Abstract: Improved apparatus and methods for determining the shear stress required for removing drilling fluid deposits formed on the walls of a well bore are provided. The methods basically comprise introducing a drilling fluid into a test apparatus which simulates a permeable section of a well bore. Drilling fluid deposits are caused to be formed on the walls of the permeable section, and the drilling fluid is circulated through the permeable section at progressively increasing flow rates for time periods whereby the pressure drop of the drilling fluid stabilizes. The stabilized pressure drop below which no appreciable erosion of the deposits takes place is determined by acoustically measuring and comparing the thicknesses of the drilling fluid deposits at each of the flow rates when the pressure drop stabilized.

Abstract: The present invention relates to methods of ensuring the maximum removal of drilling fluid from a well bore including gelled drilling fluid and filter cake deposited on the walls of the well bore prior to placing cement in the well bore between a pipe disposed therein and the walls thereof. The methods basically comprise circulating the drilling fluid through the well bore while monitoring the drilling fluid pressure, flow rate, viscosity, temperature and density. The circulating volume of drilling fluid in the well bore is then calculated and compared with the total volume available in the well bore to determine the volume of drilling fluid remaining on the walls of the well bore.

Abstract: Methods of measuring the erodability of drilling fluid deposits and the shear stress required to remove drilling fluid deposits formed on the walls of a well bore are provided. The methods basically include introducing a drilling fluid into a test apparatus which simulates a permeable section of a well bore. Drilling fluid deposits are caused to be formed on the walls of the permeable section, and the drilling fluid is circulated through the permeable section at progressively increasing flow rates to determine the pressure drop below which no appreciable erosion of the drilling fluid deposits takes place which corresponds to the minimum shear stress required to erode the deposits. The erodability of the drilling fluid which is inversely proportioned to the minimum shear stress can also be determined. Apparatus for carrying out the methods is also provided.