A Mechanistic Model To Evaluate Reservoir Souring in the Ekofisk Field

摘要

This paper presents a mechanistic approach to modeling the reservoir souring process in the Ekofisk Field, located in the Norwegian sector of the North Sea with over 6 billion STB OOIP and currently producing about 300,000 BOPD and injecting around 500,000 BWPD sea water. The objectives of this study were to determine if observed increases in H_2S concentrations from this seawater-flooded oilfield were due to microbiological activity and, if so, to estimate future H_2S production with further seawater injection and proposed produced water reinjection. Mechanisms considered in the model were imbibition and water flow through a highly fractured chalk formation; generation of H_2S due to the activity of sulfate-reducing bacteria (SRB); and partitioning of H_2S between the oil, water, and gas within the reservoir and in the topside separation system.Model-calculated H_2S production rates for individual wells, waterflood patterns, and full-field compared well to actual rates. Results indicated that both the water-oil and gas-oil ratios have a large impact on measured H_2S concentrations in the produced gas, but that increased water production is responsible for significant increases in total H_2S production. However, results also indicated that only a small fraction of the biogenic H_2S will be transported to the producer.This model presents a new approach for evaluating and forecasting the effects of souring for a naturally fractured reservoir whereby a biofilm is developed on the fracture faces and microbial nutrients are provided by incoming seawater and from formation water initially in the chalk matrix. It incorporates a mechanistic understanding of all of the key processes and is calibrated using the actual historical production rates from wells in several waterflood patterns. Presented in this paper are the model-forecasted results for the field-wide H_2S production associated with continued seawater injection.