Laboratory tests were carried out to study the removal of drilled particles in a well borehole simulated annulus. The main objectives of these tests were to determine the effect of varying: (1) the particle size and particle size distribution; (2) the annular flow rate; (3) the fluid type viscosity and density; (4) the rate of rotation of the inner pipe. Dropping tests with individual particles of various sizes were also performed to study the behavior of, and to determine the settling velocity of these particles in a quiescent Newtonian fluid medium of varying viscosity. From these experiments a general settling velocity equation applicable to all particle sizes and an expression for the cumulative mass fraction of particles removed from the annulus are formulated. It was concluded that: (1) Under a given constant state, the total mass fraction of particles removed may be less than one. (2) At low annular flow rates, the first arrival time, (tmin)ap, for any one particle size may be greater than the minimum time based on the relative average fluid and single particle settling velocities. (3) The initial number of particles of any one particular particle size affects fcmax and (tmin)ap. (4) Rotating the inner pipe does increase the rate at which particles are removed and the total number of particles removed. (5) Introducing a drilling mud into the annulus may or may not increase the rate at which particles are removed, compared to the same annular flow rates using water. The exact behavior of the particles under these conditions was not determined, and the reduction of Reynolds number from turbulent to laminar flow by use of the drilling muds does not permit a true comparison between water and drilling muds. (6) In the laminar flow of drilling muds, it appears that one must consider both the relative flatness of velocity profiles and the particle Reynolds number in evaluating the carrying capacity capabilities of these fluids. A flatter velocity profile (low power law exponent) alone does not necessarily insure better carrying capacity.