Gas Generation and Composition from Cracking of Olefin Base Fluids in a Closed System Pyrolysis

Olefin-based drilling fluids are widely used due to their thermal performance, lubricity, and operational stability. However, under thermal stress, these base fluids may undergo cracking and generate gases with distinct compositional signatures, potentially affecting the geochemical interpretation of gases detected during drilling. This is particularly relevant in drill bit metamorphism (DBM), where localized thermal stress associated with high mechanical energy input during drilling may generate thermogenic products unrelated to the geological formation. This study compared the thermal behavior of three olefin-based drilling fluids, two predominantly composed of C15 olefins and one characterized by a broader C16–C19 distribution. The fluids were subjected to anhydrous pyrolysis using high-pressure, high-temperature (HPHT) reactors at 350 and 550 °C for 72 h. Generated gas mass was determined gravimetrically, and gas conversion was calculated relative to the initial fluid mass. Gases were characterized by GC-FID for light hydrocarbons. Increasing temperature markedly enhanced gas conversion in all fluids. For the two C15-dominated fluids, conversion increased from 9.30 to 50.60% and from 15.41 to 32.47%, respectively, whereas the C16–C19 fluid increased from 8.50 to 46.38%. At 350 °C, all fluids generated wet hydrocarbon gases enriched in C2–C3 compounds, including ethene and propene, consistent with hydrocarbon species reported in association with thermal alteration of drilling fluids during DBM. Considering the normalized C1–C4 hydrocarbon fraction, methane represented 26.06, 29.41, and 25.22%, while the wet gas fraction (C2–C4) represented 73.94, 70.59, and 74.78%, respectively. At 550 °C, methane increased to 77.26, 76.21, and 86.29%, while the wet gas fraction decreased to 22.74, 23.79, and 13.71%. This transition was accompanied by strong depletion of C3–C4 hydrocarbons and of ethene and propene, indicating progressive secondary cracking toward methane-rich gas. These results demonstrate that increasing thermal severity enhances gas conversion and produces a pronounced compositional evolution from C2+-rich wet gases toward methane-dominated dry gases. The distinct responses among the fluids, including differences between the two C15-dominated formulations, indicate that nominal carbon-number distribution alone does not determine thermal cracking behavior. These findings provide experimental constraints for understanding the thermal cracking of olefin-based drilling fluids and evaluating their potential contribution to gas signatures detected during drilling.