Aphron-based Drilling Fluid: Novel Technology for Drilling Depleted Formations in the North Sea

摘要

Drilling depleted reservoirs is fraught with a host of technicaland economic problems that often make it unprofitable tofurther develop some mature fields. Most of the problemscenter around uncontrollable losses and differential sticking.Frequently, less expensive drilling fluids will be used in aparticular interval, even though it may have the propensity todamage the formation. The reasoning holds that such fluidswill offset the high costs of losing more expensive muds to theformation. If operators turn to underbalanced drilling as analternative, the extra time and equipment required for a safeoperation can seriously degrade project economics insome applications.A specialized invasion-control drilling fluid has beendeveloped to drill reservoirs prone to lost circulation. Thisfluid combines certain surfactants and polymers to create asystem of micro-bubbles or aphrons that are encapsulated in auniquely viscosified system.Aphron based systems are engineered drilling fluids thataid in well construction by controlling losses in depleted,high-permeability sands while stabilizing pressured shales orother formations. One of the more attractive features of anaphron-based system is that it does not require any of the extraequipment used in air or foam drilling. There are nocompressors, high-pressure hoses or connections to add costsand safety concerns. The system uses conventional fluidmixingequipment to form tough, flexible micro-bubbles.This paper describes the development and application ofthe specialized micro-bubbles-based drilling fluid forcontrolling downhole mud loss in a depleted reservoir in theNorth Sea. The key issues of this project were excessiveoverbalance drilling conditions (> 5,000 psi) leading to therisk of highly expensive lost circulation and open perforationsin the upper producer, requiring temporary sealing duringdrilling. The well was successfully drilled to TD without any drilling fluid losses. The authors will detail the laboratorymethods used to generate appropriate formulations, theoperational procedures, and field application.