Abstract
Inorganic barium sulphate is one of the most difficult types of scale to inhibit in oil and gas production systems, due to its physical hardness and its chemical and thermal stability. Barium sulphate scale is most commonly inhibited using either phosphonate or polymeric scale inhibitors (SIs) deployed at sub-stoichiometric concentrations. What is much less well known in the oil industry is the effect of using combinations of two (or more) SIs synergistically for enhanced scale inhibition performance. In this paper, we present a series of static barium sulphate inhibition efficiency (IE) test results in which a series of pairs of SIs have been tested synergistically at pH 5.5 and 95°C (203°F). Polymers can be blended with phosphonates, or alternatively pairs of phosphonates or polymers may be applied. In all cases, the synergistic IE is compared with the IE of each SI tested independently at the mass dosage (i.e. the same concentration in mg/L or ppm). Each separate single SI used in the work has been tested previously for barium sulphate IE at pH 5.5, 95°C (203°F) in order to determine the minimum inhibitor concentration (MIC) for each species (Shaw et al, 2012). A total of 9 phosphonate and 9 polymeric SIs have been tested individually. The MICs of the synergistic blends are compared with the normal MICs of the individual SIs. Surprisingly, we find that in most cases, the synergistic IE (for combined products) is usually higher over the range of [SI]s tested (i.e. MIC is lower), compared to that of each SI tested separately. Certain “pairs” of SIs used together yield a significantly beneficial effect (i.e. lowering of MIC; enhanced IE); e.g. DETPMP and HMTPMP. Some mechanistic reasons why these synergistic pairs work particularly well are suggested although we do not have conclusive proof of these conjectures.
