Knowing the H2S concentration early in the exploration and appraisal phase of a potential hydrocarbon discovery is critical for proper planning of the development phase. Production from a newly discovered accumulation often requires tie-in to an existing pipeline or facility, which usually has an upper limit on how much H2S can be handled, if any. The presence of H2S also affects the design of the completion and the production systems. Not discovering H2S until the production phase can halt production while the surface facilities are upgraded to handle the sour gas, which can be very expensive.
Several publications have shown that using standard wireline formation testing (WFT) to obtain fluid samples without adequate planning and preparation may result in H2S being scavenged during sampling or during transfer. In addition, standard gas chromatography (GC) analysis may measure small H2S concentrations inaccurately. Alternative methods for sampling and conserving small H2S concentrations have been proposed, but they are supported only by laboratory tests with no actual field data published.
In this paper, we present case studies of applying low H2S concentration sampling in the field. The wireline formation sampling was designed to capture samples with very small amounts of H2S under field conditions. This includes selection of the materials used in the sampling tool, tool string setup, choice of coating material for the sampling bottles, transfer of samples and analysis. The flow path inside the sampling tool was reduced to minimize the potential loss of H2S. In addition to wireline formation testing, the wells were also tested using a traditional drillstem test (DST). The results of the well test and the wireline sampling agreed well in both cases.