Wall cake. The solid material deposited along the wall of the hole resulting from filtration of the fluid part of the drilling fluid into the formation. See: cake thickness, filter cake.
Wall sticking. See: differential pressure sticking.
Warp. In a woven cloth, the direction of the wires running parallel with the loom or running the length of a roll of cloth. In wire cloth production, these are the long or longitudinal wires. See: loom.
Water-based drilling fluid. Common, conventional drilling fluid. Water is the suspending medium for solids and is the continuous phase, whether or not oil is present. See: water loss, filtration.
Water-based mud. See: water-based drilling fluid.
Water block. A reduction in the permeability of the formation caused by the invasion of water into the pores (capillaries). The decrease in permeability can be caused by swelling of clays, thereby shutting off the pores, or in some cases by a capillary block of wetted pores due to surface tension phenomena.
Water feed. Water to be added for dilution of the mud feed into a centrifugal separator. See: dilution water.
Water loss. See: filtration, fluid loss.
Water wet. Not oil wet. A surface on which water easily spreads. If the contact angle of a water droplet on a surface is less than 90 , the surface is water wet. See: hydrophilic, oil wet.
Water-in-oil emulsion. See: invert oil-emulsion drilling fluid.
Weft. See: shute.
Weight. In drilling fluid terminology, the density of a drilling fluid. This is normally expressed in either lb/gal, lb/cu ft, psi hydrostatic pressure per 1000 ft of depth, or specific gravity related to water. See: density.
Weight material. Any of the high–specific gravity materials used to increase the density of drilling fluids. This material is most commonly barite or hematite. In special applications, limestone is also called a weight material.
Weight percentage. The number of weighted parts of the total weight. Weight percentage is the most common method of reporting oil in solids discharges and mass balance calculations. See: volume percentage, ppm.
Weight up. To increase the weight of a drilling fluid, usually by the addition of weight material.
Weighted drilling fluid. A drilling fluid to which commercial solids have been added to increase the slurry weight.
Weighted mud. See: weighted drilling fluid.
Well bore. The hole drilled by the bit, also known as the borehole.
Well-bore stabilization. Maintenance of well-bore integrity, which generally
requires manipulating the properties of the drilling fluid to simulate the physicochemical environment of the rock before it was drilled.
Well logging. See: electric logging, mud logging.
Wetting. The adhesion of a liquid to the surface of a solid.
Wetting agent. A substance that, when added to a liquid, increases the spreading of the liquid on a surface or the penetration of the liquid into a material.
Whipstock. A device inserted into a well bore to cause the drill bit to exit the established path of the existing well bore. The whipstock is the tool used for the initiation of directional drilling.
Wildcat. A well in unproved territory.
Windage loss. (1) The resisting power of air or air friction acting against
a rapidly rotating armature or cooling fan to create a power loss.
(2) The resisting power of air or air friction against the rotating bowl of a centrifuge.
Wire cloth. Screen cloth of woven wire. See: screen cloth.
Working pressure (WP). The maximum pressure to which equipment should be exposed in order to comply with manufacturer’s warranty and be within industry codes and safety standards.
Workover fluid. Any type of fluid used in the workover operation of a well.
Yield. The quality of a clay in terms of the number of barrels of a given
viscosity (usually 15 cP) slurry that can be made from a ton of the clay.
Based on the yield, clays are classified as bentonite, high-yield, lowyield,
etc. Not related to yield point. See API RP 13B.
Yield point. (1) A term derived from a direct-reading viscosimeter (Fann
V-G or equivalent) based on subtracting the plastic viscosity from the
300-rpm reading. (2) An extrapolated shear stress at zero shear rate created by assuming a linear relationship between shear stress and shear rate and determining the intercept on the shear stress axis. The linear relationship between shear stress and shear rate that results in a yield point is called a Bingham Plastic model. See: viscosity, plastic viscosity, API RP 13B.
Yield value. See: yield point.
Zero-zero gels. A condition wherein the drilling fluid fails to form
measurable gels during a quiescent time interval (usually 10 minutes).
The measurements of gel are made with a direct-reading viscometer at
intervals of 10 seconds and 10 minutes. See: progressive gel, flat gel.
Zeta potential. The electrokinetic potential of a particle as determined
by its electrophoretic mobility. This electric potential causes colloidal
particles to repel each other and stay in suspension.
Zinc bromide. ZnBr2. A very soluble salt used to increase the density of
water or brine to more than double that of water. Normally added to
calcium chloride/calcium bromide mixed brines.