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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. ProposalAphron drilling fluids, which are highly shear-thinning waterbased fluids containing stabilized air-filled bubbles (aphrons), have been applied successfully worldwide to drill depleted reservoirs and other high-permeability formations. Although the performance of these fluids in the field is well documented, questions remain about how such fluids work. A study was initiated this past year under the auspices of the U.S. Department of Energy to develop some understanding of the mechanisms by which these fluids can seal loss zones with little permanent formation damage.Among the key findings of this on-going project is that aphrons can survive elevated pressures for a much longer time than conventional bubbles, though they appear to be fairly sensitive to shear. In a loss zone, aphrons that survive the trip downhole can migrate faster than the base liquid and concentrate at the fluid front, thereby building an internal seal in the pore network of the rock. A microgel network formed by particulates in the drilling fluid aids the aphrons in slowing the rate of invasion, as does, of course, the radial flow pattern of the invasion. As the fluid slows, the very high LSRV (lowshear-rate viscosity) of the base fluid becomes increasingly important; this high LSRV, coupled with low thixotropy, enables the fluid to generate high viscosity rapidly. Bridging and formation of a low-permeability external filter cake also occur during the latter part of this period, ultimately reducing the rate of invasion to that of ordinary fluid loss.Another key finding is that aphrons have very little attraction for each other or for mineral surfaces. Consequently, they do not readily coalesce nor do they stick easily to the pore walls, resulting in easy displacement by the produced fluids. In addition, the drilling fluid itself is very compatible with produced fluids and generates low capillary forces, thereby facilitating back-flow of produced fluids. The combination of these two effects is expected to result in low formation damage and minimal requirements for cleanup.

Proposal Aphron drilling fluids, which are highly shear-thinning water-based fluids containing stabilized air-filled bubbles (aphrons), have been applied successfully worldwide to drill depleted reservoirs and other high-permeability formations.Although the performance of these fluids in the field is well documented, questions remain about how such fluids work.A study was initiated this past year under the auspices of the U.S. Department of Energy to develop some understanding of the mechanisms by which these fluids can seal loss zones with little permanent formation damage. Among the key findings of this on-going project is that aphrons can survive elevated pressures for a much longer time than conventional bubbles, though they appear to be fairly sensitive to shear.In a loss zone, aphrons that survive the trip downhole can migrate faster than the base liquid and concentrate at the fluid front, thereby building an internal seal in the pore network of the rock.A microgel network formed by particulates in the drilling fluid aids the aphrons in slowing the rate of invasion, as does, of course, the radial flow pattern of the invasion.As the fluid slows, the very high LSRV (low-shear-rate viscosity) of the base fluid becomes increasingly important; this high LSRV, coupled with low thixotropy, enables the fluid to generate high viscosity rapidly. Bridging and formation of a low-permeability external filter cake also occur during the latter part of this period, ultimately reducing the rate of invasion to that of ordinary fluid loss. Another key finding is that aphrons have very little attraction for each other or for mineral surfaces.Consequently, they do not readily coalesce nor do they stick easily to the pore walls, resulting in easy displacement by the produced fluids. In addition, the drilling fluid itself is very compatible with produced fluids and generates low capillary forces, thereby facilitating back-flow of produced fluids.The combination of these two effects is expected to result in low formation damage and minimal requirements for cleanup. Introduction Many oil and gas reservoirs are mature and are becoming increasingly depleted of hydrocarbons, which makes for evermore costly drilling. While the formations above and below these producing zones typically have much higher pore pressures and require high fluid density to stabilize them, exposure of a depleted zone to this high-density fluid can result in significant loss of whole drilling fluid and differential sticking.[1–4] Furthermore, pressured shales are often found interbedded with depleted sands, thus requiring simultaneous stabilization of multiple pressure sequences. Drilling such zones safely and economically is very difficult with conventional rig equipment. Preventive measures with normal or high-density fluids generally entail use of a plugging agent at low concentration in the entire circulating system, or remediation if the rate of loss of drilling fluid exceeds some threshold level.While such techniques can be effective for controlling lost circulation in non-producing formations, the damage that these techniques can cause producing formations makes them wholly unsatisfactory for mitigating losses in oil and gas reservoirs. An increasingly popular alternative for drilling depleted and multiple pressure zones is the use of underbalanced drilling, whereby the fluid has a density low enough to balance the pore pressure in the lowest-pressure zone.However, this technique requires additional equipment and risks wellbore collapse and blowouts.Aphron drilling fluids do not have such limitations.The air that is used to generate aphrons is usually incorporated into the fluid with conventional mud mixing equipment at ambient pressure, thereby reducing costs and safety concerns associated with air or foam drilling.Because the amount of air in the fluid is very low, the density of the fluid downhole is essentially that of the base fluid.Yet, the fluid is able to seal loss zones effectively and with minimal formation damage.Consequently, aphron drilling fluids are marketed as a cost-effective alternative to underbalanced drilling.

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.

TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractAphron 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.