Biodegradability of drilling-fluid chemicals is becoming a requirement for discharge of drilled cuttings on offshore drilling operations, especially in Europe. Due to their ready biodegradability, fatty acid esters are very popular in the North Sea. While esters enjoy advantages over other synthetic-based fluids on the biodegradability issue, their drilling fluid performance has remained substandard. The ester-based fluids undergo hydrolysis in the presence of lime at temperatures exceeding 200°F (93°C).
A new approach to the stability of ester-based fluids is a new, negative-alkalinity ester-based drilling fluid that is stable up to 350°F (177°C). The thermal stability and performance characteristics of the negative alkalinity ester-based drilling fluids are superior to the conventional ester-based mud formulations. The system uses a surfactant package that is designed to perform as an invert emulsifier under negative alkalinity conditions. The negative alkalinity ester-based drilling fluid system is simple and easy to run without any exotic change in the chemistry of esters. The system can be formulated using relativity inexpensive and readily available fatty acid esters. Due to the enhanced stability of fatty acid esters in this system, the selection of the esters utilized in this system can be based on economic considerations and availability rather than hydrolytic stability of the ester base fluid.
The thermal stability and performance characteristics of the negative-alkalinity ester-based drilling fluid are presented in this paper. A comparison of stability of the various types of ester base fluids using negative alkalinity invert emulsion and conventional reserve alkalinity ester-based drilling fluid shows that the negative alkalinity ester-based drilling fluids are stable up to 350°F (177°C). The paper discusses the overall impact of negative alkalinity ester-based drilling fluid in meeting the environmental and functional requirements of drilling operations and the economic advantages derived from this innovative system.