The Effect of Wettability on Relative Permeability, Capillary Pressure,

Electrical Resistivity and NMR

Saif AL-Sayari

Prof. Martin Blunt

• Experimental and network modelling studies have illustrated that, for a given sample, changes in wettability HAVE a strong influence on relative permeability and oil recovery – in particular the work of Morrow and co-workers.

• How do we characterise wettability at the pore scale? How can we relate this to macroscopic measurements? Can we use this information to predict relative permeability and oil recovery.

Objective

• Understanding the relationship between wettability and distribution of oil and water in pore space is a necessary step in the difficult problem of quantifying wettability and its effects on oil recovery.

• Carry out a suite of experiments to obtain precise and accurate measurements of flow properties for a variety of rock samples with different wettabilities.

• Compare with numerical predictions using pore-scale modelling.

Importance of this Project

• Evaluation of the influence of contact angle and wettability on recovery are not very clearly discussed in the literature. These studies tend to focus on either experimental measurement of one property or simulation.

• We propose to combine in one study the effects of wettability on multiphase flow parameters, looking at capillary pressure, wateflood recovery, electrical properties, NMR and relative permeability.

Approach

• Perform experimental measurements on static and dynamic properties such as, Φ, k, kr, Pc and wettability on a selected set of rock samples under different conditions mimicking representative formations.

• Quantification of the influence of wettability conditions on the dynamic and static properties.

• Use of the gathered data to perform detailed pore-scale modelling on the samples and comparison with conventional approach in reservoir simulation.

Approach

• Three main set of samples that will be studied, are:

Sample Conditions

Sandpack Water-wet

Sandstone Oil-wet

Carbonates Mixed-wet

Fluids

Brine (5% NaCl and 1% KCl)

Oil (Multibar H)

Wettability

• Tendency of fluid to spread on a solid surface in the presence of other immiscible fluid.

• Wettability determination is important in reservoir characterization because it controls the distribution of the fluids in the reservoir and therefore the oil recovered.

Water-wet Mixed-wet Oil-wet

Rock Oil Brine

Wettability

• Measurement of Wettability:

Direct Indirect

Contact angle Relative Perm.

USBM Capillary Press.

Phase 1

• Sand pack – Water-wet

• Core Preparation

• Brine Preparation

• Conventional Core Analysis

• Special Core Analysis

• kr

• Pc

• Electrical Resistivity

• NMR

Relative Permeability

• Ability of the porous medium to conduct fluid flow when multiple fluid phases are present.

• Steady – State Method

* Table and Graph (William Anderson, SPE, Conoco Inc.)

Relative Permeability - Procedure

• Clean Dry core 100% saturated with brine• Measure ka for water• Establish Swi by injecting 50 P.V. oil• Establish Sor by injecting 50 P.V. brine• Oil and Brine simultaneous circulation at given rates by two metering pumps (Having a stepwise decrease of the water fractional flow).• Produced oil and brine are separated in graduated burette.

Steady-state condition is determined to be established for each step of flow by continuous monitoring of the level of oil/brine interface in the burette and the pressure drop across the core

Capillary Pressure

• When oil and water are placed together on a surface, a discontinuity in pressure exists across the interface separating them. This difference in pressure in known as Capillary Pressure

Capillary Pressure

• Apparatus for Porous Plate Method:

Capillary Presure - Procedure

• Carmen-Kozney correlation to estimate pressure range.• Put the core in the cell. Adjust the pressure and give the cell pressure difference.• After three days, measure the volume and check if no more water is being produced.• Calculate the water saturation corresponding to that pressure. 1• Increase the pressure in steps, and record the final equilibrium produced volume of the wetting phase at each step.• Draw the Capillary Pressure vs. Sw

SwSwi

Pc

Pc

Water-wet

Oil-wet

Drainage

Drainage

Electrical Resistivity

• When the rock is partially saturated with hydrocarbon, the resistivity of the rock is increased due to non-conductive properties of hydrocarbons. Wettability is an important factor for brine distribution, which contributes to the change of resistivity.

• Water-wet: brine is continuous, so the decrease of the cross-sectional area that can conduct flow will lead to the increase of resistivity.

• Oil-wet: brine is discontinuous, so the electrical resistivity will increase at a faster rate than in water-wet.

Electrical Resistivity

• Quadtech model 7600 RLC

• Frequency:10Hz-2MHz

• Pair of coaxial cables is used to connect the analyzer to two electrode conducting cell.

• Perform the frequency sweep.

Plot of Z & Rs Vs Frequency

0

50

100

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350

10 100 1000 10000 100000 1000000

1E+07

Rs

Z

NMR

• It is known that NMR resonance of fluids in the vicinity of solids can be quite different from that in bulk fluid. These effects are useful to the characterization of porous fluids and fluid distribution with NMR measurements.

NMR

• Oil Wet: Longer T2 peak value. Surface relaxitivity is smaller than WW.

• Water wet: T2 at Swi is wider than it is in OW.

0.00

0.50

1.00

1.50

2.00

2.50

0.1 1 10 100 1000 10000

T2 (ms)

Dis

trib

uti

on

0

5

10

15

20

25

30

35

40

Cu

mu

lati

ve v

ol.

Normalised T2

Normalised accumulativevolume

Aging Procedure

• It is very important to establish a wettability condition as close as possible to that found in the reservoir

Aging procedure

• Fluid: Crude oil (Acid Number Measurements, J. Buckley)

• Technique:

• Core plug is saturated with brine

• Mount the core in the hasler cell

• Flood oil (at least 10 P.V.) to both ends of the core.

• Age the core in a crude oil at elevated temperature (for Sandpack 45 deg. C)

• Leave the core for 7 weeks.

• Flip and shake the core every ten days

The Effect of Wettability on Relative Permeability, Capillary Pressure,

Electrical Resistivity and NMR

Saif AL-Sayari

Prof. Martin Blunt