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Lab Measurements of Capillary Pressure

All Roads Lead to Rome: Water Saturation atCapillary Equilibrium in the Core

Carlos A Grattoni

London Petrophysical Society - One day Seminar

14 December 2009



Slide 2®

A proper understanding of capillary pressure behaviour is

crucial for reservoir characterisation and hydrocarbonproduction

Capillary pressure play a role in:

Saturation-height models for determination of hydrocarbon in place

Calculating original saturations through depleted zones

Thin bedded zones that resistivity logs cannot resolve

Checking irreducible water saturations calculated from logs

Residual oil saturation determination

Derivation of free water level from formation pressure data

Explaining differences in contacts between wells

Sealing capacity

….



Slide 3®

Capillary PressureControlling factors

Pc = ∆ρ g h Fluids density, height



Slide 4®

Capillary Pressure Definitions

Entry pressure

Threshold,

Breakthrough,Bubbling Pressure Water blocking

Snap-off

Saturation: Fraction of the pore space occupied by a given fluid

⎟⎟⎠

⎞⎜⎜⎝

⎛ +=21

11

RRPc σ



Slide 5®

Capillary Equilibrium (water-wet)

Sw



Slide 6®

Static Water Saturation Distribution



Slide 7®

Quantitative Measurement Saturation on Cores

For core cut in water based mud,Retort Method is used toidentify potential oil and gasreservoirs

For core cut in oil based mud,Retort Method and Dean-Starkwater saturation can provideaccurate measurement of

connate water saturation



Slide 8®

• Pc

(Sw) depends on thesaturation history. Thiseffect is known ashysteresis

• Drainage: the wettingphase saturation decreases

• Imbibition: the wetting

phase saturation increases

Capillary Pressure Cycles (water-wet)

Swi

Sor

Pc

0 1

Imbibition

Primary Drainage

Secondary Drainage

Water-wet system



Slide 10®

POROUS DIAPHRAGM METHOD

Scale

Brine

Ultra-finefrittedglass disk

OilSeal

Saran tube

Crude oil

Nitrogenpressure

Neoprenestopper

Spring-

Kleenexpaper

Core

Modified from Welge and Bruce, 1947

Restored State Cell



Slide 11®

POROUS DIAPHRAGM METHOD

Laboratory procedure

•Begin with core and porous disksaturated with wetting fluid

•Use non-wetting fluid pressure to forceinto core, thus displacing wetting fluidthrough the porous disk

•The pressure difference between the fluids

equals the capillary pressure

•Wait until equilibrium is reached (i.e., nomore wetting fluid is displaced)

•Measure/ determine wetting phase saturation

in core at each pressure increment

• Repeat at successive higher pressures

Unconfined Confined stress

Water

Saturated

diaphragm

Air or Gas Gas or Oil

Water



Slide 12®

POROUS DIAPHRAGM- example

Data



Slide 13®

MERCURY INJECTION METHOD

Lucite window

Lucite window

To

0-2,000 psi pressuregauge

Regulating valve

atmosphere

U-tube monometer

Cylinder

0-200 psi pressure gauge

From Purcell, 1949

Lucite window

Lucite window

To

0-2,000 psi pressuregauge

Regulating valve

atmosphere

U-tube monometer

Cylinder

0-200 psi pressure gauge

From Purcell, 1949

Sample Chamber

Mercury Pump

Mercury

DisplacementReading

Sample

Up to 60000 psi

Fully automatic:vacuum, volume

injected, low-high

pressure



Slide 14®

MERCURY INJECTION, examples

Incremental Intrusion (mL/g)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.0010.1101000Pore Diameter (m icrometers)

Capillary Press ure (psi)

1

10

100

1000

10000

100000

0 20 40 60 80 100% Hg Saturation

Pore Size Distribution,Inference of porenetwork characteristics

Needs converting toOther fluid systems



Slide 15®

Data conversion to reservoir conditions

( ) ( )labawaw

owowyac PcPc

−−

−−=θ γ θ γ

coscos

Typically

In the lab

γa-w ≈ 70 dynes / cm

θa-w ≈ 0o

In the reservoir

γo-w ≈ 28 dynes / cm

θo-w ≈ 33 a 55o

( ) ( ) labowyacow Pcx

Pc / / 70

70.028−− =

( )( ) 43

1

/

/

aPc

Pc

labow

resow ≈−

−

Pc Conversion



Slide 16®

Typical Values ofγ , θ

PC ratio

5

8.7

1

15



Slide 17®

CENTRIFUGE METHOD



Slide 18®

Schematic drawing of the centrifuge set-up

Malla

SEALING SCREW

SCALE

CORE PLUGMESH

CENTRIFUGE TUBE

CALIBRATE

D SCALE

ROTATION VELOCITY

1500-20000 r.p.m



Slide 19®

Analysis and Calculations

LL

r nP einlet c ⎟⎠

⎞⎜⎝

⎛ −∆=2

2ρ

0

5000

10000

15000

20000

25000

30000

35000

0.00 0.20 0.40 0.60 0.80 1.00

Capillary pressure PC

Saturation

Mean saturation

n: number of revolutions per minute

L: sample lengthRe: distance from sample to rotor

Sw

Hagoort, SPEJ 1980

Reality?



Slide 20®

Combined methods (CT scans, NMR)



Slide 21®

Comparison of Main Capillary Pressure Methods



Slide 22®

Vapour equilibrium (desorption) method

• Thompson (1870) showed that the vapour pressure above a liquidsurface is a function of its curvature.

• Capillary pressure is therefore related to vapour pressure for porous

media containing water in equilibrium with its vapour.

whereP c = capillary pressure,R = universal gas constant,R H = relative humidity,

T = absolute temperature,V m = molar volume of water (0.018016).



Slide 23®

Vapour equilibrium methodPlugs placed in constant humidity chambers until weight/saturation stabilises

Saturated solutions of different salts

Very high pressures, simple and cheap technique, very slow (months)

Other systems: Glycerol - water



Slide 24®

Comparison Vapour desorption- centrifuge

Newsham et al., SPE 89866



Slide 26®

Phyllosilicate-frame analogue

(converted to Hg)



Slide 27®

Heterogeneous samples

0.1

1

10

100

1000

10000

100000

0 20 40 60 80 100

Gas Saturation

Capillary

Pressure (psi)

Sample F Sample H

Sample M Break T 2

Break T 1PcHg converted to gas-brine



Slide 28®

Capillary pressure of combined rocks



Slide 29®

Fault rocks- sealing capacity



Slide 30®

Water Saturation at Capillary Equilibrium

Do all Roads Lead to Rome?

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