Oil Muds –Oil/Water Ratio–Electrical Stability– Gas Solubility

Oil/Water Ratio

•The oil-or synthetic-to-water (O/W or S/W) ratio relates only to the liquid portion of the mud and is not affected by the solids content.
–The oil-or synthetic to-water ratio relates the oil and water fractions to the total liquid fraction.
–Generally, higher mud weights require higher ratios.
–Different conditions favor the use of different ratios, so there is no ratio that must be used for any set of conditions.
•The calculation of the oil-to-water ratio requires retort values as follows:
–Oil ratio (O) = (vol% oil)/(vol% oil + vol% water) x 100
–Water ratio (W) = 100 –oil ratio
•The O/W ratio remains constant when the mud is weighted up or solids are incorporated into the mud, even though the volume percent liquid is decreased significantly.
–A rapid decrease in the O/W or S/W ratio indicates an influx of saltwater from the formation, and a pit volume increase should have been observed.
•When using oil or synthetic muds, all water hoses on the pits should be disconnected or plugged to prevent accidental contamination with water
•The viscosity and HTHP filtrate will change with changes in the oil-or synthetic-to-water ratio.
–Changing the ratio is not used to alter either of these properties.

Electrical Stability

•The electrical stability is an indication of how well (or tightly) the water is emulsified in the oil or synthetic phase.
–Higher values indicate a stronger emulsion and more stable fluid.
–Oil and the synthetic fluids do not conduct electricity. In the electrical stability test, the voltage (electrical potential) is increased across electrodes on a fixed-width probe until the emulsified water droplets connect (i.e., coalesce) to form a continuous bridge or circuit.
•The stronger the emulsion, the higher the voltage required to break down the emulsion completing the electrical circuit to conduct electricity.
•The unit of measure for recording the electrical stability is volts.
•Factors that can influence the electrical stability are:
–Water content
–Water wet solids
–Emulsion strength
–Temperature
–Salt concentration
–Saturation
–Weight material
•Freshly mixed, invert-emulsion muds usually have low electrical stabilities when shipped from the liquid mud plant, even though they are adequately treated with emulsifiers.
–The emulsions of these systems will tighten as they are exposed to downhole temperatures and sheared through the bit.

Gas Solubility

•Oil and synthetic fluids are soluble to methane and other gases encountered while drilling.
•They have high gas solubility to natural gas, carbon dioxide and hydrogen sulfide, see pic.
•This can interfere with kick detection and well-control procedures.
–This soluble gas does not begin to come out of solution until the pressure is reduced as the mud is circulated up the annulus.
–The majority of the gas expansion occurs in the last 1,000-ft interval below the surface.
–For this reason, extra care should be taken to monitor pit levels with these systems and when handling the influx of wellbore fluids.
–It is important to be able to monitor and detect kicks to a level of about 5 bbl