Summary
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.
Background
Aphrons were first described by Sebba (1987) as unique microspheres with
unusual properties. Much of his work was carried out with microbubbles
consisting of air encapsulated in a multilayer shell created and maintained by
means of chemical equilibrium with various components in the base fluid.
Brookey (1998) described the first use of aphrons in a drilling-fluid
application. The microbubbles were created as a minor phase in a water-based
fluid. This system was used to control lost circulation and minimize formation
damage in a low-pressure, vugular dolomite reef zone. The microbubbles allowed
the zone to be drilled to total depth (TD), logged, and drillstem-tested, which
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 horizontal dolomite well, where the
bit dropped only a foot and no fluid was being returned to the surface. 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 the discovery of Sebba’s (1987) work with aphrons.
Reformulation of the drilling fluid increased stability of the aphrons
through re-engineering of the multilayer 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 had
been drilled using various fluids and techniques, including underbalanced
drilling. Because of severe depletion, lost circulation, and borehole
instability, none of these wells had been drilled successfully to TD. Ramirez
et al. (2002)described the application of aphron technology in this field,
which resulted in no drilling-fluid losses and excellent wellbore stability
even in troublesome shale sections. Conditions were so favorable that coring
was managed with over 90% recovery on the first well. Extensive wireline
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.
Fig. 1 shows the results of a Repeat Formation Test (RFT) log in this field,
which reveals the effect of compression on aphrons and shows the actual
hydrostatic pressure downhole. In Fig. 1, depth is plotted vs. pressure and
density. The red line indicates the true hydrostatic pressure, while the dark
blue line shows the theoretical hydrostatic pressure derived from surface
measurements. The difference is the approximate aphron (undissolved air)
concentration in the fluid. The light blue line shows actual formation
pressure, which drops markedly in the highly permeable sands between 5,500 and
6,700 ft true vertical depth (TVD). Thus, the fluid exerted an overbalance
across these depleted sands that was more than sufficient to stabilize the
wellbore, yet no losses were experienced.
Kinchen et al. (2001)describe drilling in a highly vugular, fractured
dolomite zone with good success, even when coring with this fluid. Besides
providing lost circulation control, these wells came on line with full
production in 4 days vs. the average of 30 to 60 days in previous wells drilled
with various fluid programs.
Gregoire et al. (2005)chronicle a program of drilling with an aphron
drilling fluid and controlling losses in a fractured granite zone, which
resulted in openhole production almost instantaneously without treatment.
Besides instant cleanup, the production rates were much higher than had been
seen before with any other drilling fluid. Other operators have recently
drilled in that area successfully.
Although aphron technology has been used in about 300 wells in South
America, North America, Africa, and the Far East over a period of several
years, not all of these operations have been successful, and the reasons for
this have not been clear. Thus, it was considered desirable to develop a deeper
understanding of the way aphron drilling fluids work and to use laboratory
techniques to optimize field applications. The initial and predominant type of
aphron drilling fluid used in the field has been a polymeric water-based
system, though a clay water-based alternative and a non-aqueous-based aphron
drilling fluid also have been developed (Growcock et al. 2003, 2004). A 2-year
research and development program was undertaken under the auspices of the US
DOE to obtain laboratory evidence for the capability of aphron drilling
fluids—primarily the polymer water-based system—to limit fluid invasion in
permeable formations with minimal formation damage and to provide a sound
scientific basis for this behavior (Growcock 2005). The following describes
some of the results of this study.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
27 December 2005
- Revised manuscript received:
15 December 2006
- Manuscript approved:
27 February 2007
- Version of record:
20 June 2007
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