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Collaborating Authors
Texas
Summary Rock brittleness plays a significant role in effective hydraulic fracturing for shale gas production, and is often related to mineralogy, mechanical properties, and microstructure features in shales. We construct a rock physics workflow to link elastic properties of shales to complex constituents and specific microstructure attributes. Multiple compositions and various pore geometries are considered using a self-consistent approximation (SCA) method. The laminated textures due to the preferred orientations of clay particles and possible laminated distribution of kerogen are considered using Backus averaging to model the anisotropy (transverse isotropy) of shales. Results based on the analysis of the rock physics templates reveal that the degree of clay lamination significantly affects Vp/Vs of shales, whereas it has little impact on acoustic impedance of shales along the vertical direction. An increasing degree of clay lamination will increase Vp/Vs, and therefore the Poisson’s ratio. With increasing porosity, the variation of mineralogy has less impact on acoustic impedance than on Vp/Vs, which illustrates that Vp/Vs is a better indicator for lithology detection. On the other hand, acoustic impedance is a more suitable parameter to discriminate porosity compared with Vp/Vs. Our rock physics model is calibrated on the well log data from the Barnett Shale and is used to find reasonable parameters to characterize the Barnett Shale. Based on the model, we generate rock physics templates for the interpretation and prediction of shale rock brittleness, mineral constituents, and porosity from elastic properties of shales. Seismic AVO analysis based on modeling data from the top and base of the Barnett Shale illustrates that AVO intercept and gradient have predictable trends according to the variation of brittleness index, mineralogy, and porosity, which means that we can predict variations of such factors in space from seismic responses.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- North America > United States > Texas > Haynesville Shale Formation (0.99)
- North America > United States > Texas > Fort Worth Basin > Barnett Shale Formation (0.99)
- North America > United States > Louisiana > Haynesville Shale Formation (0.99)
- North America > United States > Arkansas > Haynesville Shale Formation (0.99)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Shale gas (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
Summary The effectiveness of fracture stimulation techniques depends on the microstructural features which control the rock strength. We analyze brittleness index, fractures, and microstructure of the Barnett Shale for a better understanding of the correlation between mechanical properties, mineralogy, and pore geometry. The complexity of multiple minerals, pore geometries, and pore inclusions are modeled using the self-consistent approximation (SCA) model, with consideration of statistical distributions of pores and cracks in shales. The method is applied to core samples and well log data from the Barnett Shale to invert the aspect ratio (ratio of short axis to long axis) of pores, and to estimate crack density, and the proportion of stiff pores and cracks in the Barnett Shale. The inverted crack density gives an average estimate of the pore space geometry. Results show that the aspect ratio for the Barnett Shale varies between 0.01 and 1 and has a dominant value of 0.1. Analysis on the core data indicates that both quartz and carbonate minerals contribute to increased crack density. Comparison reveals good correlations between the brittleness indices defined in terms of ?+2µ)/? and Poisson’s ratio. While the brittleness index defined by Young’s modulus is not consistent with the definition in terms of (?+2µ)/? and Poisson’s ratio, Young’s modulus is a good indicator for the variation of crack density. Results of pore-type inversion show that the variation of pore types coincides with inverted crack density.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Petroleum Play Type > Unconventional Play > Shale Play > Shale Gas Play (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- North America > United States > Texas > Haynesville Shale Formation (0.99)
- North America > United States > Texas > Fort Worth Basin > Barnett Shale Formation (0.99)
- North America > United States > Louisiana > Haynesville Shale Formation (0.99)
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