The Completion of Subsea Production Wells Eased by the Use of a Unique, High-Density, Solids-Free, Oil Based Completion Fluid

Abstract

Abstract Hole stability issues and the need to reduced drillpipe torque have made oil-based drilling fluids the systems of choice for highly deviated or horizontal reservoir sections offshore Norway. However, the tendency of barite sag in non-aqueous drilling fluids has led to plugging of sand-control screens, thereby creating in some cases the need for a solids-free environment for the completion. Unfortunately, because of problems related to emulsion blocking when mixing filtrate from oil-based drilling fluid and water-based completion fluid, displacing to brine after drilling with oil-based fluid can damage productivity. This paper describes the design and successful application of a unique high-density solids-free, oil-based completion fluid. This fluid obtains its density from an emulsified internal phase of heavy brine. This eliminates the requirements for solid weight materials for fluid density, providing a solidsfree, oil-continuous environment for the completion. Introduction Statfjord Nord & Øst in the Norwegian North Sea comprise several production and injection wells that are drilled from subsea templates. The production from the wells is tied back to the Statfjord C platform for processing. The wells were drilled and completed using a semi-submersible drilling rig. Drilling challenges related to hole stability and lubricity have required the use of oil-based drilling fluids. More recently, three one-section sidetracks were planned, using sand-control screens to compensate for depleted reservoir pressure and poorly consolidated reservoir sands. The fluid density required to drill the reservoir section of these wells is 1.60 sg. After drilling, sand screens were planned to be run into the open hole before the upper completion was initiated. Once the screen is run in the hole, it can take weeks before the wells are produced back. If oilbased drilling fluid is left in the hole, barite settling raises the possibility of screen plugging. For instance, a 1.60-sg fluid contains more than 20% by volume barite weight material and drill solids, which can restrict flow when the wells are put on production. Completing the wells in a clear, heavy brine, such as CaCl2 /CaBr2-brine, would appear to be the obvious alternative. However, since the wells are drilled with oil-based drilling fluid, incompatibility between the drilling fluid and completion brine could reduce productivity. This is a direct result of the formation of emulsion from the oil-based drilling fluid filtrate and completion brine. As a result, one option was to drill the well with oil-based drilling fluid and displacing with water-based fluid for the completion. Consequently, prior to displacement with the solids-free water-based completion fluid and running the screens, the hole had to be opened with water-based drilling fluid to remove the oil-based filter cake and filtrate. This procedure would cause problems related to incompatibility between the oil and water-based drilling fluids as well as costs associated with the additional time spent on the operation. In addition, displacing to water-based fluid and brine was considered risky because of the potential for hole instability. The use of a unique solids-free oil-based completion fluid has allowed the wells to be completed immediately after drilling with an oil-based drilling fluid without having to reopen the hole to remove the filter cake. Not having to reopen the hole not only reduces rig time, but also prevents the risk of problems related to emulsion blocking and incompatibilities with the oil-based drilling fluid. This paper describes the design and successful application of this fluid in three wells completed at the Statfjord Satellites. Field and Well Description The Statfjord area is located approximately 140 km offshore Norway. The productive formation consists of sandstone with permeabilities varying from milidarcys to several Darcy. Previously, the wells were completed by cementing and perforating the 7-in. liner. Sand production prompted a change in the completion strategy to include sand screens. Fig. 1 is a schematic presentation of the completion design of the first well completed with screens and using a low-solids oil-based mud (LSOBM).