Page 42 - Primer on Enhanced Oil Recovery

Fluid-rock interaction
4

Abstract

Three material types form an oil reservoir formation rock, liquids and gases. We
hope that the liquids will be just oil and the gases would be just some light hydrocar-
bons. As it is an oil formation then we should predominantly have rock and hydrocar-
bons. But as life goes, we will have some water too. All elements interact with each
other and before we start oil and gas extraction all of them are in the thermodynamic
equilibrium. As we start extraction the equilibrioum is no more and the processes
become even more intricate.

Three material types form an oil reservoir formation rock, liquids and gases. We
hope that the liquids will be just oil and the gases would be just some light hydro-
carbons. As it is an oil formation then we should predominantly have rock and
hydrocarbons. But as life goes, we will have some water too. All elements interact
with each other and before we start oil and gas extraction all of them are in the
thermodynamic equilibrium. Equilibrium means that all chemical reaction have
already finished and any changes have stopped long time ago.
All elements in the reservoir are complex and have many subcomponents.
Description of all balances, extraction dynamics and interactions are impossible.
Still, it does not mean that we should not try to understand the most important
moments and predict with a commercial susses the whole system dynamic during
oil extraction. Water usually appears as a relatively simple liquid, but nothing is
simple about the connate water. It can be reasonably understood from three main
points water is polar liquid, it contains dissolved salts and water strongly inter-
acts with the rocks. Water dissolves many chemical salts and dynamically interact
with the rock. Dynamic interaction means that some salts in form of ions interact
with rock and stick to the rock surface. Some minerals from the rock surface dis-
solve in the water if thermodynamic conditions change. Those processes create
absolutely new rock surface which has very different chemical representation and
will interact completely differently with water and oil.
Water, oil and gases interact with rock. Let us forget about gases and say that
we only have oil, water (two immiscible fluids) and a rock surface. First phenom-
ena we encounter is wettability. Wettability in essence shows how strongly surface
interacts with a liquid. The first useful model is represented on Fig. 4.1.
The figure shows interaction (adhesion) between rock and liquids water and
oil. The model contains a droplet of water in oil on a rock surface. In the case a)
water/rock interaction is weaker than oil/rock interaction. In this case we will talk
Primer on Enhanced Oil Recovery. DOI: https://doi.org/10.1016/B978-0-12-817632-0.00004-9
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