A 1.9 SG oil-based fluid for suspending open-hole pre-perforated liner completions in 1100 m long, horizontal openhole sections, under HPHT conditions (162°C, +800 bar reservoir pressure) has been engineered for the Morvin field development. The main challenge was to provide this fluid with sufficient stability so that it still was mobile when wells were backproduced after up to 4 weeks of subsequent well completion activities.
The main drivers for the development were the constraints that alternative fluid concepts pose under such conditions. Standard oil-based fluids weighted with barite, under the prevailing conditions up to 1.4 tons per cubic-meter of fluid, showed to be highly instable under simulated reservoir conditions. Barite settled out and deposited thick immobile mud bodies that were potentially completion damaging. Other fluid alternatives such as Cesium-formate weighted water-based suspension fluids were suspected to be incompatible with the selected oil-based reservoir drilling fluid.
Based on previous experiences with low solids (barite free) oil-based completion fluids (screen running and perforation fluids), a so-called "reduced solids oil-based completion fluid" was engineered in cooperation with the fluids contractor. The main features of the fluid were a close-to 50/50 oil-to-brine ratio, a Cesium-formate internal brine phase and fine barite to weigh up the fluid to approach the required weight. Several variations of this fluid were rigorously tested to evaluate stability and mobility of fluid sample samples.
The fluid has so far successfully been used on three completions. This paper reports in detail on the fluid development process, operational experiences and well-clean-up observations.