Reservoir and Production Characteristics of Composite Coal/Sandstone Gas Reservoirs

Abstract

ABSTRACT Exploration for deep Atoka gas reservoirs during the past 10 years in the Arkoma basin of Oklahoma has provided opportunities to obtain reservoir and production data from shallow. Pennsylvanian age, Hartshorne composite coal/sandstone gas reservoirs. The Hartshorne formation by itself is not normally considered to be a potential secondary target if gas shows are evident and the deeper zone is non-productive. Independent operators, however, have been completing shallow wells (<2000 feet) in the Hartshorne formation for over sixty-five years. Four hydraulically stimulated wells, each having eight years of Hartshorne production history, were chosen to be analyzed. All four of the gas wells are situated on the northern flank of the Kiowa syncline, Pittsburg County, Oklahoma and are located within three miles of each other. The range in depth for the producing Hartshorne intervals in these wells is from about 3000 feet to 4200 feet. Geophysical well logs, seismic data, sample cutting and core descriptions, gas composition data, well test information and production decline curve analyses were used to characterize the reservoir geology and to estimate the reserves and recovery of these composite reservoirs. Reserve estimates ranged from .04 BCF to 0.5 BCF per well. Well data suggests that in-fill drilling between these four wells has the potential of recovering more gas. Analysis of the reservoir geology indicates a deltaic-fluvial depositional environment that has several lateral discontinuities in the gaseous Hartshorne coalbed. The geologic data shows that the coalbed can split into as many as three identifiable seams in some parts of the syncline.