Summary
In recent years, a number of nonaqueous delivery systems for scale
inhibitors (SI) have been developed that are designed to be applied as
low-damage, low-water-cut or pre-emptive squeeze treatments (e.g., in critical
or expensive subsea wells). The mechanisms through which nonaqueous SI systems
operate is an important technical issue. Only when a good understanding of the
transport and retention mechanisms is developed can this be built into a model
for designing such squeeze treatments (such as the SQUEEZE VI model). The
experimental work in this paper focused on a specific oil soluble version of a
standard pentaphosphonate inhibitor (DETPMP), which has been described
previously in the literature. Several floods have been carried out comparing
corresponding nonaqueous and aqueous applications of DETPMP to better determine
the main features of the transport and retention mechanism of the system. Novel
core flood experiments with tracers in both the aqueous and oleic phases have
been performed and are reported in this paper. Unique information is generated
by designing very detailed flooding cycles and carrying out tracer floods at
each stage. Results point to the formation of an immobile “emulsion-like” third
phase in this system. To confirm our proposed mechanism, nonaqueous inhibitor
has also been applied at zero residual water saturation (100% oil) to
investigate whether or not the tailing effect (or third layer deposition) could
be generated without previous oil/water partitioning of the SI. For this case,
tailing of the brine tracer only was observed. Mass balance showed that there
was still considerable retention of SI that was ultimately produced during the
aqueous post-flush.
The corresponding aqueous-inhibitor coreflood showed similar returns to that
of the previous nonaqueous experiment and no retardation of either the tracer
species or the metal ions in the brine. No formation damage occurred for either
phase after the inhibitor was injected. A further “control” flood (no SI
treatment) proved that the brine tracer tailing arose as a direct result of the
treatment.
The use of the tracer species in each (water and oil) phase is a genuine
innovation, which provides a powerful additional technique for demonstrating
the effect of chemical treatments on the flow and the retention of all fluids
in the core.
Introduction
Aqueous-based SI treatments have traditionally been the most effective way
to control mineral scale formation in oilfield applications. Water-soluble
inhibitors may be deployed by continuous injection in topside applications and
in downhole “squeeze” treatments. However, such aqueous-based treatments are
not suitable for all SI applications. In water sensitive formations, for
example, squeezing with aqueous products can cause localized increases in water
saturation and formation damage because of wettability alteration, both of
which can lead to production decline (Ravenscroft et al. 1996). Also, as
increasingly complicated subsea satellite fields are developed, where access to
individual wells after production has started is both difficult and expensive,
the need for pre-emptive squeezing at zero or very low watercuts becomes a
priority.
The application of nonaqueous SIs has been proposed to address some of these
concerns (Wat et al. 1999; Hebvey et al. 2000; Collins et al. 2000; Smith et
al. 2000). Not only can these chemistries alleviate the problems described
above, but they also can help with lifting the well on startup as a result of a
reduced hydrostatic head. Enhanced treatment lifetime has also been claimed in
some cases (Guan et al. 2006; Miles et al. 2003; Heath et al. 2004; Collins et
al 2001).
Nonaqueous systems are generally prepared by incorporating conventional
aqueous SIs, such as phosphonates, polyacrylates, or sulphonated copolymers,
into a nonaqueous medium. Many types have been reported, ranging from invert
emulsion-type technologies (Collins et al. 2000; Smith et al. 2000; Lawless and
Smith 1999), microemulsions (Guan et al. 2004), encapsulated products (Bourne
et al. 2000), amphiphilic solvent systems (Heath et al. 2003), oil solubles
(Wat et al. 1999; Collins 1998; Wat et al. 1998), and water-free materials.
Although their delivery systems may differ, the basic principle of operation is
the same. The nonaqueous package is injected into the formation and, following
a suitable shut-in period, back produced as in conventional aqueous treatments.
The inhibitor species then partitions into the aqueous phase on contact with
the reservoir or injected brines, thus protecting the well and its associated
equipment from scale formation.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
13 June 2006
- Meeting paper published:
30 May 2006
- Revised manuscript received:
9 May 2007
- Manuscript approved:
1 June 2007
- Version of record:
20 February 2008
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