Abstract

This paper, which uses large amount of data, discusses the wettability of formations and reservoirs and its characteristics by analyzing the surface tension and surface electrostatic force of different rock minerals and pore fluids, the effect of preferential wetting and preferential adsorption on reservoir wettability, and the change of reservoir wettability after water flooding. On the basis of the above discussion, three-state model of pore fluid distribution are divided into three types.

Surface Dynamic Nature of Rock - Forming Minerals, Clay and Fluids
i. Rock-forming minerals

Liquids and solids have surface tension. All the experimental materials indicate that solids have larger surface tension than liquids. The mineral grains which form reservoirs are made up of non-uniform ions with different electric properties. All the ions on the surfaces of the grains have an asymmetrically electric field as well as an asymmetrically gravitational field. These asymmetrical fields make the grain surfaces form a weak electromagnetic field. Now we shall analyze the commonest surface layer.

i) Quartz

Take the commonest - quartz for example. The crystal structure of - quartz is shown in Fig. 1. It is usually known as silicon-oxygen tetrahedron. From this we can know:

  • The tetrahedron surface is oxygen

  • The proportion between silicon and oxygen in SiO2 is 1:2.

  • Because of the above characteristics of the surface structure, the quartz surface is made to be a negative electric field. It is proved by calculation that the force of unit negative charge at the distance b (0) from the surface of - quartz is

    (1)

    where - diameter of O2 - f - Coulomb factor

    e - Charge of an electron

ii) Feldspar

In the gap of the frame structure formed by silicon-oxygen tetrahedrons and aluminium-oxygen tetrahedrons, these are full of potassium or sodium ions sometimes Calcium or barium ions (See Fig. 2). Such crystal characteristics, in addition to the actions of potassium ions, decrease the negative field intensity of the crystal surface and make the negative field intensity of Feldspar surface less than that of quartz's.

Calcite also exists in rock-forming materials of sandstone reservoirs. Analyzing of crystal structure, the polarity of calcite is smaller than feldspar's. Since water is a molecule with strong polarity, general speaking minerals which have strong field intensity have strong hydrophilic nature. Daqing Oilfield made an experiment by use of a crystal of feldspar and quartz and the same quantity of oil and water. The result is that the contact angle of oil on the quartz chip is smaller than that on the feldspar's. This indicates that quartz has a stronger hydrophilic nature than feldspar.

ii. Fluids

The fluids in the pore space are usually oil, air and water.

P. 1

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