Distribution of Hydrogen Sulphide in the Buzzard Field

Abstract

The Buzzard field, discovered in 2001, is an Upper Jurassic turbidite reservoir lying in the outer Moray Firth of the North Sea. It has an estimated STOIIP in excess of 1 Billion STB. During the appraisal drilling the fluid properties were found to be generally similar across the field with the exception of the H2S, CO2 and Mercaptan levels. During initial development drilling, using improved measurement techniques, the H2S levels were found to vary more than originally predicted. Extensive geochemical studies were performed to ascertain the charging mechanism. The results suggested that the H2S was present in the original charge oil, that the charge was very recent, and that the aquifer water was most likely having a significant effect in removing some of the H2S from the oil during migration. It was also thought that there was additional natural scavenging from the rock. These theories were also supported by equation of state modelling and stable isotope analysis. The H2S data was mapped onto the geo-model and H2S levels were simulated and controlled using tracers in the dynamic model. The results were subsequently used to determine future well locations and to predict and control the field's producing H2S levels. Since the H2S distribution was determined, 10 out of 10 production wells drilled have been successful in targeting sweet oil. This drilling program has allowed the field ramp up and plateau to be unaffected by the more variable H2S distribution. To date the field has now produced more than 155 MMstb of oil with actual H2S levels being relatively close to predictions. In summary improved H2S sampling and measurement techniques, geochemical modeling, EOS modelling and mapping in conjunction with simulation have provided a robust solution to aid effective field development. Introduction Sampling during the initial development drilling showed the H2S levels to be more variable than originally predicted from the appraisal well data. The distribution of H2S across the Buzzard field shows a trend of diminishing concentration towards the south west. There is also a correlation between carbon dioxide and hydrogen sulphide concentrations on a West - East basis though this varies with northing. Figure 1 shows the Buzzard location and Figure2 shows the measured results to date of all the initial H2S levels across the field prior to any predicted sweep in each area. Numerous analogue fields2,6 have been examined though few show comparable variability. There was a strong desire to understand the mechanism behind the distribution of the contaminants and why, given the reservoir is highly permeable, the gradients have not diffused away over geological time. The objectives were twofold; firstly to give reassurance that extrapolations from contouring could be used with confidence and so predict concentrations away from the well controls. The second was to predict likely maximum H2S levels so that remediation facilities could be sized appropriately. This paper examines the causes and distribution of H2S across the Buzzard reservoir as well as the techniques used for prediction of producing H2S levels.

The Buzzard Field: anatomy of the reservoir from appraisal to production

“…6), found H 2 S levels three times greater than expected based on appraisal well predictions. Subsequent wells throughout the field demonstrated that the H 2 S distribution varied significantly over short distances (Beilby et al 2009). As a result, a solution for handling of higher H 2 S levels later in field life was required and the decision to install a fourth platform to handle the fluids was made.…”

Scavenging Alkyl Mercaptans: Elucidation of Reaction Mechanisms and Byproduct Characterization

“…Mercaptans have been reported to exacerbate high-temperature corrosion in crude and condensate distillation units . Mercaptans are also well known to reduce the fiscal value of gas and crude oil if present in sufficiently high concentrations …”

H2S Risk in Presalt Reservoirs

“…As, for example, biochemical sulfate reduction (BSR) needs sulfate and sulfate-reducing bacteria in environments with temperatures lower than ≈ 60 to 80 °C 3 , thermal cracking of kerogen (especially type II-S), magmatic sources and possibly thermal alteration of oils 4,5,6 . Because H 2 S is a powerful poison and has great capacity for increasing corrosion, its presence can bring significant health and economic impacts for petroleum exploration and production.…”