the reliability of the original data. The black line represents an average J-function that was initially used in a reservoir simulation study for the field. Ultimately, the rock type was broken into smaller units, and five different capillary pressure relationships were used to represent the ranges of carbonate rock quality observed in the reservoir.

DEFINITION OF LEVERETT J-FUNCTION Capillary Pressure 14 DEFINITION OF LEVERETT J-FUNCTION C Pc k ( ) = J S w s q f cos J-Function is DIMENSIONLESS, for a particular rock type: Same value of J at same wetting phase saturation for any unit system, any two fluids, any values of k,f (k/f)1/2 is proportional to size of typical pore throat radius (remember k can have units of length2) C is unit conversion factor (to make J(Sw) dimensionless) J(Sw) = Leverett J-function, dimensionless Sw = water saturation, fraction Pc = capillary pressure, psia  = interfacial tension, dynes/cm  = contact angle, degrees k = formation permeability, md  = porosity, fraction

Pc(Sw) Depends on k,f High Quality Capillary Pressure 15 Core Pore Petrophysical Gamma Ray Flow Core Lithofacies Plugs Types Data Log Units Capillary f vs k Pressure High Quality 5 4 3 Function moves up and right, and becomes less “L” shaped as reservoir quality decreases 2 Low Quality 1

LEVERETT J-FUNCTION FOR CONVERSION OF Pc DATA Capillary Pressure 16 LEVERETT J-FUNCTION FOR CONVERSION OF Pc DATA J(Sw) = Leverett J-function, dimensionless Sw = water saturation, fraction Pc = capillary pressure, psia  = interfacial tension, dynes/cm  = contact angle, degrees k = formation permeability, md  = porosity, fraction

USE OF LEVERETT J-FUNCTION Capillary Pressure 17 J-function is useful for averaging capillary pressure data from a given rock type from a given reservoir J-function can sometimes be extended to different reservoirs having same lithologies Use extreme caution in assuming this can be done J-function usually not accurate correlation for different lithologies If J-functions are not successful in reducing the scatter in a given set of data, then this suggests that we are dealing variation in rock type