Improving the Value: A Case Study of Integrated Asset Modelling of a Giant Contaminated Gas Field

Abstract

The pace of natural gas discoveries globally is growing, but comes with challenges from entrained acid gases, primarily carbon dioxide and hydrogen sulfide. The development of these sour gas fields becomes more complex when there is varying gas composition along the field. Considerations are gas calorific values and corrosive fluids which are detrimental to pipelines and facilities. This paper presents a case study of an integrated asset modeling for three different compartments of varying heating values sharing a joint surface production facility. The aim is to optimize future sale gas contracts, avoid operational problems, and appraise water production effects.

The field has three hydraulically isolated compartments, each with unique compositions, with more than 6 Tscf reserves. The gas heating values range from 300 to 700 BTU/CUFT, with CO2 concentrations from 9% to 45%. Field economics requires all three compartments to be produced through a single production platform. This poses many operational challenges, e.g., production strategy to meet sale gas heating values, and pressure constraints to balance gas and water production. A major consideration is the mixing ratio of gas from each compartment. The combined gas composition and produced water will govern acidization effects in the surface facilities, which would need to be minimized. After geological and geophysical modelling, historical production data is matched. Next step is network modeling of surface facilities. The final piece is an economic analysis focused on costs of production, production improvement/remediation costs, and gas sales.

The result is the ability to maximize the rate of return on investment while keeping optimized production rates that meet the contractual requirement. Final outputs include gas reserve estimates and the optimized development scheme. The proposed scheme considers workovers, infill wells and gas compression to increase the estimated ultimate recovery and prolong field production life. The final integrated model improved the understanding of the interface between all the components of the system (i.e., reservoir, wellbore, surface, and facility). The model was validated when field production exceeded the maximum historical gas rate and fulfilled the high energy demand requirement. As expected, the field data closely matched the model predictions. The model considered the difficulty in producing the required energy rate and delivery pressure due to bottlenecks in the pipeline and predicted the need for compression due to rising water-gas ratio.

The ability to model complex reservoirs and surface production equipment in a single platform is a powerful tool for reservoir management. By carefully modelling the entire system and forecasting, the model transitions from a simple analysis tool to a test bench for altering the production strategy without impacting the actual physical assets. This allows the operator to minimize the cost of ownership and maximize return on investment.