Geology Essay Example: Reservoir Rock Wettability and Capillary Pressure

Paper Type:  Essay
Pages:  4
Wordcount:  935 Words
Date:  2021-04-01


To start with wettability definition, the whole process of wettability is determined by the compound interface boundary existing between the pore spaces of reservoir rocks. The conditions as a result of the interface wettability determine the movement associated with oil displacement. Wettability is always an issue of concern as it poses flow problems ranging from the process of oil migration from the source rocks to enhancement recovery operations. The problem of reservoir wettability is not a simple one that requires simple definition but a rather complex topic. Various procedures for measuring wettability of a surface are thus available. Water flooding has therefore been applied to mitigate reservoir rock wettability. Laboratory wettability measurement methods and wettability tests are done to find appropriate ways of consolidating the effect of wetting. The studies include determination of water-floods and determination of electrical resistivity against siltation and water saturation relationships.

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Rock Mineralogy: Capillary Pressure Measurement

Mineralogy is one of the subjects of great importance in geology. It deals with the study of the chemical formation and crystal structure together with the physical properties of minerals. Some studies would include classification of minerals and geographic distribution across the earth. Mineralogy influences reservoir wettability via contact angle between the water particle and the crystals of the minerals making up the tank. From the capillary pressure equation in which R represents the roughness of the reservoir and also known as the ratio of the surface area of the crystals of the rock making up the pool, the angle made with the horizontal in a case of contact is the apparent wettability contact angle. The obvious problem with water drops on rough reservoir surfaces is that the pressure at such reduction surfaces varies with height in agreement with the force of gravity. In any case, the solid surface is wetted by the liquid so that bulk gets retained in the surface groove of the reservoir, the receding interface and the apparent angle of contact will be microscopic. Any drop with little intrinsic angle in any case will get set on the grooves of infinite length (Mason, 2013).

Water Salinity and Temperature

Reservoirs with carbonate rock formations happen to contain more than 50% petroleum reserves. At conditions that are relevant, the reservoir rocks get positively charged. In any case, water gets formed in the reservoir, the surface area in contact with water becomes wet. Carbonate reservoirs with high temperatures happen to be wet compared to low-temperature reservoirs because there is a real relationship formed between temperature and acid number. The acid number of crude oil formed always decreases as the temperatures increase. The composition of the water in the reservoir is considered to hinder the wetting process too. Since sulfate is the most active ion in the wetting properties of carbonate rocks, any form of sulfate present in the formation of water in the reservoir affects the wetting conditions for the reservoir (Han, 2016).

Capillary Pressure of Oil and Gas

It is quite difficult for gasses to penetrate media until the capillary pressure exceeds the threshold pressure. The process depends on the size and shape of the pores of the reservoir rock and the wettability of the same. As some capillary pressure increases, saturation of water is bound to decrease in the reservoir rock. Most porous rocks are heterogeneous of which they possess laminations or fractures. Such heterogeneities give rise to bumps concerning wettability of porous surfaces (Iglauer, 2016).

Oil Composition

There are carbonate reservoirs which are mainly associated with the formation of petroleum material. The rock structure includes chalk, limestone, and dolomite. At appropriate conditions in the reservoir, the carbonate surfaces of the rocks get positively charged. In the oil phase, the negative carboxyl group gets connected to the crude oil. Substantial fractions of oil get formed with rocks like asphaltene and resin. In carbonate reservoirs, the carboxylic composition associated with heavier parts of crude oil will turn the surface of the rocks wet over time (Han, 2016).

Thickness of SWI Layer

Methods of surface forces mostly describe the wetting of the mineral surface. Multi-layer force components include electrostatic, van der Waal and structure. These effects found in the rock layers are affected by the presence of brine pH, salinity, crude oil composition and availability of minerals. Due to the disjoining pressure isotherm, surface pressure is experienced within the rock layers. Consequently, to describe rock wettability, a morphological description of the pore space of the reservoir rocks. The factor of bilayer focuses on the intermolecular surface forces that determine the wettability of the reservoir rocks (Hirasaki, 1991).


What causes increased capillary hydrostatic pressure and why is low capillary pressure desirable? With the variable interplay of diagenetic factors such as sand composition and fluid chemistry, increasing stress and temperatures may lead to a wide range of potential outcomes about reservoir porosity and permeability. With high reservoir quality, no strict sense of anomalousness would represent any form of consequences of a particular kind of combination of geological factors. With increasing effective stress through loading of sediment, there is a major physical force that drives the reduction of porosity of the reservoir rocks. There might be the occurrence of porosity of preservation due to fluid overpressures of the sandstones. With appropriate exploration strategies, desirable reservoir quality based on over-pressuring will be realized (Taylor, Giles, Hathon, & Diggs, 2010).



Hirasaki, G. (1991). Fundamentals and Surface Forces. One Petro.

Iglauer, S. (2016). Capillary pressure. PetroWiki.

Mason, G. (2013). Contact Angle Hysteresis at Smooth and Rough Surfaces. Integration.

Taylor, T. R., Giles, M. R., Hathon, L. A., & Diggs, T. N. (2010). Sandstone diagenesis and reservoir quality prediction. Models, myths, and reality, 7-15.

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