Carbon and Noble Gas Isotope Banks in Two-Phase Flow: Changes in Gas Composition During Migration

Abstract

A dramatic expansion of natural gas exploration and extraction in unconventional reserves is underway. However, there is public concern that hydraulic fracturing will also cause natural gas, reservoir brines and associated fracturing fluids to contaminate shallower groundwater reservoirs. Considerable scientific research is currently focused on attributing methane found in shallow groundwater sources to either thermogenic or low temperature bacterial sources. Attribution techniques use concentration ratios of methane, ethane and propane and their stable carbon and hydrogen isotope ratios, as well the abundance of atmospheric and crustal derived noble gases. These distinct properties can be used to differentiate bacterial and thermogenic methane assuming negligible change in composition and stable isotope ratios during transport. We use experimental results, theoretical models, and existing field data to determine whether hydrocarbon gas will show any appreciable change while migrating a distance greater than 1km. Theoretical two-phase gas displacement models predict that methane will become enriched at the front of a migrating gas plume due to mixing with dissolved biogenic methane in shallow groundwater.. Propane will dissolve more readily into the subsurface brines as the plume rises, leaving the final gas plume in the shallow groundwater heavily enriched in methane. We show that a mixture of a thermogenic gas plume with a small, dissolved biogenic methane supply in the groundwater will cause significant isotopic changes in the gas plume. Furthermore, atmospheric derived noble gases will be swept ahead of the methane pulse, leaving the main gas plume depleted of atmospheric gas components. We present results of experiments investigating these processes. All experiments used a 1m long, sand-packed steel column saturated with water containing dissolved noble gases. We then displaced the water by injecting methane, and measured the composition and carbon isotope ratio of the effluent gas. In this series of ongoing experiments, we are able to test both theory and field observations. Preliminary experimental results agree with theory and field observations, and show that dissolved gases and high volatility gases present in the injection gas are enriched in banks at the front of the displacement. These enrichment processes can be used to aid source identification of both fugitive gas plumes and migration of natural gas from source to reservoirs.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015