Sulfate-Based Scales: Why Sulfate Ion Concentrations Matter

Abstract

Waterflooding is frequently applied in oil and gas reservoirs to sustain the reservoir downhole pressure and achieve a high rate of hydrocarbons production. The main challenge of water flooding is the potential to induce mineral scale depositions, especially when the injected water contains a high sulfate content. The objective of this paper is to determine the minimum threshold of sulfate contents present in the injected water that will not lead to mineral depositions through theoretical and experimental work. This study will showcase scale simulation and laboratory experiments involving multi-compositional water interactions with various sulfate concentrations ranging from 100 to 5,000 ppm. The study scheme starts by water preparation for the proposed brines, followed by geochemical analysis using Inductive-Coupled Plasma (ICP) to confirm the physical and chemical properties. Afterwards, scale risk assessment was performed using ScaleSoftPitzer™ to evaluate the scaling tendency. Finally, static bottle tests are conducted for a 48- hour testing period at temperatures ranging up to 260°F using HPHT aging cells at different mixing ratios. The scaling tendency assessment indicated a calcium sulfate and strontium sulfate precipitation when sulfate content exceeded 300 ppm. Although 5,000 ppm of sulfate content showed theoretically a low scaling index of gypsum and celestine, the scale mass was estimated to be nearly 2,500 and 500 mg/L, respectively at equivalent mixing ratio. Also, it was observed that as the sulfate content increases, the scale severity increases as well. The experimental compatibility tests reinforced the simulation findings, which showed similar result of inorganic sulfate-based scale precipitation that was observed after 48 hrs. testing time. The paper shares a system approach for water compatibility assessment and provides different scenarios using both simulations and water-water lab tests to confirm these predictions. It will also highlight how to mitigate the potential risk of scale and formation damage during water flooding.