Monitoring production in unconventional reservoirs is crucial for assessing change from stimulated rock volume to drained rock volume over time. While reservoir engineering data has traditionally been the cornerstone for such analysis, this study posits that the geochemistry of produced fluids, particularly molecular geochemistry, offers vital insights. Focusing on diamondoids, a distinct class of ultra-stable hydrocarbons, this research aims to utilize their unique characteristics for monitoring production in reservoirs with relatively high thermal maturity. The study proposes that changes in diamondoid composition over time can be effectively correlated with the source-rock diamondoid fingerprint, offering a novel method to trace and interpret production zone variations through time.
This study employed time-series production monitoring in an unconventional muddy carbonate reservoir. Over a period of 22 months, five time-lapse fluid samples were systematically collected from the separator during the flowback phase. In the laboratory, these samples underwent geochemical analysis including sample preparation and fractionation was carried out using liquid chromatography. The targeted analysis of diamondoid compounds was then conducted using the advanced technique of gas chromatography coupled with triple quadrupole mass spectrometry. This sophisticated approach allowed for precise detection and quantification of diamondoids. Finally, the diamondoid data was integrated with additional geological and engineering data to enable comprehensive interpretation and analysis.
The analysis focused on a range of diamondoids, including lower diamondoids such as adamantane, diamantane, and trimantane. Across the fluid samples, a consistent decreasing trend was observed in their concentrations. Notably, the ratio of trimantane to methyldiamantanes demonstrated a decreasing pattern over time, punctuated by some anomalies. This trend in the lower diamondoids was complemented by intriguing patterns in higher diamondoids like triamantanes, tetramantanes, pentamantanes, and hexamantane. These higher diamondoids revealed an interesting overlap across the time-series samples, particularly between the samples from Time 3 and Time 5, each displaying a distinctive fingerprint.
The time-lapse analysis of diamondoids in unconventional reservoirs represents a significant advancement in reservoir characterization and monitoring. The ultra-stability of diamondoids, which resist alterations even under high thermal maturation, positions them as an ideal geochemical tool for tracking molecular changes in produced fluids. This approach is particularly effective in highlighting reservoir zones where there are abrupt changes in the drained rock volume. Such insights are crucial for optimizing production, as they offer a clearer understanding of fluid dynamics post-stimulation.