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Understanding Controls on EUR in the Haynesville Shale Gas Play: It's All About the In-situ Density and Pressure of Methane Gas


Despite comparatively uniform petrophysical and geomechanical rock properties, significant variations in Estimated Ultimate Recovery (EUR) are observed in many wells in the Haynesville shale gas play, NW Louisiana. Some of the differences may be attributable to differences in completion practices and Sw. However, these differences do not fully explain the observed disparities in EURs. Using micro Laser Raman Spectroscopy (mLRS), we show that the observed differences in EUR are likely due to differences in the in situ density and pressure of methane gas. Analyses of fluid inclusions trapped in bedding-parallel calcite veins suggest that early generated liquid hydrocarbons were selectively destroyed through Thermochemical Sulfate Reduction (TSR), due to an influx of high temperature (~120°C to 185°C), highly saline (~20 to 25 wt. % equivalent NaCl) Ca+Mg-rich sulfate brines into the Haynesville source rock shale during burial and thermal maturation. δ34S of H2S Sulfur in Haynesville gas is the same as the δ34S of Sulfur in anhydrite from the underlying Jurassic-aged Werner anhydrite. Integrating results of fluid inclusion thermometry with a calibrated burial history model, we show that the TSR process was associated with igneous activity of Cretaceous age (80 to 110 MYr). This igneous activity is believed to be responsible for the over-mature nature of the Haynesville source rocks and anomalous present-day high heat flow. The source of high gas saturations (Sg ~ 0.9) and geopressures (0.92 psi/ft pressure gradient) is attributed to the generation of methane gas due to late stage oil-to-gas cracking (OTGC). The destruction of liquid HCs due to TSR may explain both the presence of H2S gas in the Haynesville gas shale and differences in observed EUR, since TSR would reduce the amount of methane produced by OTGC, and should be reflected in the density and pressure of methane gas trapped in FIs. In situ methane densities were estimated from measured Raman peak shifts of methane in FIs. In situ methane pressures were estimated using an EoS for CH4-CO2 mixtures saturated with water vapor, and were obtained by iterative simulation until the measured vapor density at 25°C was matched at the fluid inclusion formation temperatures. Our mLRS data show that significant differences do exist in the in situ density of methane, supporting our hypothesis that observed differences in EUR for in different parts of the Haynesville shale are due to the lower in situ methane pressure.