Abstract
Along with exorbitant costs and safety considerations, drilling an exploratory well where conventional mud systems could not maintain wellbore stability and trouble-free drilling, also posed the additionally difficult challenge of selecting a fluid system stable against multiple contaminants and flexible for a broad range of densities. The demands are compounded dramatically when designing the system and information to support the decision is poor.
This paper details the design and performance of a silicate-based fluid, which is an inhibitive mud system formulated with a soluble silicate for maximum shale inhibition. The system has been developed to drill water-sensitive reactive shales and dispersible chalk and illite formations. The degree of inhibition provided by the silicate system is significantly greater than any other water-based system, truly approaching the level of an oil-based system.
The authors will discuss the design and the application of the drilling fluid, which was selected to drill a formation containing anhydrite, claystone, dolomite and salt.
The system was formulated to be salt-saturated to avoid dissolution of salt layers; to withstand anhydrite contamination; to be flexible for densities up to more than 2.0 kg/dm3 while maintaining its high level of inhibition. Though comprehensive lab tests were performed, the possible depletion of silicate while drilling the anhydrite-containing formation remained. Nevertheless, this challenge was accepted with tight wellsite engineering planned to maintain the fluid properties.
Six wells have now been drilled with the proposed silicate system, with each showing very good performance and stable behavior in the above mentioned environment. The authors will present its effectiveness, formulation, properties, maintenance and lessons learned, along with the coordination and pre-planning that contributed to its successful application.
