Gas Production from Non-Fractured Shale
David K. Davies, Richard K. Vessell, 2003. "Gas Production from Non-Fractured Shale", Siltstones, Mudstones and Shales: Depositional Processes and Characteristics, Erik D. Scott, Arnold H. Bouma, William R. Bryant
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The long-term production history of some offshore and onshore Gulf Coast reservoirs reveals that gas production exceeds assigned reserves for no readily apparent reason. The distinct possibility exists that surrounding shales contribute significant quantities of gas during the reservoir lifecycle. Direct evidence for economic gas production from non-fractured shale intervals comes from the Devonian of the Appalachian Basin and from Tertiary-Pleistocene reservoirs in the Gulf of Mexico. Intergranular pores occur between detrital clay particles in true shales of the Devonian interval. The ability of this intergranular pore system to transmit gas (permeability) is controlled primarily by the microfabric of the shale. It is reasonable, therefore, to expect that Gulf Coast shales with similar internal characteristics will yield sufficient gas to impact reservoir economics.
The Gulf Coast area contains significant proportions of sediments deposited in distal deltaic and deep water environments. These environments produce thick, fine grained “shale” intervals that, in reality contain numerous thin (<1 inch) laminations of porous and permeable silt and/or sand separated from one another by layers rich in clay minerals (true shales). Given a large number of silt/sand interbeds, sufficient permeability thickness can be developed in the interval to yield gas at high rates. Routine methods of log analysis fail to resolve the thin-bedding in these pay intervals, many of which are therefore bypassed.
Reserve calculations can also be significantly effected by gas production from true shales in traditional reservoir settings (such as the Wilcox Formation). The amount of gas recovered in reservoirs developed in relatively thin sand bodies (generally <50ft) can be increased by gas migration from surrounding shales during production-related pressure depletion of the main reservoir body. Improved reserve calculations require that potentially productive shales are included in all aspect of reservoir evaluation, from petrophysics to simulation.
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Siltstones, mudstones and shales have been studied mainly with regard to clay mineralogy and general transport-deposition. Recent studies on deepwater deposits from cores and outcrops have shown that fluid flow properties of deepwater reservoirs are greatly affected by the presence of finer-grained deposits in the reservoir. Initial analysis indicates that the majority of these finer grained deposits have a large silt component and are closer to siltstones rather than mudstones, commonly called shales To date, little attention has been given to their characteristics resulting from different depositional processes.