ABSTRACT

A low enthalpy geothermal system consisting of a water production and injection well faced serious injection obstruction problems within two-and-a-half months after start-up. The obstruction was so severe that the operation was suspended and research was done in order to determine the cause. To solve the obstruction problem the system was treated downhole using acid and biocide. During this treatment dangerous amounts of H2S were released. A study was initiated to understand the origin of the H2S and to give options for microbiological treatment and growth prevention. The results showed high amount of bacteria in the obstruction material indicating that microorganisms caused the obstruction. The detected species were typical thermophilic species with optimal growth temperature at 40-60 °C. During the 2,5 month period the casing was unprotected in high corrosive environment and bacteria and iron oxides, iron sulphides (FeS) and iron-copper sulphides (FeCuS2) were identified suspected to have caused the obstruction. The H2S formation was likely caused by a combination of biological FeS and CuFeS2 formation (due to Microbiologically Influenced Corrosion; MIC) and the release of H2S by the addition of acid during cleaning. To prevent this type of issues biocide treatment was applied, however, further investigation showed that microorganisms were still able to grow in the system. Identification of the microorganisms in the production water showed that several MIC related species are already present in the production water. Continuous monitoring and adjustment of kill strategies are essential for safe operation.

INTRODUCTION

A geothermic system in the Netherlands faced serious injection obstruction problems two-and-a-half months after start-up. The obstruction was so severe that the operation was suspended and research was done in order to determine the cause. The conclusion was that the obstruction was caused by a combination of slime forming bacteria and iron and copper oxides/sulphides (possibly formed by bacteria). To solve the obstruction problem the system was treated downhole using acid and biocide. During this treatment dangerous amounts of H2S were released. The hypothesis is that H2S was formed by the activity of (sulphate reducing) bacteria and/or by the chemical reaction during the acid treatment. Due to QHSE (Quality, Health, Safety and Environment) regulations and lack of knowledge about what has happened in this system, there is the need to understand which processes have taken place and what can be expected in the future with regard to obstruction issues, H2S formation and Microbiologically Influenced Corrosion (MIC). To understand the issues better, the microbiology in the injection well was identified and interesting bacterial targets were further used to quantify its activity. To prevent microbiology related issue in the future, biocide was dosed once monthly in the injection well with the aim to prevent microbiological growth. However, after several months the injection pressure started increasing again slowly. Although the injection pressure was well within the production requirements, after more than two years of operation, additional tests were done and further research was performed to understand the reason for this increase in pressure and to get a better understanding and control of risks related to microbiology and MIC threat based on the detected microbiology. This report described the results of this study.

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