Multi-Domain Integrated Workflow for Reservoir Souring Modeling and Prediction to Effectively Define and Mitigate H2S Production Risk in Offshore Developments Undertaking Waterflooding
Authors: Chaban Fady , Garduno Jorge , Osorio Nelson , Jordan Lee 
Published: October 2020

This workflow evaluates reservoir souring risk for a deepwater development, taking into consideration dynamic reservoir characteristic effects over time, and incorporating detailed flow modelling of the system to assess H2S partitioning effects and to determine their design and operational impact. The objectives are: Showcase an integrated multi-domain approach to souring modeling from reservoir to facilities, provide a reservoir souring modeling workflow applicable to new and existing offshore waterflooding projects, and present a case study in which this workflow was successfully implemented at two distinct points of (pre-first oil and after eight years of production)of an existing development

This unique workflow takes a holistic approach to reservoir souring prediction. This process may be implemented either pre-first oil or during field production in order to optimize souring mitigation strategies and operations, such as sulfate removal and nitrate injection. If the evaluation is to be undertaken before production operations, data will be more limited, however, this methodology has proved to still be effective in determining the risk associated with souring for that particular reservoir. All aspects of the system are considered: reservoir fluid characteristics, reservoir static and dynamic models, production system thermo-hydraulic and produced fluids thermodynamic performance, and the critical subsea and topsides design considerations.

The workflow has been proven to simulate the souring process, the growth and respiration effect of SRB leading to H2S formation in the reservoir, as well as the biological reactions related with the injection of nitrate to inhibit these phenomena for a series of conditions using the latest modeling tools available.

The approach helped understand a complex set of bio-chemical reactions involved in the production of H2S and its potential inhibition to avoid reservoir souring in a particular field. A sensitivity analysis using the integrated special reservoir simulation model was able to predict the effectiveness of nitrate injection and has also been used to guide the design phase, while supporting the subsequent optimization of the mitigation strategy supportbased on the evaluation of actual production data.

The comprehensive modeling approach employed considers for variation in biological system characteristics, reservoir conditions, and hydrodynamics of the production system that affect the production of hydrogen sulfide. Production of elevated levels of H2S can generate material integrity issues including corrosion, health and safety issues, and loss of hydrocarbon value, since high H2S contents will reduce the monetary value of produced oil or gas.

The field case study presents souring models calibrated and validated with actual data collected from the production streams during eight years of history and a set of sensitivity scenarios were analyzed to consider uncertainties in various simulation parameters. The results show that the dosage of calcium nitrate injected in this instance successfully controlled SRB growth and that no reservoir souring was detected. However, the application of the developed workflow allowed the operator to optimize the current Calcium Nitrate dosage, enabling a considerable reduction in the injection rate, which represented a key factor for operational costenabling subsequent operating expenditure reductions.

The value added with this workflow, during waterflood management using seawater injection, allows operators to timely and clearly determine reservoir souring behavior and anticipate potential risks associated with it. The findings are also useful for supporting major design and operational decisions, including the development and optimization of souring management and mitigation techniques.