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

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

The analysis of chemical processes in reservoirs on the basis of the identification of injection-water fraction in produced brine

Summary The time and subsequent evolution of injection-water breakthrough are two of the main indicators monitored by production chemists. After injected water breaks through, the risk of scaling may change significantly, and scale-mitigation procedures should be planned accordingly. The fraction of the injection water in the produced brine may be ascertained only from analysis of the produced-water samples. However, to date, there has been little discussion about other applications of injection-water-fraction tracking. In this paper, new applications that follow on from accurate knowledge of injection-water fraction are proposed. The calculated injection-water fraction may be applied To quickly and accurately identify when injection-water breakthrough has taken place, at which time remedial action to prevent scale damage needs to be implemented. To identify which ions are involved in in-situ geochemical reactions, and the degree of relative ion deviations (i.e., to identify ion-exchange reactions). To detect the formation or formations from which a well is producing, and to determine (potentially) the amount of flow from each layer without the use of downhole flowmetering. Strong evidence of the involvement of barium, sulfate, and magnesium ions in reactions, on the basis of the calculations of the relative ion deviations, has been shown for field data. In another case, application of injection-water fraction prompted a re-evaluation of formation-water compositions, and as a result, it was discovered that the well was producing from a different formation after reperforation. The significant new developments presented in this paper allow analysts to obtain an indication of which ions are involved in the reactions, and the degree of relative ion deviations. Additionally, a technique is proposed that identifies the formation or formations from which the well is producing.</jats:p

Predicted and Observed Evolution of Produced-Brine Compositions and Implications for Scale Management

Summary Produced water was sampled and measured repeatedly during production from an offshore field, and an extensive brine-chemistry data set was developed. Systematic analysis of this data set enables an in-depth study of brine/brine and brine/rock interactions occurring in the reservoir, with the objective of improving the prediction and management of scale formation, along with improving its prevention and remediation. A study of the individual-ion trends in the produced brine by use of the plot types developed for the reacting-ions toolkit (Ishkov et al. 2009) provides insights into the components that are involved in in-situ geochemical reactions as the brines are displaced through the reservoir, and how the precipitation and dissolution of minerals and the ion-exchange reactions occurring within the reservoir can be identified. This information is then used to better evaluate the scale risk at the production wells. A thermodynamic prediction model is used to calculate the risk of scale precipitation in a series of individual produced-water samples, thus providing an evaluation of the actual scaling risk in these samples, rather than the usual theoretical estimate, on the basis of the endpoint formation- and injection-brine compositions and the erroneous assumption that no reactions in the reservoir impact the produced-water composition. Nonetheless, the usual effects of temperature, pressure, and brine composition are accounted for in these calculations by use of classical thermodynamics. The comparison of theoretical and actual results indicates that geochemical reactions taking place in this given reservoir lead to ion depletion, which greatly reduces the severity and potential for scale formation. However, ion-exchange reactions are also observed, and these too affect the scale risk and the effectiveness of scale inhibitors in preventing deposition. Additionally, comprehensive analysis by use of a geochemical model is conducted to predict the evolution of the produced-brine compositions at the production wells and to test the assumptions about which in-situ reactions are occurring. A good match between the predictions from this geochemical model and the observed produced-brine compositions is obtained, suggesting that the key reactions included in the geochemical model are representative of actual field behavior. This helps to establish confidence that the model can be used as a predictive tool in this field.</jats:p

Streamline Simulation of Barium Sulfate Precipitation Occurring Within the Reservoir Coupled With Analyses of Observed Produced-Water-Chemistry Data To Aid Scale Management

SummaryIn waterflooded reservoirs under active scale management, produced-water samples are routinely collected and analyzed, yielding information on the evolving variations in chemical composition. These produced-water chemical-composition data contain clues as to the fluid/fluid and fluid/rock interactions occurring in the subsurface, and are used to inform scale-management programs designed to minimize damage and enable improved recovery.In this interdisciplinary paper, the analyses of produced-water compositional data from the Miller Field are presented to investigate possible geochemical reactions taking place within the reservoir. The 1D and 2D theoretical model has been developed to test the modeling of barium sulfate precipitation implemented in the streamline simulator FrontSim. A completely 3D streamline simulation study for the Miller Field is presented to evaluate brine flow and mixing processes occurring in the reservoir by use of an available history-matched streamline reservoir-simulation model integrated with produced-water chemical data. Conservative natural tracers were added to the modeled injection water (IW), and then the displacement of IW and the behaviors of the produced water in two given production wells were studied further. In addition, the connectivity between producers and injectors was investigated on the basis of the comparison of production behavior calculated by the reservoir model with produced-water chemical data. Finally, a simplified model of barite-scale precipitation was included in the streamline simulation, and the calculation results with and without considering barite precipitation were compared with the observed produced-water chemical data. The streamline simulation model assumes scale deposition is possible everywhere in the formation, whereas, in reality, the near-production-well zones were generally protected by squeezed scale inhibitor, and, thus, the discrepancies between modeled and observed barium concentrations at these two given wells diagnose the effectiveness of the chemical treatments to prevent scale formation.</jats:p

Studies of the Affinity of Rickettsia mooseri for Tunica vaginalis I: Affinity for Tunica vaginalis of Guinea-pig

It is said that Neill-Mooser reaction in the scrotum of the male guinea-pig is specific to R. mooseri, but R. prowazeki also sometimes causes the non-specific reaction, so-called false Neil-Mooser reaction. The author studied on the substance of Neill-Mooser reaction by investigation of the figures of cells in the exudate of the tunica vaginalis of scrotum. The results are summarized as follows: 1) In every case, the tunica reaction by R. mooseri was markedly positive and lasted for a long time. The reaction by R. prowazeki was accidental and not specific. 2) Even inthe cases of inoculation of R. mooseri into male guineapigs, the reaction did not always appear typically in those weighing under 400 g. 3) In the positive phase of the scrotal reaction in guinea-pigs inoculated with R. mooseri, activation of fibrohistiocyte system appeared in the tunica vaginalis, and the great majority of rickettsiae became visible in the serous cells. In short, the affinity of R. mooseri for the tunica vaginalis of the guinea-pig was specific, while that of R. prowazeki was not specific