Aphron drilling fluids are being used globally to drill depleted reservoirs and other underpressured zones. The primary features of these fluids are their unique low-shear rheology and the presence of aphrons, which are specially designed pressure-resistant microbubbles of air. However, how aphron drilling fluids work is not well understood, which limits acceptance of this technology. Recently, a study was undertaken under the auspices of the US Department of Energy (DOE) to gain some understanding of aphron drilling fluids and provide guidance about running these fluids in the filed to optimize performance.
Various laboratory techniques were applied to determine the physicochemical properties of aphrons and other components in the fluid and how they affect flow through permeable and fractured media. These included wettability and surface tension, bubble stability, radial and dynamic flow visualization, and fluid displacement tests.
One key discovery was that aphrons can survive compression to at least 4,000 psig, whereas conventional bubbles do not survive pressures much higher than a few hundred psig. When drilling fluid migrates into a loss zone under the drill bit, aphrons move faster than the surrounding liquid phase and quickly form a layer of bubbles at the fluid front. The bubble barrier and radial-flow pattern of the fluid rapidly reduce the shear rate and raise the fluid viscosity, severely curtailing fluid invasion.
Another key finding is that aphrons show little affinity for each other or for the mineral surfaces of the pore or fracture. Consequently, the seal they form is soft, and their lack of adhesion enables them to be flushed out easily during production. Equally important, the interfacial tension between the base fluid and produced oils or gases is quite low, so that produced fluids do not create a formation-damaging high-viscosity emulsion; instead, they channel through the drilling fluid with relative ease.
Depleted wells, which are very expensive to drill underbalanced or with other remediation techniques, have been drilled overbalanced with the aid of aphron drilling fluids.