How Scale Inhibitors Work: Mechanisms of Selected Barium Sulphate Scale inhibitors Across a Wide Temperature Range
- K.S. Sorbie (Heriot-Watt University) | N. Laing (BP Exploration, Aberdeen, Scotland)
- Document ID
- Society of Petroleum Engineers
- SPE International Symposium on Oilfield Scale, 26-27 May, Aberdeen, United Kingdom
- Publication Date
- Document Type
- Conference Paper
- 2004. Society of Petroleum Engineers
- 4.3 Flow Assurance, 4.2.3 Materials and Corrosion, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale
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Scale inhibitors (SI) have been applied very successfully over many years in oilfields to prevent the formation of mineral scale. Both barium sulphate and calcium carbonate scales may be prevented using inhibitors, although in this work we will focus on the more difficult barite inhibition problem. A number of publications have appeared discussing the mechanisms by which barium sulphate scale inhibitors operate to prevent or retard scale formation. The mechanisms are discussed here in terms of (a) nucleation inhibition where the scale proto-crystals forms but are then disrupted or redissolved by the action of the inhibitor molecules, and (b) crystal growth inhibition where the inhibitor is thought to adsorb or interact with the active crystal growth sites (growing edges or spirals) hence retarding or stopping the crystal growth process. Both of these mechanisms are consistent with the inhibition of mineral scale at "threshold" levels, typically for MIC=0.5 - 20 ppm (MIC=minimum inhibitor concentration for a defined level of inhibition for a given test procedure). The MIC is always considerably below stoichiometric values in terms of the scale inhibitor to mineral scale molar ratios. It is known that most inhibitor types from the small molecular phosphonates (e.g. DETPMP) to polymeric species (e.g. PAA, PVS, PPCA) actually operate through both of the above mechanisms although one of these may predominate for specific species. Previous work has established that, broadly speaking, smaller phosphonates operate principally as crystal growth inhibitors and polymeric species work mainly as nucleation inhibitors.
A number of factors are known to affect the inhibition efficiency (IE) of scale inhibitors against barite formation, the main ones of concern here being pH, temperature and the calcium and magnesium levels in the scaling brine mixture. The general observed effects of these factors have been described in the literature and will be discussed in detail in this paper. However, no complete description of the mechanism of barite inhibition has appeared which clearly and consistently explains all of the observed effects of these parameters for different scale inhibitor types. It is the central aim of this paper to present a complete and consistent set of mechanisms for barite inhibition which may vary in degree for different inhibitor types. Our proposed mechanisms are based on a wide range of observations from the open literature analysed with our own experimental and modelling results.
To develop a set of mechanisms for barite inhibition, we use a number of experimental techniques and data sources. Since the experimental techniques used are well known, we refer the reader to literature descriptions for details.
Static inhibition (bottle) tests and dynamic (tube blocking) tests are both very well known techniques for determining inhibition efficiency (IE) and are described in detail elsewhere1,2. The brine compositions used in the new static IE results presented here are given in Table 1. The scale inhibitors used in this study are: diethylene triamine penta (methylene phosphonic acid) (DETPMP), phosphino polycarboxylic acid (PPCA) and polyvinyl sulphonate (PVS) and their structures are given in Fig.1
Crystal structure measurements of the a-axis deformation of the barite lattice from this work and the literature are used here3,4. The theory behind this is given in Ref. 5.
We also present some simple calculations of the equilibrium system containing calcium ions, magnesium ions and scale inhibitor. Results are given for the Ca/Mg/DETPMP system close to MIC levels for which the stability constants are known6.
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