Hole Cleaning Performance of Oil and Water based Drilling Fluids in Circular and Non-Circular Boreholes.

Wells in the petroleum industry are drilled for distances more than 10.000 meters and a large part of such a long well will be horizontal. It is crucial to have an efficient removal of cuttings to ensure the operation. This removal is done by complex drilling fluids designed for each specific operation. The project have through experimental activity provided improved understanding of drilling fluids capacities to transport cuttings out of a wellbore. Mainly fundamental differences in hole cleaning capabilities between oil based and water based fluids was investigated. Industry experience indicates that oil based fluids have advantages in hole cleaning. On the down side cuttings transported out of the well by an oil based fluid requires most costly handling. If improved understanding of these capabilities is aquired, this may facilitate more efficient operations with water based fluids with both economical and environmental benefits. Key findings in the project show that oil and water based fluids, of similar properties as measured according to industry standards, have significant difference in cuttings transport capabilities. Since many models are derived from experiments using water based fluids these findings may require re-visiting and improvements of many existing models.

Drilling fluids are one of the most important factors for safe, efficient and economic oil well drilling. Hole cleaning capacity is one of the essential functions of the drilling fluid. Current field experience states that oil based drilling fluids are su perior with respect to more efficient hole cleaning than water based drilling fluids, even if the fluid properties are equal as measured in accordance with API specifications. Furthermore, drilling progress is 5-30% faster if oil based drilling fluids are used than if water based drilling fluids are used. Some theories exist in the industry why oil based drilling fluids are superior to water based drilling fluids, but none of these have been properly verified. Experience show that improvement of the dril ling fluids down hole performance frequently result in an increased waste handling operation on the surface. This waste handling operation is one of the more costly operations during drilling. To be able to design a drilling fluid with good down hole per formance and with good waste treating properties it is necessary for the industry to understand why oil based drilling fluids perform significantly better than water based drilling fluids. This application addresses the knowledge need to understand diff erences in oil based and water based fluid behaviour, including rheological and hydraulic behaviour. Fluids performance will be evaluated in an experimental test rig with both circular and non-circular wellbore geometry. This knowledge can be used to opti mise oil based drilling fluid design and to construct water based drilling fluids that can be a real alternative to oil based fluids.