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The AAPG/Datapages Combined Publications Database
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Abstract
Cumella, S. P.,and J. Scheevel,
DOI:10.1306/13131054H33104
2008. The American Association of Petroleum Geologists. All rights reserved.
The Influence of Stratigraphy and Rock Mechanics on Mesaverde Gas Distribution, Piceance Basin, Colorado
Stephen P. Cumella,1 Jay Scheevel2
1Bill Barrett Corporation, Denver, Colorado, U.S.A.
2Scheevel Geo Technologies, Grand Junction, Colorado, U.S.A.
ACKNOWLEDGMENTS
Many thanks from S. Cumella to the geologists and engineers he has worked with at Barrett Resources, Williams, and Bill Barrett Corporation, especially Kurt Reinecke, Robert Mueller, Doug Ostby, Jon Cantwell, Terry Barrett, Taryn Frenzel, Chris Hansen, Mark Rothenberg, and Chad Odegard. The U.S. Geological Survey has done much fine work in the Piceance, and we have benefited from discussions with Ron Johnson, Mark Kirschbaum, and Ben Law. Keith Shanley and Paul Devine provided useful guidance on stratigraphic concepts, and Fred Meissner helped with concepts on overpressuring caused by hydrocarbon generation. Much of the supporting data for the rock mechanics component of this study were previously published or provided to Jay Scheevel directly by past workers on the MWX project. Their contributions are gratefully acknowledged, especially those of Norm Warpinski at Pinnacle Technologies, John Lorenz at Sandia Laboratory, Steve Laubach at the Texas Bureau of Economic Geology, and Larry Teufel at New Mexico Tech. The analytical solutions benefited in their early stages from discussions with Greg King with Chevron in Luanda, Angola. Phillip Nelson at the U.S. Geological Survey in Denver carefully reviewed an early manuscript outlining the elastic theory, and Jon Olsen of the University of Texas at Austin provided critical review and valuable suggestions that condensed and clarified the final work. John Robinson and Eric Nelson provided comments that substantially improved the chapter. We are grateful to Jane Rerecich for her help with the drafting of some of the figures. We thank Bill Barrett Corporation for its support for this publication.
ABSTRACT
A regionally extensive basin-centered gas accumulation in the Mesaverde Group of the Piceance Basin is currently being actively developed. Daily production has increased from less than 200 MMCFGD in the year 2000 to more than 1 BCFD currently. Most gas production in the Piceance Basin is from discontinuous fluvial sandstones of the Williams Fork Formation of the Mesaverde Group. In some areas of the southern Piceance Basin, 10-ac (4-ha) well density has proven successful. Estimated ultimate recoveries (EURs) of typical wells in these areas range from 1 to 2 bcf per well, resulting in reserves of about 60–120 bcf per section (1 mi2; 2.5 km2). The depth limits to the commercial gas accumulation are poorly defined, but it is possible that much of the deeper part of the basin may have commercial gas reserves. Within the area of commercial gas production, most gas is produced from a continuously gas-saturated interval in the Williams Fork. Productive intervals can attain gross thicknesses of more than 3000 ft (900 m). The gas-saturated interval thins toward the basin margins, where the Williams Fork gas reserves become subeconomic.
This tremendous gas resource exists because of several important geologic circumstances. Large volumes of gas were generated from thick Mesaverde coals as they achieved high thermal maturity. Migration
of this gas was inhibited by the very low permeability and discontinuous nature of the Mesaverde sandstone reservoirs. The rate at which gas was generated and accumulated in the reservoirs outpaced the rate at which gas could escape, resulting in overpressure. Eventually, the pressure of the gas phase in the pore system exceeded the capillary pressure of the water-wet pores, and water was expelled from the pore system, resulting in the development of an overpressured, gas-saturated reservoir with little movable water.
In addition, the overall distribution and pressure of the gas in the Williams Fork is probably the direct result of pore-pressure–assisted fracturing and subsequent migration
through the resulting natural fracture system. The orientations of the fracture populations are predetermined by the orientation of tectonic stresses at the time that the fractures formed, but the distribution and intensity of fracturing are mostly influenced by the history and magnitude of overpressuring during gas charging.
Fractures in the Mesaverde in the Piceance Basin are primarily opening-mode fractures (also called extension fractures or joints). Some of these fractures are found to have opened, then they have been cemented, and then they opened again. In some cases, this process occurred repeatedly. It is well known that open fractures are a significant factor in the movement of subsurface fluids. In-situ permeabilities of the Mesaverde from well flow testing are measured to be up to three orders of magnitude higher than the matrix permeabilities measured from core. Because most fractures in the Mesaverde of the Piceance Basin are extension fractures, our analysis emphasizes the impact of the magnitude of pore pressure on effective stress and strain boundary conditions and how these may control extension fracturing. We find that high pore pressure compresses and shrinks the individual sand grains uniformly in all directions (poroelastic effects). As pore pressure increases, the lateral normal stress decreases, until under some conditions of pore pressure, the rocks experience tensile effective stress and fracture. These conditions were common during maturation of the coal-bearing lower Williams Fork, where pressures were high enough to fracture most of the rock types. Some parts of this section remain highly overpressured to the present day. As pressures decrease upward, away from the coal-bearing section, only fracture-prone sandstone lithologies fractured, causing stratigraphy to be a more important factor in the upward and lateral migration
of gas.
This gas-migration
process is dominant below the top of continuous gas saturation. Above the top of continuous gas saturation is a transition zone that contains both gas- and water-bearing sandstones. Gas-charged sandstones in the transition zone were probably sourced by
migration
along major fault and fracture zones. The sandstones in the transition zone commonly have better porosity and permeability than those in the continuously gas-saturated interval. Conventional trapping is probably active in many transition-zone gas sandstones.
Significant erosion of overburden has occurred in the Piceance Basin after peak gas generation. Gas expansion during exhumation probably had a significant effect on gas saturations. Gas expansion is significant when the gas accumulation is tightly sealed, when the reservoir depth after exhumation is shallow (6000 ft;
1800 m). Gas expansion in discontinuous sandstone reservoirs has the potential to displace water into surrounding shales, resulting in reservoirs with no apparent downdip water.
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