Calcium Sulfate Scaling Risk and Inhibition for a Steamflood Project

Abstract

Abstract Steamflooding has been used as one of the emerging techniques to enhance heavy oil production. Scale control during steamflooding, on the other hand, can be challenging since the high temperature of the steamflood can decompose thermally unstable inhibitors and/or lead to the precipitation of metal-inhibitor pseudo-scale. In this paper, we present the analysis of the scaling risk and scale inhibition for a pilot steamflood project in a Middle Eastern oilfield. The formation of this field is a dolomite formation interbedded with anhydrite (CaSO4) streaks. Anhydrite has been observed to be the predominant scale form, which is presumably formed due to increased production system temperature as a result of steamflooding and/or mixing of steam condensate with incompatible connate water. Anhydrite is inherently difficult to control due to its high solubility and the high temperature conditions under which it forms. Compared to barite and carbonate, only limited knowledge has been acquired for anhydrite control. To predict the scaling tendency and inhibitor need in different wells of this field with different supersaturation level and temperature, a scaling risk model has been developed. To build such a model, detailed and revised laboratory procedures have been developed to study nucleation and precipitation kinetics of anhydrite at 125-175 ºC, different supersaturation, water composition, and long reaction time. Predictions of this scaling risk model suggest a saturation index (SI) of 0.8 as a critical SI for anhydrite control at >125 ºC. For example, when the SI is above 0.8, anhydrite will be difficult to control in the presence of threshold inhibitor. Model predictions were found to be consistent with scale occurrence observations in different wells of this field. With the recommended inhibitor concentrations, anhydrite scale has been controlled in this field, which provides validation that the proposed scaling risk model is a powerful tool to optimize scale treatment plan.