STABLE ISOTOPES OF PRODUCTION GASES FROM HORIZONTAL WELLS OF THE MIDLAND BASIN, TX: PREDICTION OF GAS-OIL RATIO AND SETTING A BASELINE FOR THE FUTURE

Stable carbon isotopes (δ¹³C) have been used to characterize hydrocarbons for exploration, development and production since the 1960s, and have re-emerged as predictive tools with the development of unconventional “tight” oil and gas plays. In low-porosity unconventional systems, where organic-rich shales can act as both source and reservoir for hydrocarbon fluids and gases, carbon isotope fractionation through desorption may contribute to produced gas isotope variability throughout the lifecycle of an unconventional well. Here we show time-series data of gas composition (C1-C5, C6+, N₂, O2/Ar, CO₂, He, H₂) and carbon isotopes (δ¹³C of C1-C5) of five newly completed horizontal wells proximal to one another in the Midland Basin (Howard County, Texas). Wells were sampled for production gas at monthly intervals, for periods of five to nine months. Production data show periods of production decline were captured in the time series analysis. Gas composition was similar for the all wells. Gas wetness (%C2-C5) ranged from 26.3% to 34.4% and averaged 30%. Carbon isotope values showed little change throughout the study. An average of gases showed δ¹³C values (±2σ) of C1 = -53.0±0.4‰, C2 = -38.7±0.9‰, C3 = -32.6±0.8‰, iC4 = -33.0±1.0‰, nC4 = -30.1±1.0‰, iC5 = -28.4±0.7‰, and nC5 =-29.8±1.2‰. No trends between production declines and carbon isotope values were observed in any of the wells. These results contradict fractionation observed in desorption studies of core samples, suggesting a more uniform “free gas” model in the petroleum system. Production gas carbon isotopes did not covary with temporal gas-oil ratio (GOR) trends in any of the wells. The C1-C3 carbon isotopes of production gas and mud gas values for the study area do fall on cumulative GOR correlations developed in the Denver Basin, suggesting mud gas and production gas carbon stable isotopes may be a useful predictor for long-term production trends across multiple basins. Texas results will be presented in conjunction with additional data from the Appalachian and Denver Basin. Overall, the stability of mud gas and production gas carbon isotopes show value in setting baseline data for not only production and exploration applications, but also for future environmental forensics in cases of stray gas.