Page 147 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Effect of Gas Wettability on the Surface Properties CHAPTER 4 131

that of water-wet quartz sand by one order of magnitude. Analyzing the
reason, normally, the surface area of bulk solids is smaller, so the effect of
surface energy is not obvious, and the gas adsorption phenomenon is also
not remarkable. But, for highly dispersed solid powder and porous
medium, the surface area of the substance per unit weight is considerable
and the effect of surface energy, i.e., adsorption effect, seems very signifi-
cant. In addition, the inner surface area of interior pores of grains is usu-
ally larger than the exterior surface area by several orders of magnitude.
The greater number of pores and smaller sizes result in a larger surface
area. The quartz sand is used to simulate and constitute the framework of
reservoir rocks in this experiment, which truly reflects effects of wettability
alteration on gas adsorption capability compared to measuring with cores,
and eliminates interference of surface area and pore structure in the experi-
mental results.

4.1.3 Quantum Chemistry Theory of Gas Adsorption
Capacity of Rocks
All solid substances possess large or small capacity for adsorbing molecules,
atoms, or ions from surrounding media on their surfaces. This property is
called adsorption property of substances. Researchers in China and overseas
have conducted many theoretical studies on the interaction of H 2 O, CH 4 ,CO 2
and N 2 , with coal surfaces. It was agreed that the adsorption sequence for
coal-surface models on various gas molecules is H 2 O . ＞CO 2 . ＞CH 4 . ＞N 2
[3 7]. However, there are few studies on the interaction of reservoir rocks sur-
faces with various wettabilities and gases.
This section establishes two types of atom group models: liquid wetting rock
surfaces and gas wetting rock surfaces with Gaussian03 software and quantum
chemistry. It studies the interaction among molecules of rock surfaces of liquid
wettability and gas wettability, and different fluids (gas phase and water phase,
totalling four fluids), and states the effect of wettability on fluid adsorption
property from a micro perspective.
4.1.3.1 ADSORPTION OF DIFFERENT FLUIDS ON SOLIDS

Different chemical compositions cause soild surfaces to have different adsorp-
tion capacities, which is mainly due to the different interactions between
chemical composition and solids. The interaction relates to physicochemical
parameters, including boiling points of various adsorbing substances, critical
temperature, critical pressure, ionic potential, intrinsic dipole moment, etc.,
under atmospheric pressure, as shown in Table 4.5.
Based on theory, the adsorption sequence of the above four fluids on solid
surfaces is H 2 O . ＞CO 2 . ＞CH 4 . ＞N 2 . However, for physical adsorption (the
acting force between adsorbents and adsorbate is constituted by Debye
induction force and London dispersion force, and adsorption heat is nor-
mally less than 20 kJ/mol), the depth of adsorption potential relates well to