Removing water from crude oil often requires additional processing beyond gravitational separation. In selecting a treating system, several factors should be considered to determine the most desirable methods of treating the crude oil to contract requirements. Some of these factors are:
1. Tightness of the emulsion.
2. Specific gravity of the oil and produced water.
3. Corrosiveness of the crude oil, produced water, and casing head gas.
4. Scaling tendencies of the produced water.
5. Quantity of fluid to be treated and percent water in the fluid.
6. Paraffin-forming tendencies of the crude oil.
7. Desirable operating pressures for equipment.
8. Availability of a sales outlet and value of the casing head gas produced.
A common method for separating this “water-in-oil” emulsion is to heat the stream. Increasing the temperature of the two immiscible liquids deactivates the emulsifying agent, allowing the dispersed water droplets to collide. As the droplets collide they grow in size and begin to settle. If designed properly, the water will settle to the bottom of the treating vessel due to differences in specific gravity.
The process of coalescence requires that the water droplets have adequate time to contact each other. It also assumes that the buoyant forces on the coalesced droplets are sufficient to enable these droplets to settle to the bottom of the treating vessel. Consequently, design considerations should necessarily include temperature, time, viscous properties of oil that inhibit settling, and the physical dimensions of the vessel, which determine the velocity at which settling must occur.
Laboratory analysis, in conjunction with field experience, should be the basis for specifying the configuration of treating vessels. The purpose of this chapter is to present a rational alternative for those instances when laboratory data do not exist or, if it is desirable, to extrapolate field experience.
