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 1.972.952.9435. AbstractThe need for an efficient displacement of drilling fluids from cased wells prior to installation of completion equipment has been relatively undisputed in the past.Several different fluid systems has been applied as well as a variety of different mechanical tools to achieve what is considered to be a clean well in order to safely run advanced completion equipment in the well. In contradiction to the requirements related to running more advanced equipment in the wells is the fact that several of the fields on the Norwegian shelf are entering the mature phase and tail end production. For these fields the margins are less and a significant focus is put on cost efficient solutions. In many cases, the need for casing cleaning in conjunction with displacement of drilling fluids is questioned.The current paper are reviewing in detail the results from drilling fluids displacement and cased hole cleaning operations performed on 192 wells on the Norwegian continental shelf over the six past years.These wells cover a variety of different completion scenarios where both water based and oil based drilling fluids have been utilised.The results from the survey are presented with a focus on the requirements towards a clean well and the ability to measure the efficiency of the different fluid systems and operational procedures utilised.The paper discuss different casing cleaning requirements versus well completion scenarios and displacement techniques.

Open hole gravel packing of reservoir sections drilled with oil based fluid is traditionally performed with an aqueous carrier fluid. This typically involves displacing oil based fluid to aqueous fluid once the gravel pack screen is in place. In reservoirs with swelling or unstable shale this approach reduces the risk associated with open hole exposure to aqueous fluid over time. However experience has shown that instability can still occur resulting in an incomplete, or even an aborted gravel pack. In addition, mixing of incompatible oil and water based fluids downhole has the potential to generate very viscous emulsions that negatively impact gravel pack efficiency and well productivity. The objective of the new technology was to maintain borehole stability, eliminate fluid incompatibility and enable a complete gravel pack.An oil based carrier fluid has been developed and qualified using laboratory and yard scale testing. The fluid is a solids free invert emulsion that exhibits near Newtonian rheological behavior; thereby promoting settling of proppant during gravel packing. The density of the fluid is controlled by adjusting the volume fraction and density of the brine phase. The fluid has been qualified up to a density of 1.25 SG with further potential to achieve a density of 1.63 SG.The oil based carrier fluid has been introduced on a mature field, with a long history of gravel pack completions. Progressive reservoir depletion has created operational challenges, resulting in inconsistent gravel pack performance. Consequently, several procedural changes have been implemented over time. Reservoir inclination is typically up to 50°, open hole length up to 200 meters, and bottom hole static temperature around 90°C. Gravel packs were most recently performed with a 1.10 SG aqueous carrier fluid.The new carrier fluid has exhibited stable properties during implementation on multiple well completions. Gravel pack efficiencies have been consistently good, at 100% or higher. As a result, well productivity expectations have consistently been achieved or exceeded. The operational time for installing the lower completion compares well with the traditional approach with aqueous fluid.The implementation of oil based gravel packs in multiple wells, allows a comparison with brine based gravel packs in the same field. It is therefore considered to be an industry first.

TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractThe need for an efficient displacement of drilling fluids from cased wells prior to installation of completion equipment has been relatively undisputed in the past.Several different fluid systems has been applied as well as a variety of different mechanical tools to achieve what is considered to be a clean well in order to safely run advanced completion equipment in the well. In contradiction to the requirements related to running more advanced equipment in the wells is the fact that several of the fields on the Norwegian shelf are entering the mature phase and tail end production. For these fields the margins are less and a significant focus is put on cost efficient solutions. In many cases, the need for casing cleaning in conjunction with displacement of drilling fluids is questioned.The current paper are reviewing in detail the results from drilling fluids displacement and cased hole cleaning operations performed on 192 wells on the Norwegian continental shelf over the six past years.These wells cover a variety of different completion scenarios where both water based and oil based drilling fluids have been utilised.The results from the survey are presented with a focus on the requirements towards a clean well and the ability to measure the efficiency of the different fluid systems and operational procedures utilised.The paper discuss different casing cleaning requirements versus well completion scenarios and displacement techniques.