How Aphron Drilling Fluids Work

Abstract

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.

Recent Advances in Aphron Drilling-Fluid Technology

“…When aphrons become smaller than about 50 m in diameter, they become less stable, and when they reach a diameter of 25 to 35 m, they suffer catastrophic loss of nitrogen (Belkin et al 2005). Clearly, the mechanism for nitrogen loss changes below this critical size and does not follow the diffusion behavior described above.…”

“…Within minutes of preparing an aphron drilling fluid, the oxygen fraction of the entrained air is lost through reaction with a cellulosic component of the mud (Belkin et al 2005), leaving the aphrons filled primarily with nitrogen. For a drilling fluid that contains 15 vol% surface air, the concentration of oxygen is only 0.015 lbm/bbl, whereas the concentration of the cellulosic oxygen scavenger is 300 times greater.…”

“…However, for aphrons, the diffusion rate was found to be proportional to the surface area. Thus, as an aphron decreases with size (because of increased pressure, the passage of time, or both), the rate of nitrogen loss slows to a greater extent than for conventional bubbles (Belkin et al 2005). For aphrons, the increase in driving force that normally accompanies a decrease in bubble size 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 (Belkin et al 2005). This may seem counterintuitive, because slow changes would be expected to permit the surfactants and polymers in the aphron shell to rearrange more completely when the pressure (aphron size) is changed.…”

“…In previously reported tests (Belkin et al 2005), aphrons were easily comminuted by passing the drilling fluid through various types of filter media. Indeed, with increasing number of passes through a filter, the bubble size distribution became increasingly finer and narrower.…”