We will now describe fluid displacement, or how one fluid replaces another in a porous medium. We will start with a pore space that is entirely saturated with the wetting phase and allow a non-wetting phase to enter. This is a primary drainage process. The word primary (meaning first) indicates that this is the first time the non-wetting phase enters the pore space: we start with a wetting phase saturation of 1. Drainage, in general, refers to the displacement of a wetting phase by a nonwetting phase.

There are three common examples of primary drainage in natural systems, listed below.

1. (1) The migration of oil and gas from source rock (shale) to a hydrocarbon reservoir. Here the hydrocarbon migrates upwards under buoyancy (it is less dense than the brine in the pore space) until it encounters a barrier to movement, under which it then collects. This accumulation becomes the reservoir. Locally, oil invades the pore space driven by the buoyancy force caused by the density difference between oil and water.

2. (2) The injection of CO2 into a saline aquifer. Here CO2 is forced into the pore space of the rock, displacing brine, for long-term underground storage. The driving force, providing the local capillary pressure, comes from the CO2 pressure in the injection well, which is higher than in the resident brine. Furthermore, as in oil migration, the CO2 is less dense than brine and will rise in a storage aquifer and again collects under impediments to flow. This is also a primary drainage process.

3. (3) Mercury injection capillary pressure (MICP) measurements. These are measurements routinely performed on small (cm-sized) cylindrical rock samples (called plugs) to assess the pore-size distribution, as discussed below. The rock is cleaned and dried and placed in a vacuum that acts here as the wetting phase. Mercury is the non-wetting phase. It has a very high surface tension, since it is metallic (see Chapter 1), and a correspondingly high interfacial tension with the solid, since not only the bonds in the solid are broken, but the bonding in the metal too. Indeed, the interfacial tension between mercury and the solid is higher than the solid and a vacuum (where only the solid bonds are broken), resulting in a contact angle that is greater than 90° when measured through the mercury: this can be seen from the Young equation (1.7).