TOTAL NEUTRON SCATTERING EVALUATION OF METHANE BEHAVIOR IN THE NIOBRARA FORMATION AT WET-GAS THERMAL MATURITY CONDITIONS: PERSPECTIVES ON WELL PRESSURE MANAGEMENT

Within many unconventional source-rock reservoirs at intermediate thermal maturities [e.g., late oil (R_o ~0.9%) to wet gas windows (R_o~1.5%)] the distribution and behavior of methane (CH₄) and alkanes (C₂-C₅) associated with gas condensates in mesopores (diameters 2 to 50 nm) and micropores (diameters <2 nm) are unclear. However, this pore size range may represent a significant portion of available pore volume in the reservoir and are within the size regime where densification effects are known to occur. Hence, understanding petroleum compound (e.g., CH₄) distribution within these settings informs our understanding of resource generation, movement, and recoverability, and may aid in developing improved estimates of ultimate recovery. Here, perdeuterated methane (CD₄) behavior was investigated in a sample from the Late Cretaceous Niobrara Formation at the late oil/wet gas window using wide Q-range total neutron scattering at reservoir pressures (200 – 750 bar) and temperature (60°C). The Niobrara Formation is an active petroleum-producing formation within the Denver-Julesburg Basin, United States, and is currently producing ~650,000 barrels of oil per day. Neutron scattering data show that larger mesopores (5 to 25 nm) within the Niobrara Formation sample are generally accessible to CD₄, e.g., ~80% of 25 nm diameter pores are accessible. In contrast, scattering signals corresponding to small mesopores and all micropores (nominal pore diameters from 0.5 to 5 nm) fluctuate with increased CD₄ pressure, inhibiting an accessibility estimation. We interpret these signal fluctuations to signify the creation of organic porosity, likely through deformation of sample organic matter by supercritical CD₄, and/or the solvation of CD₄. CD₄-induced porosity represents a pore volume increase of at least ~8% within the sample. Additionally, densification of CD₄ within the sample is clearly indicated by a shift in the CD₄ intermolecular scattering peak to higher Q-values compared to bulk CD₄. These results provide insight into fluid properties within source-rock reservoirs at intermediate thermal maturity, especially as they relate to pore connectivity, and are discussed with regard to their application in the pressure management of gas condensate wells.