Inverted oil muds have the ability to control the expansion of water-sensitive clays, and they must be used whenever wellbore stability is unattainable with water-based muds. However, the initial production rate of a well drilled with inverted oil mud is often lower than expected.
The prevalent damage mechanisms have been found to be: Inversion of wettability of the rock matrix from water-wet to oil-wet, and formation of stable emulsions inside the porous medium. Reductions to the mobility of oil between 60% and 98% have been found.
A process, with the trade name ULTRAMIXTM was developed which includes a mixture of surfactants and solvents, each one having a specific function against the effects of the components of the inverted oil mud filtrate, and a procedure for the proper placement of this mixture in the porous medium surrounding the wellbore.
Laboratory tests of this process showed a damage removal up to 96%. Full scale field treatments conducted over 5 years yielded production increases ranging from 60 to 300%, sustained over long periods.
Oil based drilling muds that contain more than 50% v/v gas oil and form emulsions in which this component is the continuous phase are known as inverted oil muds. These emulsions contain two different types of surfactants primary surfactant, whose function is to stabilize the emulsion at high temperatures, and secondary surfactant to ensure that all the solids added to the mud and drilled from the well will be preferentially oil wet. The dispersed phase of these muds is water containing up to 300,000 ppm CaCl2. Organophillic clay and weighing materials such as barite or hematite are added.
When the drilling mud is pushed against a permeable formation, due to the hydrostatic pressure being greater than the formation pore pressure, a damage or blockage to the effective oil permeability can be originated, due to several mechanisms.
The acting mechanisms can be:
Plugging of the formation due to solids invasion, this mechanism being prevalent in very high permeability or naturally fractured formations.
Invasion of the liquid part of the mud, known as mud filtrate, which can alter the formation wettability and generate very viscous emulsions.