Aphron drilling fluids are being used globally to drill throughdepleted reservoirs and other under-pressured zones. Theprimary features of these fluids are their unique low-shearrheology and aphrons (specially designed pressure-resistantmicrobubbles of air). However, how aphron drilling fluidswork is not well understood, which limits acceptance of thistechnology, along with efforts to optimize the system'sperformance. Recently a study was undertaken under theauspices of the U.S. Department of Energy to gain someunderstanding of the workings of aphron drilling fluids. Thoseresults are presented here.Various laboratory techniques were applied to determinethe physicochemical properties of aphrons and othercomponents in the fluid and how they affect flow throughpermeable and fractured media. These included wettabilityand surface tension, bubble stability, radial and dynamic flowvisualization, and fluid displacement tests.One key discovery was that aphrons can survivecompression to at least 4000 psig, whereas conventionalbubbles do not survive long past a few hundred psig. Whendrilling fluid migrates into a loss zone under the drill bit,aphrons move faster than the surrounding liquid phase andquickly form a layer of bubbles at the fluid front. At the sametime, the shear rate of the fluid continually decreases and theviscosity is rapidly rising. The combination of the bubblelayer and the rapidly increasing viscosity of the liquid severelycurtails fluid invasion. Another key finding of the study isthat aphrons show little affinity for each other or for themineral surfaces of the pore or fracture; consequently, the sealthey form is soft and their lack of adhesion enables them to beflushed out easily during production.Depleted wells which are very expensive to drillunderbalanced or with other remediation techniques can nowbe drilled overbalanced. This study has provided a soundtechnical basis for the success of aphron drilling fluids and is providing guidance on the way to run these fluids in the fieldto optimize their performance.
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