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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 63 (1979)

Issue: 3. (March)

First Page: 414

Last Page: 414

Title: Natural Gas Stability in Deep Subsurface: ABSTRACT

Author(s): Colin Barker, Marwin K. Kemp

Article Type: Meeting abstract

Abstract:

The trend from oil to gas with increasing reservoir depth is well established, but depth limitations on the occurrence of natural gas (methane) have yet to be determined. The controlling factor at depth is the chemical stability of the methane and its reactions with water and the rock matrix. This problem has been studied using a computer program which calculates the equilibrium composition in a multiphase, multicomponent system that simulates the rock-water-gas combination in deep reservoir. The program accepts thermodynamic data, rock mineralogy, and gas-water ratios as input data. Within these constraints all possible combinations of compounds are considered and the equilibrium composition established using minimum free energy criteria.

Methane alone has considerable thermal stability but in the natural system it occurs in a water-wet environment that decreases its stability. At 30,000 ft (9,000 m) approximately 5% of the methane is destroyed, but for high geothermal gradients this can be as much as 40%. Reservoirs that contain methane derived from the breakdown of crude oil contain a carbonaceous residue. This residue can interact with the water in clastic reservoirs and produce increased amounts of carbon dioxide. However, in carbonate reservoirs (or clastic reservoirs with carbonate cement), methane is destroyed with increasing depth, although carbon dioxide remains a major component in the gas phase. Free sulfur and many sulfur compounds dramatically reduce the stability of methane and generate high concentration of hydrogen sulfide.

Computer calculations help define the role of rock composition in controlling the stability, and therefore the distribution, of natural gas in the deep subsurface. This understanding will become more important as the average well depth increases.

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Copyright 1997 American Association of Petroleum Geologists