A Stable Isotope Ranking Method Useful for Addressing Bradenhead Gas Sources in Legacy Wells, SW Weld County Colorado

In advance of directional well development within the Niobrara shale gas resource play, operators are systematically plugging and abandoning vertical legacy wells. Many of these vertical wells have uncemented annular spaces above the Niobrara Formation that can accumulate gases from non-commercial Pierre Shale sand horizons (e.g. Sharon Springs and Sussex Sands). These gases are compositionally distinct from underlying produced gases. Either or both can migrate into bradenheads.

Results of dual inlet mass spectrometer stable isotope analyses show that, for any given stable isotope parameter, both the mean absolute value (MAV) of the difference between sample pairs and the standard deviation (SD) of those differences are similar regardless of gas sample source (lab standard or consecutive production gas samples). This allows statistical ranking of the difference between bradenhead (BHD) and produced (PRD) gas sample pairs from any single well. Each of 8 possible stable isotope parameters in any sample is scored with a value of 1 when |δ_BHD-δ_PRD| is less than MAV+3*SD. Totaling the score (maximum of 8 parameters in the methane through pentane range) helps engineers address the source of bradenhead gas.