8/16/2019 Two Sem Techniques to Investigate Reservoir-rock Wettability, Robin-cambes-culec, 1998

1/2• NOVEMBER 1998 77

E & P E X C H A N G E

In 1989, Inst. Français du Pétrole initiated a research program onthe use of cryo scanning electron microscopy (CSEM) in wettabil-

ity studies. The program led to important results concerning therole of mineralogy (presence and nature of clays) and pore-size dis-

tribution (particularly in the case of carbonates) in the coexistence

of two networks (water wet and oil wet) in so-called mixed-wetta-bility systems.1

The study established that kaolinite found in some clayey sand-

stone reservoir rocks that is originally preferentially water wet

could, after aging under initial-water-saturation conditions, consti-

tute an oil-wet network, while quartz and illite remain water-wet.In carbonate rocks, the study led to the deduction that a water-wet

network was made of small pores, while large pores were at the ori-

gin of the oil-wet network.

This technique also has been used successfully to study the effect

of wettability and spreading on the distribution of three phases(water, oil, and gas) within the pore structure and the efficiency of 

a gas injection at the pore scale.2 The shortcomings of the tech-

nique, however, are that it requires freezing of the rock/fluids and

that it gives a static picture of wetting.

Another research program was launched in 1994 in collabora-tion with Marne-la-Vallée U. to evaluate the potential of environ-

mental SEM (ESEM) to determine the distribution of oil and water

within a porous medium and to observe the dynamics of wetting.

This new method allows investigation of rock samples in the pres-ence of liquids (both oil and brine) without freezing and coating.

Also, fluids can be introduced directly into the observation cham-ber; consequently, dynamic experiments can be observed (e.g., cap-

illary rise of a wetting fluid in the presence of a nonwetting fluid).For these observations, the contrast is mainly topographic, pre-

senting a great depth of field (Fig. 1). A slight chemical contrast is

superimposed because of the presence of backscattered electrons.

Chemical contrast is directly related to the variation of the mean

atomic number Z, allowing differentiation between the mineral

(Z=11, light), the brine (Z=7, medium gray), and the oil (Z=5.5,dark gray).

Experiments first were performed with very simple model

porous media consisting of sintered-glass beads specially treated to

become strongly oil wet. Spontaneous oil imbibition in such cores

saturated with brine was observed on the top of the sample as darkrings appearing at the edge of the beads (Fig. 2).

Robin et al.3 conducted similar studies on reservoir rocks of 

intermediate wettability (spontaneously imbibing either brine or

oil). Their studies showed wettability heterogeneity at the pore

scale, leading to a mixed wettability at a macroscopic scale, and

TWO SEM TECHNIQUES TO INVESTIGATERESERVOIR-ROCK WETTABILITY Michel Robin, SPE, Inst. Français du Pétrole; Rossana Combes, Marne-la-Vallée U.;

and Louis Cuiec, SPE, Inst. Français du Pétrole

Fig. 1—Dry sintered-glass sample. Fig. 2—Oil imbibition in oil-wet sintered glass.

(SPE 52526)

8/16/2019 Two Sem Techniques to Investigate Reservoir-rock Wettability, Robin-cambes-culec, 1998

2/2

• NOVEMBER 1998 79

also demonstrated the influence of composition, size, and distrib-

ution of minerals on physicochemical behavior of the sampletoward fluids.

In the mixed-wettability sandstone, oil capillary rise occurredthrough kaolinite zones (Fig. 3), while for carbonate, oil capillary

rise occurred through the largest pores (Fig. 4). This was in agree-

ment with the static observation made with CSEM.Both CSEM and ESEM techniques must be considered as very ef-

ficient tools for studying heterogeneous systems. They offer new

perspectives for understanding the influence of surface-properties

distribution on two- and three-phase displacements.

REFERENCES

1. Fassi-Fihri, O., Robin, M., and Rosenberg, E.: “Wettability Studies at the

Pore Level: A New Approach by the Use of Cryo-Scanning Electron

Microscopy,” SPEFE (March 1995) 11.2. Vizika, O., Rosenberg, E., and Kalaydjian, F.: “Study of Wettability and

Spreading Impact in Three-Phase Gas Injection by Cryo-Scanning Elec-

tron Microscopy,” J. Petroleum Science & Engineering, (June 1998) 189.

3. Robin, M. et al.: “Wettability of Porous Media from Environmental Scan-

ning Electron Microscopy: From Model to Reservoir Rocks,” paper SPE

37235 presented at the 1997 SPE International Symposium on Oilfield

Chemistry, Houston, 18–21 February.

Fig. 4—Middle East carbonate, appearance of oil in a large pore.Fig. 3—North Sea sandstone, appearance of oil in a pore filled

 by kaolinite.