Abstract: A microprocessor controlled rotating-cylinder viscometer for measuring the shear stress at a given shear rate of a fluid is disclosed. The fluid is contained between a rotatable outer cylinder and a rotatable inner cylinder. The inner cylinder rotates in response to the torque produced by rotation of the outer cylinder in the fluid. The amount of rotation of the inner cylinder is related to the shear stress of the fluid. A fast slewing accurate motor speed control is provided for maintaining rotation of the outer cylinder at the selected shear rates. An absolute value shaft encoder monitors the amount of rotation of the inner cylinder to obtain the shear stress readings. The steady state value of the shear stress is obtained by averaging a predetermined number of measurements taken over a sample interval to obtain a stress reading. Consecutive readings are compared until the difference between readings in less than .+-.1.degree., the current shear stress then taken on the steady state value.

Abstract: A new weighting material for use in controlling the specific gravity of fluids employed in the drilling, completion, perforation and workover of oil and gas wells is disclosed. The material siderite (FeCO.sub.3), has the unique property of being removable from the bottom of well boreholes and producing formation zones by virtue of almost complete solubility in both mineral and organic acids. This weighting material is compatible in both oil and water muds and has a density of 3.8 g/cc which allows formulation of fluid weights up to 19 lbs./gal.