Abstract
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.
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.
| Original language | English |
|---|---|
| Pages (from-to) | 306-317 |
| Number of pages | 12 |
| Journal | SPE Production and Operations |
| Volume | 27 |
| Issue number | 3 |
| DOIs | |
| Publication status | Published - Aug 2012 |