Calcite Scale Prediction and Management in a Chalk Reservoir – Field Cases

Abstract

Abstract This paper demonstrates a field case study where large volumes of deferred oil, due to calcite deposition, have been reduced by effective scale modeling and a strong focus on optimised well performance. The South Arne field, operated by Hess Denmark, on behalf of the South Arne Partnership, is an oil field in the Danish sector of the North Sea. Production is from a chalk reservoir using de-sulphated seawater injection for pressure support. Calcite deposition has been problematic in some of the wells with a variety of mitigation and clean-up methods being employed. The deferred production peaked with 100,000 barrels of oil in 2006. Since then, the oil deferment has been reduced by 75 % by continuously improving the calcite scale prediction and prevention for the wells. Modeling carbonate scale deposition is challenging and many scale prediction models can be over-conservative, leading to unnecessary chemical injection, excessive use of chemicals and sub-optimal oil production. A new methodology for predicting calcite scale has been applied to the South Arne field, becoming a successful tool for downhole scale management. This new approach is based on scale kinetics as a function of temperature, integrated with detailed pressure-temperature traverse data derived from the interpretation of well tests and gas lift performance. Results have been used to guide well operation decisions and to support a new scale prevention strategy based on deep injection of large volumes of lift gas. Accurate calcite scale prediction is sensitive to the carbonate alkalinity. The approach described here is based on simulating the calcite scaling kinetics from reservoir to wellbore. Model results show that produced water is close to calcite saturation from deep reservoir to the wellbore, and this has been used as the basis for calculating carbonate alkalinity in the produced water. Hence in this paper the calcite saturation index for the onset of scaling has been defined using calcite kinetics to give a fixed scaling rate rather than the more customary use of a fixed saturation index. In addition, well performance models were completely re-worked to include observations from recent well interventions and new data input parameter validations. The calcite scale prediction methodology described here has provided the South Arne field with benchmarked operating scale envelopes that on a regular basis are utilised to optimise well performance in terms of risk and reward.