1. n. [Drilling Fluids]
The escaping tendency, or vapor pressure, of water molecules in an aqueous solution compared with that of pure water, typically abbreviated aw. Activity is expressed mathematically as the ratio of two vapor pressures: aw = p/po, where p is vapor pressure of the solution and po is vapor pressure of pure water. The ratio ranges from near 0 to 1.0 and corresponds to percent relative humidity (% RH) of air in equilibrium with the aqueous solution. For pure water, aw = po/po = 1.00 and RH = 100%. By increasing the concentration of salt (or other solutes) in the solution, aw decreases, because vapor pressure of the solution decreases. However, aw never reaches zero. Known-activity, saturated-salt solutions are used to calibrate RH meters. Measuring RH of air above an oil mud is a simple way to measure the activity (salinity) of its water phase. Adjusting the salinity of the water phase is a way to control movement of water into or out of shales that are being drilled with an oil mud. Chenevert related aw in oil mud to RH above the mud sample and devised a practical test using an electrohygrometer to measure RH, called the "Chenevert Method."