51316-7848-Calcium Sulfate Scale Formation and Inhibitor Selection for a Steam Flood Project

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Product Number: 51316-7848-SG

ISBN: 7848 2016 CP

Author: Fangfu Zhang

Publication Date: 2016

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Steam flooding is an emerging technology to enhance the production of heavy oil from various geological formations. A pilot study to test the steam-enhanced production has been conducted in a Middle Eastern oilfield for several years. The formation is a dolomite formation imbedded with anhydrite streaks. Calcium sulfate is one of the dominant scales in this field and scale control has been included in the program. At high temperatures hemihydrate (CaSO4•0.5H2O) and anhydrite (CaSO4) are the predominant sulfate scales formed while gypsum (CaSO4•2H2O) scale may form at low temperature. At temperatures close to the steam flood condition anhydrite has been observed to be the predominant scale form. In this paper a case history of laboratory scale inhibitor testing and field implementation will be discussed. A total of over 100 chemicals were tested including the initial screening. The chemistry of these inhibitors includes phosphonates polycarboxylates and their P-taged versions sulfonated polycarboxylates and their P-taged versions and polyvinylsulfonates. Only a handful of inhibitors passed the initial screening and were selected for brine compatibility and thermal stability testing. The critical limit of supersaturation index where a scale inhibitor can be effectively used to control scale is established to be about SI = 1.0. Above this supersaturation index a high inhibitor dosage is needed to control scale. For example less than 3 mg/L of an inhibitor is sufficient to control anhydrite scale formation at SIanhydrite < 1. When the SI is between 1 and 1.2 at least 10 mg/L is required. When the SI is above 1.2 anhydrite scale will be difficult to control and at least 30-40 mg/L of an inhibitor is required. The thermodynamics and kinetics of calcium sulfate scale(s) formation the envelope of their phase transition the limit of effective scale inhibition and the analysis of a trace level of polymeric inhibitor are also discussed.