Oil Muds – Osmotic Theory and Borehole Stability-Water Activity– Water Phase Salinity

Osmotic Theory and Borehole Stability

•Osmosis is the net movement of water across a selectively permeable membrane driven by a difference in solute concentrations on the two sides of the membrane •In reference to Invert emulsion oils muds the water phase is saturated with Calcium Chloride –While drilling occurs and new shales are exposed to the fluid, the water captured in the shale will move with osmosis into the fluid effectively drying out the shale –Using this theory, the shale in the borehole cannot hydrate therefore it becomes more stable

Water Activity

•Water activity (AW) is a measure of the chemical potential for water to be transferred between mud and shales. –Activity is measured using the vapor pressure (relative humidity) of shale or mud. –Activity can also be estimated based on the chemical composition of the brine (salinity). –Pure water has an AWof 1.0. –Calcium chloride brines used in most non-aqueous emulsion muds have an AWbetween 0.8 (22% wt) and 0.55 (34% wt). –Lower values for activity are more inhibitive.

Water Phase Salinity

•Calcium chloride is added to increase the emulsified water phase salinity to provide inhibition of shales and reactive solids. •The range for calcium chloride content is usually 25 to 35% by weight. •The CaCl2content should be determined by titration and can be calculated by: •% CaCl2 (wt) =(Ag x 1.565)/((Ag x 1.565) + %H2O) x 100 •Where: –Ag = cm3 0.282 N silver nitrate per cm3of mud –% H2O = Volume % water from retort •The concentration can be adjusted by adding powdered calcium chloride over several circulations. –Powdered CaCl2is preferred over flake CaCl2, because the larger flake particles do not readily dissolve in oil and synthetic muds. –Flaked salts must first be dissolved in water before being added to a non-aqueous system. –The powdered form is generally available as 94 to 97% active material.