Scale-Inhibitor Consumption in Long-Term Static Barium Sulfate Inhibition Efficiency Tests

Abstract

Summary In typical bulk barite inhibition-efficiency (IE) experiments testing scale inhibitors (SIs), the IE is determined by comparing the concentrations of the barium ion in the mix to that in an uninhibited "blank" solution. That is, [Ba2+] vs. time is monitored for both a blank and an inhibited solution. The fate of the SI itself over this test time period is rarely monitored, although this can give some important insights into the inhibition mechanism. This paper presents a series of long-term (up to 96 hours) barium sulfate static IE experiments in which both the supernatant SI concentration, [SI], and [Ba2+] are assayed at multiple sampling times after initial mixing of seawater (SW) and formation-water (FW) brines. In standard IE tests to determine SI minimum inhibitor concentration (MIC), sampling is normally carried out at 2 hours and 22 hours only. The long-term IE experiments described in this paper involve up to eleven sampling times, typically at t = 0.5, 1, 2, 3, 4, 5, 6, 22, 48, 72, and 96 hours after mixing. The aim of this type of experiment is to determine the fate of the SI over time and to determine whether the IE and percentage of SI (%SI) in solution vs. time profiles correlate with each other. Clearly, SI removed from solution is consumed into the forming barium sulfate lattice in the barite precipitate. IE and solution SI often continue to decline beyond 22 hours, up to 96 hours after the mixing stage. In these experiments, the SI concentration is set at a pre-2-hour MIC threshold concentration, such that the IE and %SI consumption profiles will both decline sufficiently over time, possibly even as early as one-half hour after mixing. The SI consumption profiles obtained by testing an extensive range of phosphonate and polymeric SIs are compared and discussed. Normally, a phosphorus-containing SI is selected for evaluation to facilitate assay of SI by means of inductively coupled plasma (ICP) spectroscopy. However, some novel results are also presented in which selected polymeric SIs were assayed by non-ICP analytical techniques. We interpret this extensive series of SI lattice-consumption experiments in terms of the mechanisms of barite inhibition and Type 1/Type 2 SI classifications presented in previous papers (Shaw et al. 2012a, b). The relevance and practical application of these findings are also discussed.