The development of anaerobic setups and analytical techniques for the static testing of the oilfield sulphide scales

Abstract

The occurrence of oilfield sulphide scale has contributed to the decrease in production and injection rates by reducing the inner diameter of flow lines and causing failures to downhole equipment. The most common remedy for oilfield scale is inhibition, and the design of a scale inhibition treatment usually requires investigative laboratory tests to evaluate and understand the scale inhibition and formation mechanisms. The static jar test is one of the primary tests carried out in an evaluation study of a scale inhibitor, where two or more solutions are mixed that form or inhibit the formation of the subject scale(s). Further advancements in the technique for the static tests for sulphide scale have been required, as they are sensitive to oxygen, i.e. the tests result in iron (II) sulphide precipitation, and are prone to evolve H2S(g) at low pH levels (pH < 8), heightening further the need for special logistical and safety measures. Several developments of the conventional static bottle test were evaluated, which ultimately led to the development of an anaerobic bench-top static test setup, consisting of airtight tubes and vials. The primary purposes of this setup were to enable the control and monitoring of aqueous sulphide concentrations, as well as minimising the oxidation of sulphide species. This apparatus allows experimentalists to accurately pH-adjust sodium sulphide solutions and retain the evolved H2S(g), while continuously and nonintrusively quantifying the sulphide concentrations via the UV-Vis spectrophotometer technique. This anaerobic static test setup offers a genuine alternative to costly investment in anaerobic capabilities, e.g. glovebox facilities. The anaerobic setup was used in the determination of the effect of oxygen on the aqueous sulphide, which showed minimum influence in the oilfield scale studies. The formation of FeS, ZnS and PbS was tested under various conditions using the anaerobic setup and the results were validated against a scale prediction model. Dynamic formation tests of iron sulphide showed increasing iron-deficiency with time, which recommends more frequent cleanout treatments to increase dissolution efficiency. The reproducible pHadjustment of the aqueous sulphide had a significant impact on the scale inhibition tests, which can optimise inhibition treatment design. These results were attained using a safe, compact and highly repeatable setup developed at a fraction of the cost of other anaerobic testing systems.