Removal of Water Base Mud From Well Bores in Permafrost Zones

In completing an Arctic oil or gas well, it is sometimes desirable to remove water-base drilling mud from a casing-casing or casing-tubing annulus to minimize the possibility of casing damage resulting from freezing and thus expansion of the fluid. The proposed field procedure involves an initial water wash step, where the mud is washed from the annulus by water, followed by the pack placement step where the water is displaced by an oil-base gelled and weighted casing pack. The casing pack fills the entire annulus and also acts as a thermal insulator by suppressing convection currents with its high gel strength. The study was performed to examine the fluid displacement mechanics involved in the field operations and to determine the methods of operation which should be adopted. The operations were studied in 10-ft long model annuli of various sizes in the laboratory and then in a 900-ft test hole in the back yard. Findings from these studies were then applied to the displacement procedures in a number of field operations at Prudhoe Bay, Alaska. Data gathered from these field operations were analyzed, and they compared satisfactorily with predictions from laboratory data. It was found that, for the water-wash step, two system volumes of water was sufficient to remove virtually all the drilling mud from the annulus. For the pack placement step, two system volumes of pack was sufficient to remove virtually all the water. Both displacements should be carried out with the water in turbulent flow. A system of commercially available electronic instruments was developed for monitoring water contamination of the pack returns. It provides instant indication of excessive contamination in case of displacement failure due to equipment breakdown or other unforeseen circumstances, so that remedial actions may be taken immediately.