How Aphron Drilling Fluids Work

Abstract

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.

Recent Advances in Aphron Drilling Fluids

“…9 Clearly, the mechanism for nitrogen loss changes below this critical size and does not follow the diffusion behavior described above. This agrees with Sebba's hypothesis that aphrons smaller than 25 μm diameter may not be able to survive, due to the inability of the surfactant tri-layer in the aphron shell to assume a very high radius of curvature and pack properly around the gas core.…”

“…Within minutes of preparing an aphron drilling fluid, the oxygen fraction of the entrained air is lost via reaction with a cellulosic component of the mud. 9 This leaves the aphrons filled primarily with nitrogen. This eliminates concerns about corrosion downhole.…”

“…the rate of loss of nitrogen from aphrons decreases more rapidly with decreasing size than is the case for conventional bubbles. 9 For aphrons, the increase in driving force that normally accompanies a decrease in size of the bubble may be nullified by a decrease in permeability as the shell is compacted.…”

“…Rapid changes in pressure appear to affect the stability of aphrons much less than slow changes in pressure. 9 This may seem counter-intuitive, inasmuch as slow changes would permit the surfactants and polymers in the aphron shell to rearrange more completely when the pressure (aphron size) is changed. One explanation is that, during the initial stages of pressurization, the aphron shell compacts and contains an excess of surfactants and polymer, which is expected to decrease its permeability.…”

Herschel–Bulkley rheological parameters of lightweight colloidal gas aphron (CGA) based fluids

“…Currently, the use of CGA based fluids is increasing rapidly in the petroleum industry because these fluids exhibit properties that are desirable in many field operations (Brookey, 1998;Gregoire et al, 2005;Ivan et al, 2001;Ramirez et al, 2002;Rea et al, 2003;White et al, 2003). The potential of aphrons as components of drilling fluids rests in their ability to form a solid free, tough, and elastic internal bridge in pore networks or fractures to minimize deep invasion by means of air microbubbles (Belkin et al, 2005;Growcock et al, 2003). This microenvironment seal readily cleans up with reservoir flow-back as production is initiated, thereby reducing cost associated with stimulation processes (Growcock et al, 2006;Ivan et al, 2002).…”