SPE Production & Operations
Volume 27, Number 3, August 2012, pp. 306-317

Summary

Conventional phosphonate-type scale inhibitors (SIs) are commonly applied for barite-scale prevention in oil fields. The barite forms when the injection water (IW), which is usually sulfate rich, is injected into a barium-containing formation water (FW). The inhibition efficiency (IE) of barite-scale inhibitors is affected by the barium sulfate saturation ratio (SR) of the brine mix and, additionally, by the presence of divalent cations Ca²⁺ and Mg²⁺. What is less well known is that the precise balance between these factors (SR and Ca²⁺/Mg²⁺ ratio) can vary significantly for different phosphonate species. This paper presents novel IE experimental results for phosphonate-scale inhibitors DETPMP and HMTPMP (penta-phosphonates), OMTHP (hexa-phosphonate), and HMDP (tetra-phosphonate). Minimum-inhibitor-concentration (MIC) levels for each SI are established by testing a wide range of brine IW/FW mixing ratios, which changes (i) barite SR and precipitated mass, (ii) molar ratio of Ca²⁺/Mg²⁺, and (iii) the ionic strength of the brine mix.

The phosphonate SIs were categorized into two types on the basis of their MIC-vs.-percent-North Sea-seawater (NSSW) behavior. Type 1 (e.g. DETPMP and OMTHP) are affected principally by SR and are rather less sensitive to Ca²⁺/Mg²⁺ ratio although they do show some sensitivity to the latter factor. Type 2 (e.g., HMTPMP and HMDP) are severely affected by brine Ca²⁺/Mg²⁺ ratio as well as SR. To demonstrate these effects conclusively, a series of IE experiments is presented with varying [Ca²⁺] and [Mg²⁺] (which normally occurs in the field as the IW/FW ratio changes over time) and then similar experiments are repeated at a fixed Ca²⁺/Mg²⁺ molar ratio. The MIC level measured for both types of phosphonate SI always correlates very well with the barite SR at fixed Ca²⁺/Mg²⁺ molar ratio (fixed case). In addition, we find the MICs of both types of SI are much lower in the fixed-case experiments (higher Ca²⁺/Mg²⁺ molar ratio), compared with the base case, because of the beneficial effect of higher [Ca²⁺]. The effects observed are important for field application of phosphonate SIs because they show how the various species are sensitive to the changing scaling problems as the %NSSW increases, in terms of SR and Ca²⁺/Mg²⁺ molar ratio. These results also give some important insights into the mechanism by which different phosphonates actually work in inhibiting barite scale.

© 2012. Society of Petroleum Engineers

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History

• Original manuscript received: 24 June 2011
• Meeting paper published: 27 May 2010
• Revised manuscript received: 19 August 2011
• Manuscript approved: 6 December 2011
• Published online: 26 July 2012
• Version of record: 7 August 2012