Abstract

By mid 2000 H2S content in gas from the Chihuido Lomitas field separator started to increase significantly up to an average of 2000 ppm.

A multidisciplinary team was formed by Engineering staff of Rincon de los Sauces and Repsol- YPF Applied Technology Center to address the problem using different approaches.

While measurements were thoroughly checked and a sampling survey at key points was designed, different alternatives were discussed to treat the gas by traditional methods. However, the problem was deemed to be very likely due to bacterial activity in the reservoir. Therefore, a group focused on the issue of concern which is generally known as reservoir souring. It was concluded that using sulphur isotope measurement techniques was one of the few tools available to effectively determine the cause of the problem.

A group of specialists in isotope assessments from a Research Institute was called to join the team.

Based on determinations supported by a geochemical model it was shown that H2S generation in the Troncoso and Avile formations was due to the activity of sulphate reducing bacteria living into the reservoir. The model developed relates increasing bleeding water injection to the phenomenon studied Innovative H2S removal techniques are currently being applied. A pilot test on two production wells started in late October.

Introduction

Reservoir souring is one of the main problems that can occur during the exploitation of a given oil field. H2S sources must be recognized in order to solve or mitigate their pernicious effects. Causes include microbial sulfate reduction (BSR), thermochemical sulfate reduction (TSR), thermal hydrolysis, hydrolysis of metal sulfides and desorption of H2S from formation sediments. References in this matter include Hunt1996; Krouse,. & Grinenko.; 1988 Krouse, 1977; Ligthelm, et al., 1991 Machel, 1987; Machel 2001; Machel, et al. 1995; Machel &.Foght, 2000; Orr, 1977; Thode, 1981; Thode, 1981; Worden et al., 1995 Worden et al. 1995.

By mid 2000 H2S content in gas from the Chihuido Lomitas field separator started to increase significantly up to an average of 2000 ppm. The reservoirs are located in the Neuquen Basin, the main oil-gas producer region of Argentina. The producer reservoirs are in the Mendoza Group (Late Jurassic - Early Cretaceous), including de Agrio Formation and Avile member within this unit.

A multidisciplinary team was formed by Engineering staff of Rincon de los Sauces and Repsol- YPF Applied Technology Center to address the problem using different approaches. While measurements were thoroughly checked and a sampling survey at key points was designed, different alternatives were discussed to treat the gas by traditional methods. However, the problem was deemed to be very likely due to bacterial activity in the reservoir.

It was concluded that using sulphur isotope measurement techniques was one of the few tools available to effectively determine the cause of the problem.

METHODS

The collection of H2S was carried out by a sampling device developed at INGEIS laboratories, which includes a flow-meter and two H2S traps with two on line flasks containing AgNO3 or NaOH. The devices, called MAX 001-4, were checked in laboratory previous to the field work.

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