Scale

A model for predicting scaling
Principal Investigator: Knud Dideriksen

Barite, BaSO4, can deposit in oil wells when Ba rich formation water is mixed with seawater during production. These scales are costly to remove. We have developed a PHREEQC based model to calculate the rate at which scale accumulates in oil wells using recently established methods to predict barite solubility and growth kinetics. Based on the composition of the produced water analysed from samples collected at the surface, the model aims to predict the evolution of the solution composition and the barite scaling rate in the subsurface by calculating back in time. When barite growth is assumed to occur unhindered on pipeline walls, the predicted scale accumulation occurring for a well during breakthrough of the seawater largely reproduces the mean value estimated from scale milling and subsequent calliper measurement. However, important discrepancies between predicted values and those from measurement exist for the near surface part of the well, indicating that this type of model does not function effectively. Incorporation of a scheme to describe heterogeneous nucleation in the calculations results in predictions for scale accumulation that more closely resemble those from measurement, although discrepancies still remain in for the near surface region. Using a temperature dependent effective surface free energy for the nuclei results in minimal differences between the calculated amount of scale as a function of depth and that derived from measurement.