Summary
Conventional testing of scale inhibitors (SIs) usually focuses only on the
inhibition of bulk depositional processes (e.g., in jar tests of inhibition
efficiency). In previous work, we have extended the study of inhibition
efficiency to examine both bulk and surface depositional processes and their
inhibition using SIs. Where the minimum inhibitor concentration (MIC) is well
exceeded, both bulk and surface scaling are prevented. However, we have
demonstrated that the presence of an inhibitor, at levels marginally below the
MIC, actually enhances surface scale growth over a range of temperatures
typically encountered in the production system. Where this precise
concentration lies with respect to the MIC is not generally known for scaling
systems, and it is therefore difficult to predict where potential problems with
sub-MIC SI levels would occur.
An experimental program has been conducted to assess the efficiency of PPCA
(a polyphosphinocarboxylic acid inhibitor species) in inhibiting barium sulfate
scale formation in the bulk solution and on a metal surface over a range of
concentrations and test temperatures. The concentration range tested was chosen
(a) to allow identification of the MIC at which both surface and bulk scale
formation is controlled and (b) to establish where potential problems with
enhanced surface scaling arise at sub-MIC levels. The results obtained from
this study have allowed trends in surface and bulk inhibition to be established
and allowed the region of sub-MIC concentrations where surface scaling is
enhanced to be identified for three temperatures. We summarize our results
using some simple schematic models of the bulk and surface scaling regimes and
the corresponding inhibition process.
Introduction
In addition to understanding how SIs affect the bulk solution precipitation
process, it is important to assess how efficient inhibitors are in controlling
the nucleation, growth, and adhesion of scale on metal surfaces.
Scale-inhibition efficiency of barium sulfate (BaSO4) is normally measured in
bulk jar tests, with one of the principal aims being to determine the MIC,
which gives some acceptable level of inhibition (e.g., 95% at 2 hours).
However, this test refers mainly to the bulk precipitation of barium sulfate
scale from solution, rather than its deposition onto a mineral or metal
surface.
Studies of surface scaling have received much less attention than studies of
bulk precipitation processes, despite the fact that scaling problems are
normally associated with the deposition of scale onto tubulars or equipment
surfaces. However, in the last 5 years, there has been increased emphasis on
this by our research group (Graham et al. 2001; Labille et al. 2002; Morizot
and Neville 2000a; Morizot 1999; Morizot et al. 1999) and others (Quddus and
Allam 2000; Quddus 2002). An important driver for this has been the realization
that scale kinetic and inhibition data from bulk precipitation are not always
directly transferable to surface processes (Hasson et al. 1997). These studies
of surface scaling have revealed several important points; for example, (i)
scale-inhibition efficiency varies between surface and bulk processes (Morizot
et al. 1999); (ii) the effect of inhibitor on crystal morphology varies between
surface and bulk processes (Morizot 1999); and (iii) surface scaling can be
promoted by the addition of inhibitor at sub-MIC levels (Graham et al. 2001;
Morizot and Neville 2000a). Clearly, the position of this sub-MIC level of
enhanced scale deposition is important and is the subject of this paper.
A knowledge of precipitation kinetics (nucleation and crystal growth) is
required to predict scale formation and its control using inhibitors (He et al.
1999). Crystal growth and nucleation rates can depend on a number of factors
including supersaturation (Sp), local hydrodynamic conditions, and temperature
(Aoun et al. 1999). The temperature in the production system can range from as
low as 4°C at the seabed [in deep-sea operations (Graham et al. 2002)] to 95°C
and above in reservoirs (Graham et al. 1997). Therefore, it is important to
study scale formation over a range of temperatures to establish the scaling
tendency of the brine at different stages of production. If inhibitor
performance is compromised through temperature changes in the production
system, buildup of scale on the surfaces of essential equipment such as
electrical submersible pumps (ESPs), safety control valves, and gas lift
mandrels could occur, resulting in electrical and mechanical failure.
Therefore, understanding how temperature can affect the MIC, particularly where
surfaces are present, will allow operators to compensate for reduced (or
significantly altered) inhibitor performance.
© 2006. Society of Petroleum Engineers
View full textPDF
(
1,165 KB
)
History
- Original manuscript received:
6 December 2004
- Revised manuscript received:
1 August 2005
- Manuscript approved:
2 August 2005
- Version of record:
20 February 2006
View Cart
