Oil-Based Drilling Fluids

These fluids, because of their special nature of being a mixture of two immiscible liquids (oil and water), require special treatments and testing procedures.

Dispersed Phase: The liquid present in the form of finely divided droplets.

Continuous Phase: The liquid present in the form of the matrix in which the droplets are suspended.
To keep these liquids stabilized (i.e. to keep the dispersed phase from coalescing and settling out of the mixture), an emulsifier is added to form an interfacial film around the dispersed phase which causes them to repel each other, so they remain dispersed.
The effectiveness of an emulsifier depends on the alkalinity and electrolytes (chloride content) of the water phase, and the temperature of the drilling fluid.

Electrical Stability

The electrical stability (E.S.) of an oil-based drilling fluid is the stability of the emulsions of water in oil, or the amount of current required to break the emulsifier down and allow the saline water to coalesce.

1. An electrical probe is inserted into the drilling fluid and the voltage increased until the emulsion breaks down

a. the measure of emulsion breakdown is indicated by current flow
b. relative stability is recorded as the amount of voltage at the breakdown point

2. E.S. is recorded as the voltage reading and temperature of the drilling fluid sample

a. adding emulsifier will raise the E.S. readings
b. normal “fresh” mud is about 300 or higher
c. during drilling, the E.S. can increase to 800 or higher

Oil: Water Ratio

The Oil: Water Ratio is defined as the percent oil in the liquid phase and the percent water in the liquid phase. The percentages can be determined from a retort analysis of the drilling fluid.



Aniline Point

Another common term used when dealing with oil-based drilling fluids is the aniline point of that fluid. The aniline point is the temperature below which an oil containing 50% by volume aniline (C6H5-NH2) becomes cloudy. The solvent powers for rubber are related to the solvent power for aniline. Oils having an aniline point above 140oF are considered acceptable to use.