Recent Advances in Aphron Drilling Fluids

Abstract

Aphron drilling fluids are being used globally to drill through depleted reservoirs and other under-pressured zones. The primary features of these fluids are their unique low-shear rheology and aphrons (specially designed pressure-resistant microbubbles of air). However, how aphron drilling fluids work is not well understood, which limits acceptance of this technology, along with efforts to optimize the system's performance. Recently a study was undertaken under the auspices of the U.S. Department of Energy to gain some understanding of the workings of aphron drilling fluids. Those results are presented here. 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 4000 psig, whereas conventional bubbles do not survive long past 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. At the same time, the shear rate of the fluid continually decreases and the viscosity is rapidly rising. The combination of the bubble layer and the rapidly increasing viscosity of the liquid severely curtails fluid invasion. Another key finding of the study 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. Depleted wells which are very expensive to drill underbalanced or with other remediation techniques can now be drilled overbalanced. This study has provided a sound technical basis for the success of aphron drilling fluids and is providing guidance on the way to run these fluids in the field to optimize their performance. Background Aphrons were first described by Sebba1 as unique microspheres with unusual properties. Much of his work was done with microbubbles consisting of air encapsulated in a multi-layer shell created and maintained via chemical equilibrium with various components in the base fluid. Brookey2 described the first use of aphrons in a drilling fluid application. In this case, the microbubbles (as they were then called), were created as a minor phase in a water-based fluid. This system was used as a means of controlling lost circulation and minimizing formation damage in a low- pressure vugular dolomite reef zone. The microbubbles allowed the zone to be drilled to required TD, logged and drill stem-tested; this had not been possible previously. How did the fluid system work? Many at that time thought that density reduction was responsible, since the application resulted in a lower mud weight on surface. The next application was in a fractured dolomite horizontal well, where the bit dropped only a foot and all returns were being lost. In this application, full returns were resumed as soon as the microbubbles reached the bit. Obviously density reduction was not the reason these losses were controlled. This experience led to further research in the area of foams and aerated fluids and to the discovery of Sebba's work with aphrons. Reformulation of the drilling fluid led to increased stability of the aphrons through re-engineering of the multi-layer shell and enhancement of the low-shear-rate viscosity (LSRV), which made the fluid more effective in downhole applications. This new system was applied in South America in an area where six wells were drilled using various fluids and techniques, including underbalanced drilling. Because of severe depletion, lost circulation and borehole instability, none of these wells was successfully drilled to TD. Ramirez3 described the application of aphron technology in this field, which resulted in no drilling fluid losses and excellent wellbore stability even in previously troublesome shale sections. Conditions were so favorable that coring was done with over 90% recovery on the first well. Extensive wire-line logging was carried out with no problems. Even cementing was highly successful, with full returns throughout, though severe cementing problems had been the norm. After drilling the first three wells in this field, the operator was able to eliminate the intermediate string and drill from surface casing to TD successfully.

Insight into selection of appropriate formulation for colloidal gas aphron (CGA)-based drilling fluids

“…Aphrons form a major part of the drilling fluid at atmospheric pressure (usually 10-15 vol%) (Growcock et al 2006). Concentration of aphrons in drilling fluid is controlled through the surfactant and polymer concentrations.…”

“…Concentration of aphrons in drilling fluid is controlled through the surfactant and polymer concentrations. At downhole pressures, aphrons form an insignificant volume, e.g., a mud sample containing 12 vol% nitrogen at atmospheric pressure will contain less than 0.06 vol% nitrogen at 3000 psig (Growcock et al 2006).…”

Effectiveness of Colloidal Gas Aphron Fluids Formulated with a Biosurfactant Enhanced by Silica Nanoparticles

“…The velocities of invading fluids tend to slow as a consequence of radial flow around the wellbore and the bridging action of the CGA bubbles. This causes their shear rates to decrease and viscosities to rise [73].…”

Recirculated aphron-based drilling fluids

“…It is reported that the aphrons can survive exposure to elevated pressures much better than conventional bubbles, and can survive compression to at least 27.7 MPa. Also, the volume of aphron can recover nearly 90 % during depressurization (Berkin et al 2005;Ivan et al 2002;Wang et al 2007;White et al 2003;Growcock et al 2006).…”