Abstract

Calcite (CaCO3) is one of the most common scales in oilfield, and its formation can cause a decrease in production. Pressure drop can cause the pH and calcite saturation index (SI) to increase, similarly increase in temperature increases calcite SI. We have developed a new approach to simulate the water composition (e.g., TDS, Ca, and alkalinity) for laboratory tests with ScaleSoftPizer (SSP) software based on desired temperature, pH, and SI. The evaluation of calcite formation and inhibition in the laboratory is also a challenge because the release of CO2 gas increases the pH and saturation index (SI) of the solution, and must therefore be controlled. Accordingly, this study aimed to develop a new and well-controlled test method for calcite inhibition under simulated oilfield conditions (i.e., pH, SI, Ca/HCO3 ion ratio). A new rapid kinetic turbidity method was developed in a closed system for calcite inhibition study. Good consistency was obtained for triplicate calcite nucleation and inhibition experiments, which demonstrated the reproducibility of the new method. Calcite nucleation experiments were conducted, and a new calcite nucleation model is proposed. The effect of acetic acid and inhibitors on calcite kinetics was also evaluated with this new method. This is the first time to accurately measure calcite inhibition under a broad range of production conditions, especially under neutral-to-acidic conditions (pH = 5.5-7), using simple and low-cost kinetic turbidity test methods. By avoiding the release of CO2, this new laboratory test method strictly controls pH, SI, and Ca/HCO3 ion ratios, and thereby the field conditions for calcite scales can be accurately simulated. In addition, barite inhibition experiments were conducted, and inhibition models for eighteen different inhibitors were determined and put onto a self-consistent algorithm, with many practical advantages: speed, reliability, internal consistency, and relative effectiveness for common conditions, to mention a few. Finally, a protocol has been developed to characterize the inhibition of essentially any new inhibitor, or combination, with minimal to no testing.

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