Publications

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Natural gas is a clean fuel that will be the most important domestic energy resource for the first half the 21st centtuy. Ensuring a stable supply is essential for our national energy security. The research we have undertaken will maximize the extractable volume of gas while minimizing the environmental impact of surface disturbances associated with drilling and production. This report describes a methodology for comprehensive evaluation and modeling of the total gas system within a basin focusing on problematic horizontal fluid flow variability. This has been accomplished through extensive use of geophysical, core (rock sample) and outcrop data to interpret and predict directional flow and production trends. Side benefits include reduced environmental impact of drilling due to reduced number of required wells for resource extraction. These results have been accomplished through a cooperative and integrated systems approach involving industry, government, academia and a multi-organizational team within Sandia National Laboratories. Industry has provided essential in-kind support to this project in the forms of extensive core data, production data, maps, seismic data, production analyses, engineering studies, plus equipment and staff for obtaining geophysical data. This approach provides innovative ideas and technologies to bring new resources to market and to reduce the overall environmental impact of drilling. More importantly, the products of this research are not be location specific but can be extended to other areas of gas production throughout the Rocky Mountain area. Thus this project is designed to solve problems associated with natural gas production at developing sites, or at old sites under redevelopment.

Natural fractures in Jurassic through Tertiary rock units of the Raton Basin locally contain conjugate shear fractures that are mechanically compatible with associated extension fractures, i.e., they have a bisector to the acute angle that is parallel to the strike of associated extension fractures, normal to the thrust front at the western margin of the basin. Both sets of fractures are therefore interpreted to have formed during Laramide-age thrusting from west to east that formed the Sangre de Cristo Mountains and subsequently the foreland Raton Basin, and that imposed strong east-west compressive stresses onto the strata filling the basin. This pattern is not universal, however. Anomalous NNE-SSW striking fractures locally dominate strata close to the thrust front, and fracture patterns are irregular in strata associated with anticlinal structures within the basin. Of special interest are strike-slip style conjugate shear fractures within Dakota Sandstone outcrops 60 miles to the east of the thrust front. Mohr-Coulomb failure diagrams are utilized to describe how these formed as well as how two distinctly different types of fractures can be formed in the same basin under the same regional tectonic setting and at the same time. The primary controls in this interpretation are simply the mechanical properties of the specific rock units and the depth of burial rather than significant changes in the applied stress.

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Recent studies have observed compaction zones pass through porous rock under axisymmetric compression. An initially thin, compacted layer appears at the yield point of the stress-strain curve and then grows by thickening in the direction of maximum compression at constant stress. Strain localization theory has been applied to compaction to explain the formation of these features. This paper describes the growth of the compaction zones, that is, the propagation of their boundaries, in terms of shock wave analysis. The ratio of the applied shortening rate to the velocity of the boundary is related to the porosity change across the boundary. Certain features of the stress-strain curve are explained by the model.

Compaction bands are thin, tabular zones of grain breakage and reduced porosity that are found in sandstones. These structures may form due to tectonic stresses or as a result of local stresses induced during production of fluids from wells, resulting in barriers to fluid (oil, gas, water) movement in sandstone reservoirs. To gain insight into the formation of compaction bands the authors have produced them in the laboratory. Acoustic emission locations were used to define and track the thickness of compaction bands throughout the stress history during axisymmetric compression experiments. Narrow zones of intense acoustic emission, demarcating the boundaries between the uncompacted and compacted regions were found to develop. Unexpectedly, these boundaries moved at velocities related to the fractional porosity reduction across the boundary and to the imposed specimen compression stress. This appears to be a previously unrecognized, fundamental mode of deformation of a porous, granular material subjected to compressive loading with significant implications for the production of hydrocarbons.

Energy production, deformation, and fluid transport in reservoirs are linked closely. Recent field, laboratory, and theoretical studies suggest that, under certain stress conditions, compaction of porous rocks may be accommodated by narrow zones of localized compressive deformation oriented perpendicular to the maximum compressive stress. Triaxial compression experiments were performed on Castlegate, an analogue reservoir sandstone, that included acoustic emission detection and location. Initially, acoustic emissions were focused in horizontal bands that initiated at the sample ends (perpendicular to the maximum compressive stress), but with continued loading progressed axially towards the center. This paper describes microscopy studies that were performed to elucidate the micromechanics of compaction during the experiments. The microscopy revealed that compaction of this weakly-cemented sandstone proceeded in two phases: an initial stage of porosity decrease accomplished by breakage of grain contacts and grain rotation, and a second stage of further reduction accommodated by intense grain breakage and rotation.