The Evaluation of Enhanced (Carbonate/Sulfate) Scale-Dissolver Treatments for Near-Wellbore Stimulation in Subsea Production Wells, Gulf of Mexico

Abstract

Dissolver technology has been developed and applied with varying degrees of success over the past few years to clean carbonate and the more challenging sulphate/sulphide scales from production tubing and process equipment. The intention of this proposed application was to select a chemical that could be applied into the near wellbore region of the reservoir to recover scale induced damage while not creating any secondary precipitation or negative wettability effects. The impact of dissolver chemical formulation on the reservoir formation's wettability and the potential for mobilization of fines (both reservoir silicate and scale) has not been outlined before. In this paper reservoir condition coreflood studies are outlined where scale is formed within the cores to allow assessment of sulphate dissolver performance. This will form a critical part of the technical evaluation of the dissolver formulation suitability for field application. Results from these coreflood studies will be compared to those generated for the same chemicals in conventional static dissolver studies. The practical concerns about deployment methods within subsea production wells are also emphasized. Specific reference will be made to the impact of metallurgy on chemical selection and the need to reduce corrosion risk within developments which utilize both carbon steel and super duplex requiring the development of higher pH non acid carbonate dissolvers. This paper shows that simple dissolver tests can give misleading performance information as they only show dissolution rates, impact of concentration of active chemical or the increased dissolution possible with solvents. The use of pre scaled reservoir cores to assess the impact of dissolver performance and the potential for secondary formation damage allows the most effective dissolver with minimum formation damage potential to be selected for field application. Introduction. Oilfield scale formation and control. The precipitation of insoluble mineral scale deposits (scaling) is a common cause of near wellbore formation damage during the production of hydrocarbons and associated produced water. Precipitation of scale can occur within the formation, near wellbore or in the production tubing. The physical obstruction/restriction caused when scale is deposited within the near welbore and production tubing can lead to sever reduction in near wellbore permeability and reduce the internal diameter (ID) of the production tubing. The reduction in flow area can significantly increase flowing bottom hole pressure and reduce production rates. Two mechanisms of scale formation are commonly encountered. Carbonate scales (Calcite CaCO3, Siderite FeCO3) can be formed when pressure or temperature changes in a flowing fluid leads to the break out of dissolved carbon dioxide which is accompanied by the scale deposition. The mixing of incompatible brines can lead to the formation of sulphate scales. The common sulphate scales include Barite BaSO4, Celestite SrSO4, Anhydrite CaSO4 and Gypsum CaSO4.2H2O though of most concern with seawater injection is Barite owing to its very low solubility[1–4].

Nonacid Solution for Mineral Scale Removal in Downhole Conditions

“…Scale dissolvers are normally tested using static tests that allow the selection of the most suitable chemical and the estimation of the optimum soak period (Ramstad, 1997;Jordan et. al,;2006).…”

“…Scale dissolvers are typically aqueous solutions of either acids/chelants for carbonate scale or alkaline chelating agents for sulfate scales Frenier, 2001;Benson et al, 1997;Weintritt et al, 1967;Schutte, 1987;Clemmit et al, 1985;Jordan et al, 2006) also, carbonate scale can be removed by using corrosion-inhibited hydrochloric acid (Bakken et al, 1996;Ramstad, 1997;Benson et al, 1997;Rhudy, 1993). This paper will focus on the chemical removal of calcite and siderite scale associated with formation damage in oil fields in Colombia and the process used to develop a scale control strategy that includes the use of a material with neutral pH, high dissolution capacity, and no re-precipitation issues.…”