Page 143 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Effect of Gas Wettability on the Surface Properties CHAPTER 4 127

4.1.1.4 FACTORS AFFECTING ADSORPTION CAPACITY
4.1.1.4.1 Temperature
Gas adsorption is an exothermic process, so irrespective of whether it is physi-
cal or chemical adsorption, adsorption capacity decreases when the tempera-
ture rises.

4.1.1.4.2 Pressure
Irrespective of whether it is physical or chemical adsorption, an increase in
pressure promotes adsorption capacity.

4.1.1.4.3 Property of Rocks and Gas Molecules
Rocks adsorb polar gas molecules more easily. Normally, the more complex
gas molecules are, and the higher boiling point is, the stronger adsorption
capacity is. The pore structure and pore size of rocks have significant effect on
adsorption rate and adsorption capacity.

4.1.2 Experimental Studies on Gas Adsorption Capacity
of Rocks
The wettability of oil and gas reservoir rocks affects not just location and
distribution of liquids in pores, but the adsorption effect on gas as well. This
section details an experimental study on methane gas adsorption capacity
before and after water-wetting alteration by adopting AST-2000 simulation
experimental apparatus for bulk sample coalbed gas adsorption/desorption
with quartz sand simulating oil and gas reservoirs. The methane adsorption
capacity of quartz sand after gas wetting alteration is reduced by one order of
magnitude, the main reason being that the decrease in surface energy weakens
gas adsorption capacity.

4.1.2.1 RELATIONSHIP BETWEEN SURFACE WETTABILITY AND
SURFACE FREE ENERGY OF SOLIDS
There is a great difference in structure, chemical composition, and atoms’
interaction forces between solid surface and the internal bulk phase. The
interaction of atoms within the bulk phase reaches statistical balance, while
the atomic force-field on the surface is unsaturated with residual force field on
the free surface. Also, various phenomena, such as lattice defects, vacancy, and
dislocation, etc., lead to universal nonuniformity of solid surface due to irregu-
lar crystals. The residual force-field and nonuniformity of solid surface result
in different types of surface energy of the solid surface. However, oil and gas
reservoir rocks are typically high-energy surfaces [1].
Surfaces have a tendency to reduce surface energy spontaneously, but atoms or
molecules on solid surfaces cannot move freely at normal temperature. They
don’t reduce surface energy by shrinking like liquids, but by decreasing interfa-
cial tension. When a liquid is in contact with a solid surface, if the liquid has