H₂S Detection While Drilling: A New Approach

The presence of contaminant gases in new discoveries is becoming more and more common and among these gases hydrogen sulfide (H₂S) is the most frequent. H₂S detection, together with the identification of formations and well intervals bearing it, is crucial for Oil Companies, since it heavily impacts completion strategies and field development. For many years, inorganic scavengers were used and through mud acidification it was possible to release H₂S and to get an evaluation of its concentration. Nowadays, the use of organic scavengers which bound irreversibly to H₂S, makes impossible its detection and hinders all the information regarding well sourness. In the current paper we present a novel methodology which aims to overcome this problem by core/cutting analyses. Methodology development started from the hypothesis that part of the formation gas is still preserved in cutting pores. Experimental evidences, collected in the last few decades, show that gas (hydrocarbons and contaminant gases, including H₂S) is mostly released by the rocks while drilling into mud (so-called mud gas); nevertheless, some further gas can be released by cuttings e.g. in sealed vials (so-called head space gas) and, finally, a last portion is still preserved in cuttings. Our experimental approach implies the analysis of this last portion, also called interstitial or residual gas, that can be recovered by grinding the cuttings in a sealed mill, gas recovery from the gastight chamber and its analysis by GC-TCD detector. It must be highlighted that the obtained H₂S concentration value is not only depending on its abundance but also on many other concurring factors which are difficult to precisely evaluate and quantify, such as rock permeability, sample preservation and treatment and rock-gas interactions. Therefore, the detected concentrations cannot be interpreted on a strict quantitative basis, but they still give us an important indication of H₂S presence and of its relative abundance. Analyses of samples of ditch cuttings, stored for years, pointed out that this methodology is applicable not only for fresh samples. Case histories will support the presented methodology and shed light about the potentialities of such analysis kind.