Visualizing Pores and Nonwetting Phase in Porous Rock
Authors: Swanson B.F. 
Published: December 2007

This paper presents scanning electron photomicrographs showing the spatial distribution of nonwetting phase in several example reservoir rocks. Intrusion porosimetry is used to control the saturation of nonwetting Wood's metal. The photographs enhance our concepts of nonwetting phase distribution in rock. Fluid flow implications and pore features affecting residual oil saturation are discussed.

Introduction

Few realistic visual aids exist for envisioning the tortuous paths taken by fluids flowing through porous rock. The paths taken by fluids flowing through porous rock. The distribution of a nonwetting phase, such as crude oil in water-wet rock, can be even more difficult to envision. We have found that by combining scanning electron microscopy (SEM) with Wood's metal porosimetry, concepts of nonwetting phase distribution become clearer. Our concepts of pore distributions are elevated to the level provided by the SEM when studying grain structure in rocks.

Several researchers have made use of casts of pores in rock to demonstrate features of the pore systems. Dullien and Dhawan used Wood's metal for their casts, whereas Wardlaw used plastic for impregnation. For the most part, these efforts produced pore casts at very high saturation. The resulting casts were viewed mostly in cross-sections or polished surface form. By using a somewhat different sample preparation sequence and taking SEM photographs, we can achieve highly magnified, in-depth views of the distribution of nonwetting fluid in rock at various nonwetting phase saturations.

Wood's Metal Impregnation Procedure

Wood's metal is an alloy of bismuth containing lead, tin, and cadmium with a melting point of 70 degrees C. Wood's metal intrusion into rocks is performed in nearly the same manner as in mercury intrusion porosimetry, except that the temperature is higher than 70 degrees C.

Comparison of Wood's metal capillary-pressure data with those for mercury indicates that the alloy acts as a nonwetting liquid with an effective surface tension of about six-tenths that of mercury. The actual value is not needed here.

Fig. 1 shows the cell used for impregnations. A cylindrical rock sample of known pore volume is cemented to the bottom of a glass vial so that it will not float on the molten Wood's metal. Chunks of Wood's metal are placed above the sample. A closely fitting "float" rests on the Wood's metal.

The cell is closed, evacuated, and placed in an oil bath at 90 degrees C. As the Wood's metal melts and surrounds the sample, the float drops and becomes supported by the surface tension of the liquid metal. Float motion is detected by the linear variable differential transformer (LVDT).

When the float position is stable, injection begins by bleeding the vacuum in steps while observing LVDT output for evidence of float movement. Nitrogen is used for pressures above 1 atm. By knowing the inside diameter of the glass vial and the linear float movement, the volume of Wood's metal injected into the rock can be calculated. This volume divided by pore volume is the Wood's metal saturation. In this way, we control the nonwetting-phase saturation by controlling capillary pressure. pressure.When the desired saturation is reached, the cell is cooled while still at the final pressure. The sample is removed by breaking the glass.

JPT

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