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Abstract: Reservoir Heterogeneity as Measured by Production Characteristics


Well-production parameters, including peak-monthly production (PMP), peak-consecutive-twelve month production (PYP), and cumulative production (CP), are tested as tools to understand the heterogeneity of hydrocarbon reservoirs in older gas fields (fields with annual production less than 10% of CP). Variation coefficients (defined as VC= (F[5]-F[95]) / F[50], where F[5], F[95], and F[50] are the 5th, 95th, and 50th (median) fractiles of a sample distribution) are calculated for PMP, PYP, and CP and examined with respect to internal consistency, type of production parameter, conventional versus unconventional accumulations, and depth. Production parameters measure the net result of complex geologic, engineering, and economic processes. Our hypothesis is that such data carry information about subsurface heterogeneity that would be impossible to obtain using geologic tools with smaller measurement scales such as cores and well-logs.

Well-production data for this study were compiled for 15 gas fields in the Pennsylvanian Morrow Sandstone of the Anadarko basin, Oklahoma, one gas field each in the Cambrian-Ordovician Arbuckle, Ordovician Simpson, Pennsylvanian Atoka, and Devonian Hunton Groups of the Anadarko basin, and one gas field in Late Cretaceous reservoirs of north-central Montana. These data are supplemented by data for three areas of the Devonian-Mississippian Bakken Formation (unconventional) oil accumulation in the Williston basin, North Dakota and Montana.

Our analysis shows that: (1) Variation coefficients derived from CP of older wells are larger than those for PYP or PMP, implying that heterogeneity associated with reservoir volume accessed by wells is greater than heterogeneity associated with the plumbing system controlling flow rates to a well; (2) Of the fields studied, Bakken fracture-controlled shale reservoirs of the Williston basin are most homogeneous in terms of production characteristics and are thus the most predictable; (3) Depth exerts little or no control on heterogeneity for the fields in this study. Results such as these indicate that quantitative measures of production variability, expressed as dimensionless coefficients, are potentially valuable tools for documenting reservoir heterogeneity in older fields for field redevelopment and risk analysis.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado