(85d) Transforming An Emulsified Drilling Fluid to Be Removed From Unwanted Location Into a Low-Viscosity Single Phase System

Drilling fluids are generally emulsions, either O/W or W/O, with an aqueous phase containing various electrolytes and an oil phase ranging from hydrocarbons to polar oils. Such multiphasic systems are stabilized by surfactants and co-surfactants whose nature is related to the emulsion morphology. A stable O/W (respectively W/O) emulsion exhibits an unbalanced generalized formulation of at least 2-3 units (as measured by the surfactant affinity difference SAD or the hydrophilic-lipophilic deviation HLD) with a negative (respectively positive) sign.

The filter cake (thin packed multiphase cake around the wellbore) formed by the W/O drilling fluid; as well as the fluid invasion that plugs the porous medium by capillarity has to be removed after the drilling and completion process to assure the expected production of the wells. This usually requires the formation of a bicontinuous microemulsion with a surfactant system exhibiting the highest solubilization of both, oil and water phases, a situation that takes place when SAD or HLD becomes exactly zero. However, it should also occur with an amount of surfactant as small as possible. This is yet an unsolved problem. Recent developments indicate that there are several trends toward performance improvement that can be used in parallel or together, and this is what has been used in practical cases.