Variations of Carbon Isotopes during Shale Gas Production from the Horn River Basin, British Columbia, Canada

Abstract:

Chemical and stable isotope compositions of natural gases are key parameters for characterizing gas and hydrocarbon reservoirs. Produced gases were obtained from eight wells at multi-well pad sites located in the Horn River Basin (HRB), NE British Columbia. Shale gas wells were drilled and completed in the Devonian Muskwa, Otter Park and Evie Formations of the HRB, and gases collected as time series over short term (~50 days) and long term periods (~ 1250 days).

δ¹³C of gases from HRB formations confirm high thermal maturity and the shale gases frequently showed partial or full isotope reversals among hydrocarbon components. A 10‰ variation in δ¹³C values of methane was observed during production. In general, during early phases of production shale gases appear enriched in ¹²C compared to gases sampled at later stages and δ¹³C_methane values were approximately between -38‰ and -35‰ during times up to 50 days. The majority of cases of carbon isotope reversals between methane and ethane components of gases (δ¹³C_methane > δ¹³C_ethane) were observed at times greater than 100 days, while ethane and propane reversals were common throughout production.

Gas production rates differed significantly among the sampled wells from ~ 50 to 400 e³m³/d. Higher rates were frequently associated with gases showing ¹²C_methane enrichment. Subsequent to periods of well ‘shut in’ a change in the carbon isotope composition was detected with enrichment in ¹³C_methane of gases. Carbon isotope signatures of produced gases likely reflect a combination of both the in-situ shale gas isotope signature as well as effects of isotope fractionation which may occur during transport through pores and fractures of the shale.