Selecting the components of fluids for reservoir drilling and completion can require extensive laboratory work to determine the most compatible fluid. Well temperatures in excess of 300°F (150°C) create additional challenges, as the additives required to give water-based reservoir drilling fluids the rheological and fluid-loss characteristics needed to successfully drill and complete long horizontal wells degrade at such elevated temperatures. In addition, supplying the necessary additives to a drilling operation in a remote location can be a major logistical burden, leading to a compromise in the fluid formulation.
The Belanak field lies off the coast of Indonesia in the Natuna Sea. The reservoir temperature is high (315°F), and the reservoir sections are drilled horizontally, typically between 3,500 and 4,500 ft and often feature particularly tortuous well paths. A low-solids, brine-based reservoir drilling fluid was required because the wells use premium screens for sand control.
Six wells were drilled using the sodium formate-based reservoir drilling and completion fluids. The particle size distribution and concentration of the calcium carbonate bridging solids were monitored closely while drilling to ensure that filter cake quality was not compromised. A chelating agent-based breaker was used to break down the filter cake prior to the onset of production.
The laboratory work required to optimize the fluids for this project had to take into account not only the requirements for the best fluid technically, but also the effect of the limitations created by working in a remote location. Considerations included minimum loading of calcium carbonate required to deposit a clean, high-quality filter cake and the effect of mixing brine for the reservoir drilling fluid or completion using seawater in case of a shortage of drill water.
Fluid testing carried out in the laboratory included drilling performance at temperature, fluid compatibilities, bridging solids optimization, scale inhibition testing, and breaker selection. The fluid selected was based on sodium formate – the first application of a formate-based fluid in Indonesia.