Barium Sulphate Inhibition in Subsea Systems – The Impact of Cold Seabed Temperatures on the Performance of Generically Different Scale Inhibitor Species

Abstract

Inorganic precipitates (scales) are one of the major flow assurance concerns in offshore oil and gas production, and lead to significant reductions in productivity and costly workovers if allowed to form uncontrolled. Scale prevention by the use of chemical inhibitors, applied either by continual injection or by squeeze treatment into the near wellbore formation, has generally been regarded as the most cost effective solution to the problem. The increased development of subsea satellite production brings with it a number of additional challenges to the chemical control of inorganic scale. The requirement to control scale at low seabed temperatures (4 - 5°C) over long residence times presents a particular challenge. This paper examines the performance of three generically different scale inhibitor species including a polyvinyl sulphonate (PVS), a phosphino polycarboxylate (PPCA) and a penta phosphonate (DETPMP) under typical North Sea Scaling conditions. The relative performance of the different inhibitor species against Barium Sulphate scale is compared at temperatures between 5°C and 5°C and up to 22 hours residence times. Results show that the performance of different inhibitors are significantly affected and to different degrees as a function of temperature, and that this is not simply related to changes in supersaturation against barium sulphate precipitation. The performance of the phosphonate species DETPMP is significantly reduced at lower temperatures and this is related to reduced adsorption at the barium sulphate surface. However the performance of the PVS is significantly improved, indicating that such species offer the potential for improved scale control at lower seabed temperatures. PPCA is only marginally affected by reduced temperatures in a manner more consistent with the increased supersaturation against barium sulphate. Introduction and Background As the search for new hydrocarbon reserves and the need to maximize recovery from mature fields continues, many operators are moving away from conventional platform developments in favour of subsea wells. In these situations, wellheads are frequently located subsea, with long seabed flow lines connecting the well to production facilities such as an FPSO or an existing platform some distance away. Such installations are invariably complex, with many satellite wells tied back to a single central facility, where fluids from several wells may be mingled in a shared flow line, making access to individual wells for workovers, squeeze treatments or fluid analysis extremely difficult and prohibitively expensive, especially in deepwater development systems as have been described previously.1–6 The increased development of subsea satellite production, coupled with the extremely high intervention costs associated with such wells, brings with it a number of additional challenges to the chemical control of inorganic scale. Water is most dense at the seabed, and will usually be in the range 3.3°C - 6.1°C (38°F - 43°F), so the produced fluids in non-insulated lines experience significant cooling before arriving at the production facilities. The extreme length of subsea flow lines particularly in deepwater systems (the longest reported to date is 60 miles in the Mensa field operated by Shell in the Gulf of Mexico3) increases residence times to up to twenty four hours, compared to the minutes or hours associated with conventional platform developments. These low temperatures and long residence times combine to make a formidable challenge for scale inhibition. The supersaturation of a given solution of barium sulphate increases at lower temperatures due to the reduced solubility of the mineral. In addition, the performance of certain chemical inhibitors at low temperatures has raised some concern. In particular, different workers have demonstrated that the performance of a conventional phosphonate DETPMP (di-ethylene tri-ammine penta-methylene phosphonic acid) can be significantly reduced at lower test temperatures with certain polymers being effected affected much less significantly.5,7,8

Synergistic Properties of Phosphonate and Polymeric Scale Inhibitor Blends for Barium Sulphate Scale Inhibition

“…In such tests, a number of variables can be changed, for example, brine composition (notably Ba 2ϩ , Ca 2ϩ and Mg 2ϩ ), the barite Saturation Ratio (SR), salinity, pH, temperature, etc. (Boak et al, 1999;Boak, 2013;Cushner et al, 1988;Graham et al, 1997Graham et al, , 2002Graham et al, , 2003Laing et al, 2003;Read and Ringen, 1982;Shaw, 2012;Shaw et al, 2012aShaw et al, , 2012b2012c;Shaw and Sorbie, 2012, 2013c, 2014bSorbie et al, 2000;Sorbie and Laing, 2004;Yuan et al, 1997aYuan et al, , 1997b1998;Yuan, 2001Yuan, , 2002. In some cases, [SI] may also be assayed over time in addition to IE -these experiments are referred to as SI consumption tests (Shaw, 2012;Shaw and Sorbie, 2013c).…”

Scale Management in Deep and Ultradeep Water Fields

“…These inhibitors must still be stable at low temperatures of the seabed since the wells are located at a distance of ten kilometers from the production unity. Main active material from inhibitors available commercially is presented in Figure 1 (7,8) .…”

Meeting the Flow Assurance Challenges of Deepwater Developments: From Capex Development to Field Start Up

“…32,33 Conventional static bottle tests at 75C and 40C were performed. 30,31 The remaining question is whether or not this type of evaluation is an accurate reflection of the actual production environment. 10 Prior to mixing, the test brines were equilibrated at the respective temperatures.…”