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. Introduction After finishing the drilling and cementing operations of an oil/gas well it is necessary to displace the drilling fluid system to a completion fluid system in the well. Well completion requires the use of a packer fluid in the annulus between the casing and the production tubing. Different well completion designs and reservoir conditions set limits on the content of particles in the packer fluids. The required cleanliness of this fluid can vary from several hundred Nephelometric Turbidity Units1 (NTU) down to 10–20 NTU. The displacement of a drilling fluid with a completion fluid is normally performed through several steps. First the drilling fluid is displaced by a casing cleaning fluid. In addition to act as a displacement fluid, the casing cleaning fluid shall remove, or dissolve the remaining oil based drilling fluid and possible debris remaining after the drilling operation. Normally, seawater follows the casing cleaning fluid. Finally the seawater in the well is displaced by the packer fluid. The complete displacement of a fluid by another by hydraulic forces, only, is difficult to achieve in an annulus2. To obtain an efficient displacement of the drilling fluid with a packer fluid, it is common to use different types of chemical additives in the cleaning fluids pumped between the drilling fluid and the packer fluid3. The function of these chemicals is to disperse and break different components of the drilling fluid as well as decrease the surface activity between the two fluids. If oil or synthetic based drilling fluids were used these chemicals would be oil dissolving soaps. If water based drilling fluids were used, it is possible to rely more on displacement efficiency alone. Soaps are not required, however, there may possibly be a need for dispersants in some cases.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCleaning of offshore wells prior to completion operations has previously been associated with a relatively high instant environmental impact. Even though there has been an improvement of the environmental profiles of the chemicals used in well cleaning operations over the past years the discharges of fluids during well cleaning operations has not been significantly reduced. The most common solution has been to have enough storage capacity to collect the volumes of spent fluids offshore and thereafter send the fluids onshore for waste treatment or re-inject the fluids into the subsurface formation.This paper describes a well cleaning system designed to meet the requirements for reduced environmental impact. This has been achieved by applying environmental engineering as a positive aid in the design process identifying the opportunities instead of focusing on the restrictions. The main parameters in the well cleaning design are based on a reduction in discharges, re-use and recovery of fluids, albeit an improvement of the environmental profiles of the chemicals is still very important. The reduction in discharges is achieved by modifying procedures, equipment and chemicals.The paper focuses upon how to obtain an improved well cleaning fluid based on technical and chemical modifications which allow for re-use and recovery of the fluids at the same time as the fluids have their desired well cleaning effects. The improvement this system represents related to environmental impact, has been verified in both laboratory tests and field experiences, and will be presented in the paper.