How Minimum Inhibitor Concentration (MIC) and Sub-MIC Concentrations Affect Bulk Precipitation and Surface Scaling Rates
- Anna L. Graham (Heriot-Watt University) | Lorraine S. Boak (Heriot-Watt University) | Kenneth S. Sorbie (Heriot-Watt University) | Anne Neville (University of Leeds)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- February 2006
- Document Type
- Journal Paper
- 19 - 25
- 2006. Society of Petroleum Engineers
- 4.3 Flow Assurance, 5.3.1 Flow in Porous Media, 1.8 Formation Damage, 4.3.4 Scale, 4.2.3 Materials and Corrosion, 5.3.2 Multiphase Flow, 3.1.6 Gas Lift, 3.1.2 Electric Submersible Pumps, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc)
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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.
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.
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