Analysis and modelling of in situ geochemical reactions in oil fields based on produced Brine chemistry data.

Abstract

Management of mineral scale precipitation is one of the major challenges faced by the oil industry. Total costs of scale prevention can exceed £1 million for a field or even sometimes for a single well. Identification of the injection water fraction in the produced brine stream is of importance to production chemists involved in mineral scale prevention. This data is required to determine the onset and the severity of barium sulphate precipitation, one of the most challenging flow assurance issues in the oilfield due to the very low solubility of the mineral. This body of work develops a solution to the problem of how to determine the injection water (IW) fraction in the produced brine. A robust and accurate method for calculating IW fraction in produced water samples is presented. The method has been named the “Reacting Ions” method. The Reacting Ions method is based on interactions between ions during reactions, by correctly taking account of ion losses that will occur due to precipitation. The proposed new method allows injection water fraction to be calculated from concentrations of the ions involved in reactions, which has never been done before. In addition, the new method incorporates as a limiting case the Ion Track method - the most widespread method currently used in the industry. The Reacting Ions method removes the limitation that only conservative ions can be used to track injection brine in produced water. This Reacting Ions method is applied to a synthetic produced water case, generated using a reservoir simulator, where the “correct” IW fraction is known, and a very good match is achieved, even when significant noise is applied to the synthetic data. An additional outcome of the synthetic case tests is that conventional use of sulphate in the Ion Track method leads to a late detection of injection water breakthrough, while the Reacting Ions method based on barium and sulphate is significantly more accurate. Delayed detection of injection water breakthrough can lead to the onset of scaling before preventative measures have been taken. The Reacting Ions method was applied in the analysis of produced brines for more than 100 wells in several regions of the North Sea. Results of the study presented here show that the method is generally more effective in detecting IW fractions than conventional ion tracking techniques, especially at low IW fractions soon after breakthrough occurs. Using barium and sulphate, the new Reacting Ions method benefits from near zero end-point concentrations of iii these two ions that is typical for North Sea brines, and is consequence of the low solubility of barite. The more accurate identification of IW fraction has led to the development of three applications that use the Reacting Ions method. In the first, the relative ion deviations are used to identify whether an ion is conservative, precipitating or part of a dissolution reaction. This information can be applied by production chemists to predict possible types of mineral scale occurring. The second application assists in detecting which formation or formations the well is producing from, which gives incremental information about the reservoir itself. In the third, a method to analyse squeeze treatment response is proposed. The impact of scale inhibitor placement on the ion concentrations is evaluated, and thus a judgement can be made regarding the overall effect of the squeeze treatment in stopping the identified scale reactions from happening. All three new applications were successfully applied to field data.