Selection and Application of Scale Inhibitor for Application in Oxygenated Produced Water, Norwegian Sector, North Sea

Abstract

Scale control within produced fluids both topside and downhole in oil/gas production facilities is critical to the effective production of hydrocarbons in a safe, economic and environmentally acceptable manner. This paper will focus on the scale challenges associated with production of produced water that contains significant amounts of barium, sulphate and dissolved iron ions. While this challenge is not unusual in production operations the presence of dissolved oxygen in the topside process just prior to discharge has resulted in a very challenging environment for barium sulphate scale inhibition. This paper will outline the challenge of scale control in the presence of ferrous and ferric ion. The inhibitor selection program undertaken will be outlined as well as the critical factors that had to be taken into account given this application was for offshore Norway where environmental characteristics of production chemicals can restrict the choices of chemical suitable for this application. The development of an application strategy for the selected environmentally acceptable chemical will be outlined along with the monitoring program to ensure not only that the chemical is applied at the correct treatment rate but that it performs as expected from the laboratory performance testing results and can be optimised as brine composition changes with time. Introduction The subject field is located in the Norwegian sector of the North Sea and came on stream in December 1998. The 8 production and 3 injection wells are tied into the wellhead platform and the oil processed at an FPSO. The distance between the well head platform and the ship is 1 km. The water depth is 86 m. The oil is exported by tankers and the produced gas re-injected. Map of the fields location is shown in figure 1. Water injection utilises seawater for reservoir support and is incompatible with the formation water. Over the years scale squeeze treatments have been applied to production wells to control sulphate scale. This is supplemented by application of scale inhibitor on the wellhead platform to prevent further deposition during transport of fluids to the FPSO and during its processing prior to discharge to sea. The field's formation water and a typical produced water chemistry arriving at the FPSO is presented in Table 1. The "extreme PW" outlined in Table 1 refers to the produced water expected when a well with high concentration of formation water is flowing with a well with a high proportion of seawater. The range of process conditions is presented in Table 2.