Summary
Chemical scale inhibitors are commonly used to prevent or inhibit scale
formation in production. The most economic treatment of scale inhibitor is
normally through chemical squeeze. However, there is little agreement regarding
the primary mechanism by which the threshold scale inhibitors are retained in
producing oil or gas formations as a result of squeeze procedures. Recent
advances in phosphonate/rock interaction research at the Rice U. Brine
Chemistry Consortium have significantly improved our knowledge of what controls
inhibitor placement in the formation. It is commonly suggested that reservoir
type determines how an inhibitor is retained in a formation. Our research
suggests that the pill chemistry is also an important determinant for retention
of carbonate reservoir. Acidic pills are mostly retained near the wellbore,
while more neutralized pills move farther into the formation. Three calcium
nitrilomethylenephosphonate (NTMP) solid phases, an amorphous phase, and two
crystalline Ca2.5HNTMP phases with pKsp = 22.6 and
pKsp = 24.2, are particularly important for inhibitor retention. The
relative sizes of these solid phases formed are governed by the pill
composition and acidity. These results can be explained by a
solution-phase-controlled sequence of reactions. All of this information has
been incorporated into a new squeeze-design software program,
SqueezeSoftPitzer.
Introduction
Threshold scale inhibitors, such as the phosphonates, polyacrylates,
and polymaleates have been used to control scale since 1936 (Rosenstein).
Preferable to sequestering agents, which must be added in stoichiometric
amounts to inhibit precipitation, chemical threshold scale inhibitors generally
require the addition of only a few milligrams per liter to inhibit scale.
Inhibitors are commonly applied to production wells through an inhibitor
squeeze. An inhibitor squeeze is performed by pushing scale inhibitor solution
into a producing formation and fixing the inhibitor in the formation. When well
production begins, inhibitor is produced along with the formation water.
Following the squeeze, scale inhibitor flows back at low concentrations, which
increase and peak rapidly to some value and then decline within a few days to a
low plateau concentration, which comprises the bulk of the squeeze duration.
The plateau flowback inhibitor concentration is normally sufficient to inhibit
scale.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
30 June 2004
- Revised manuscript received:
4 August 2005
- Manuscript approved:
15 August 2005
- Version of record:
20 September 2006
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