Fundamentals of Fluid Flow in Porous Media
Multi-phase Saturated Rock Properties:
Wettability: Laboratory Determination
There are several methods for determining wettability of a rock to various fluids. The main ones are:
Microscopic observation: This involves the direct observation and measurement of wetting angles on small rock samples. One of the most popular methods for measuring the contact angle is the ‘sessile drop method’, which involves depositing a liquid drop on a smooth solid surface and measuring the angle between the solid surface and the tangent to the drop profile at the drop edge. In most of the case the drop is small enough, thus, the gravity action can be neglected. The deposited drop deforms from its initial spherical shape, flattens to form a small cap of liquid and eventually reaches its equilibrium state. Figure 2‑39 shows a schematic diagram of a liquid drop spreading on a solid surface. During the spreading process, the liquid drop will form the so-called ‘dynamic contact angle’ with the solid surface, θc(t), and spread out along the horizontal axis. With the increase of the contact radius of the drop, R(t), the drop will become thinner and its central height h(t), will decrease in order to meet a constant value. The spreading process will continue until the so-called ‘equilibrium contact angle’ is achieved. This contact angle represents the wettability of the solid–liquid–fluid system and can be related to the interfacial tensions of the system by the Young equation.
Figure 2-39: Liquid Drop Spreading on a Solid Surface
The measurements are extremely difficult, and good data relies more on luck than judgment.
There are several issues that must be addressed when measuring contact angles for determining reservoir properties:
- Surface roughness and history of which fluid first contacted the surface will affect the measured value of the contact angle.
- Rock-fluid interaction (e.g. solubility, pH, ions in the aqueous phase, polar groups in crude oil, etc.) will affect the value of the contact angle.
- Polished solids (quartz, calcite) may not be representative of solid surfaces in porous media.
- Time to reach equilibrium (when the contact angle is independent of time) may vary from seconds to days or years. Consequently, the contact angle measured in the laboratory may not represent the natural wettability of the system under examination.
Amott wettability measurements: This is a macroscopic mean wettability of a rock to given fluids. It involves the measurement of the amount of fluids spontaneously and forcibly imbibed by a rock sample. It has no validity as an absolute measurement, but is industry standard for comparing the wettability of various core plugs.
The Amott method (Figure 2‑40) involves four basic measurements. Figure 2‑42 shows how to use measured Amott test data:
- The amount of water or brine spontaneously imbibed = the amount of produced oil in step 1, AB.
- The amount of water or brine forcibly imbibed = the amount of produced oil in step 2, BC.
- The amount of oil spontaneously imbibed = the amount of produced water in step 3, CD
- The amount of oil forcibly imbibed = the amount of produced water in step 4, DA