| Popis: |
Historically, wells drilled on the Machar Field in the North Sea have experienced huge downhole losses and remedial techniques to stem losses have been marginally successful. One well experienced oil-based mud (OBM) losses exceeding 26,000 bbl. Furthermore, the massive losses of oil-based mud had threatened the future of this project. A new phase in the development of the field was planned that entailed drilling two producers and one injector well that would be tied to an existing subsea manifold, which in turn would connect to the production platform. A drilling fluid formulation was requested that would bridge off the wide range fracture widths and stop the massive ingress of well fluids, especially in the reservoir where back-flow would be hindered and remediation difficult. The design and successful application of a fit-for-purpose drilling fluid system to deal with highly fissured limestone of varying fracture widths was a key planning element for the new Machar Field drilling campaign. The knowledge gained from drilling fractured formations in the Middle East and the experience using stress cage principles in depleted reservoirs of the Southern North Sea have been adapted to successfully drill and seal limestone formations and prevent damaging losses. This paper will discuss the drilling fluids design process and the laboratory testing used to predict the bridging and sealing efficiencies needed to seal fractures ranging from 50 to 1000 microns. Testing was conducted using formulations with different materials of varying size, shape and resilience. The basis of design and knowledge gained in the laboratory was transferred to the field where losses were dramatically reduced to only six bbl in the 6-in. section. The first well was successfully flowed at the production target level and the second well surpassed the expected production index by a factor of three. |
