The effect of pH on static barium sulphate inhibition efficiency and minimum inhibitor concentration (MIC) of generic scale inhibitors

This paper discusses the effect of pH on the static barium sulphate inhibition efficiency (IE) of four scale inhibitors: DETPMP and HMTPMP (both penta-phosphonates), EDTMPA (a tetra-phosphonate) and PPCA (a phosphino poly carboxylate). pH is an important parameter with regard to SI functionality. The effect of pH on each SIs IE is explained in terms of SI speciation and binding to Ca2+ and Mg2+ (and possibly Ba2+, Sr2+). In some cases there is an “optimum pH” for the effective operation of a specific SI, depending on the nature of the functional groups present in the chemical structure.

All static IE experiments were carried out at 95oC and using a North Sea Seawater/Formation Water (NSSW/FW) mixing ratio = 80/20. pH levels of 4.5, 5.5, 6.5 and 7.5 were chosen for testing since this covers the entire range which threshold level SI concentration might normally experience in a reservoir. The IE was determined for each SI at each pH level, where the [SI] was chosen close to the threshold MIC (i.e. pre-2 hour MIC) such that the specific effects of varying pH on 2 and 22 hour IE were clearly visible. In some tests MIC levels were reached, and in these cases, findings are presented in terms of MIC versus pH. For all four species tested, IE versus pH charts are presented. Furthermore, two brine mix compositions (maintaining NSSW/FW = 80/20) were examined, one with [Ca2+] = 742ppm and [Mg2+] = 1242ppm (base case) and one with [Ca2+] = 2000ppm and [Mg2+] = 739ppm (fixed case). The base case [Ca2+] and [Mg2+] result from the normal mixing of NSSW and FW in an 80/20 ratio, molar ratio Ca2+/Mg2+ = 0.36. In the fixed case mix, molar ratio Ca2+/Mg2+ = 1.64. This is done in order to compare and context results within an extensive earlier series of IE results for SIs which were tested in this manner (Shaw et al, 2010a, 2010b).