Scale Prediction and Mineral Solubility Under HPHT Conditions

Accurate scale prediction modelling is only possible when reliable mineral solubility data are available under the required conditions. It is recognised that the relative paucity of high pressure, high temperature (HPHT) solubility data can result in inaccurate predictions as current models extrapolate from data obtained under more conventional conditions. This paper describes the generation of additional fundamental solubility data under HPHT conditions and comparison of the obtained values with several existing models. A purpose-built laboratory test rig capable of making mineral solubility measurements up to 250 °C (480 °F) and up to 30,000 psi has been used in this work. Experimental solubility data have been generated for calcium sulphate at different temperatures and the methodology has been investigated to ensure that equilibrium conditions have been reached. In this work, barium sulphate solubility data have also been generated at conditions up to 200 °C (390 °F) and 19,000 psi. Notably, the solubilities have been determined in the presence of relatively high concentrations of additional ions, e.g., calcium, as it was recognised that available data were limited for more oilfield-representative brine compositions from HPHT reservoirs. The data generated were also compared against solubility predictions for a range of industry models to assess their accuracy in these circumstances. The results obtained for calcium sulphate solubility indicate the importance of validating the test methodology, not just for each mineral, but also under the required temperature and pressure conditions, to verify that equilibrium solubility conditions have been achieved. Barium sulphate solubility increases with the addition of other divalent ions but the extent of the increase is at present not accurately predicted by existing scale prediction models at HPHT conditions. In some cases, the predicted barium sulphate solubility was up to three times greater than the experimentally determined value. It is apparent that there is considerable scope for improvement of scale prediction models under HPHT conditions particularly in complex brine systems and that further fundamental solubility data are required to facilitate this. This paper provides additional data for mineral solubility under HPHT conditions but, more importantly, shows data for complex brines that are more representative of those produced in oilfields. The work further demonstrates the limitations of existing scale prediction modelling software under HPHT conditions, particularly in the presence of other divalent ions, and illustrates areas where additional data and model development is critical to enable more accurate modelling of scale risk under these conditions.