Abstract
The removal of drilling fluids from the cased well prior to installation of critical completion equipment often involves use of special fluids to ensure a complete removal of both the drilling fluid itself and particles attached to the casing wall. This is a critical operation where failure can lead to excessive complications in the subsequent completion operation with stuck packers or formation damage due to particle invasion into the formation as a consequence. Furthermore, these operations have the highest attention from the environmental authorities because casing cleaning operations are considered as highly polluting operations during drilling and completion of a well. Modern drilling fluids used in the North Sea region are designed to meet occupational hygiene requirements and environmental criteria. These drilling fluids are relatively difficult to remove from the casing surface.
The current paper describes in detail a well cleaning fluid system designed to meet these technical objectives with special focus on reducing environmental impact. The casing cleaning fluid system is designed using a partly unstable emulsion, where the surfactants are partly dissolved in the water phase and partly used as emulsifiers. Since the emulsion is unstable, the individual components can be separated after the operation. This allows for re-use and recovery of the fluid components at the same time as the fluids has optimized the well cleaning efficiency. The removed base oil can be used to formulate new drilling fluid or re-used in casing cleaning fluid systems.
The improvement of new designed fluid has been verified in both laboratory tests and North Sea field experiences. Field experience from approximately 30 operations show that the average operation time has been significantly reduced. The paper compares field experience obtained using different casing cleaning operation techniques.
