Accurate Measurement of the Hydrogen Sulfide Content in Formation Fluid Samples—Case Studies

Abstract

Abstract The Hydrogen Sulfide (H2S) content of subsurface hydrocarbon reservoirs has a profound impact on completion, surface production, and process design. The H2S content of crude oil directly impacts the sale value of the produced hydrocarbons, while the H2S content of produced water can result in significant production problems, which can lead to detrimental effects on hydrocarbon transport and sales specifications. In theory, the H2S content of reservoir fluids can be determined from samples collected by wireline formation tester tools. In practice, however, measurement is complicated by the partial-to-total loss of the gas by absorption/adsorption onto the metal components of the downhole tool, storage bottles, and transfer/analysis system, particularly those made from iron. H2S readily forms non-volatile and insoluble metal sulphides by reaction with many other metals and metal oxides, and analysis of the fluid samples can therefore underestimate the true H2S content. Therefore, many practical challenges must be overcome to accurately determine H2S levels in formation tester samples. These include metallurgical design considerations, sampling environment and system selection as well as the timing and type of H2S analyses. This paper details our successes in accurately measuring both high and low concentrations of H2S in downhole formation tester samples. It describes an integrated workflow by which ultra low H2S concentrations can be captured and preserved in formation tester samples until the tool is brought back to surface where the samples can be analyzed. This workflow includes a number of modifications to some commercial formation tester tools and sample chambers to ensure that the bulk of the H2S does not get absorbed by the time the tool arrives to the surface. Downhole measurements made close to the fluid entry into the sampling tool are used to cross check and verify the surface measurements. Finally, on surface, a number of techniques are used to measure H2S concentration in the sample bottles as soon as practical after the formation tester tool is brought to the surface. The paper demonstrates, through a number of case studies, that by following this comprehensive workflow, it is possible to capture and analyze hydrocarbon samples with minimal loss of H2S, regardless of the levels of H2S being measured.