ALBERT

Description: This study presents a workflow that combines an isotropic and an anisotropic effective medium model with a grid-search method to invert for the reservoir properties (porosity, composition and pore shape) of the Haynesville Shale. The reservoir properties inverted from this workflow closely matched the observed data, and they provide very useful information in determining locations with relatively high porosities and relatively large fractions of brittle components favourable for hydraulic fracturing. The isotropic effective medium model represents a complex medium as a single homogeneous medium by including grains and pores of different shapes and sizes. The anisotropic effective medium model introduces vertical transversely isotropic media through aligned fractures. After building the relationships between the reservoir properties and P - and S -wave velocities, we used grid searching to obtain porosity, composition and pore shape distributions conditioned by the rock-physics models. The modelled seismic velocities that satisfied criteria from objective functions provided estimated reservoir properties. The porosity and composition estimations at the well location matched the observations from log and core data quite well. The pore shape estimation suggested that the pores, cracks and fractures within the Haynesville Shale have elongated shapes. Future application of this workflow at the seismic scale will provide continuous spatial distributions of these reservoir properties.

Description: Most subsurface formations of value to exploration contain a heterogeneous fluid-filled pore space, where local fluid-pressure effects can significantly change the velocities of passing seismic waves. To better understand the effect of these local pressure gradients on borehole wave propagation, we combined Chapman’s squirt-flow model with Biot’s poroelastic theory. We applied the unified theory to a slow and fast formation with permeable borehole walls containing different quantities of compliant pores. These results are compared with those for a formation with no soft pores. The discrete wavenumber summation method with a monopole point source generates the wavefields consisting of the P-, S-, leaky-P, Stoneley, and pseudo-Rayleigh waves. The resulting synthetic wave modes are processed using a weighted spectral semblance (WSS) algorithm. We found that the resulting WSS dispersion curves closely matched the analytical expressions for the formation compressional velocity and solutions to the period equation for dispersion for the P-wave, Stoneley-wave, and pseudo-Rayleigh wave phase velocities in the slow and fast formations. The WSS applied to the S-wave part of the waveforms, however, did not correlate as well with its respective analytical expression for formation S-wave velocity, most likely due to interference of the pseudo-Rayleigh wave. To separate changes in formation P- and S-wave velocities versus fluid-flow effects on the Stoneley-wave mode, we computed the slow-P wave dispersion for the same formations. We found that fluid-saturated soft pores significantly affected the P- and S-wave effective formation velocities, whereas the slow-P wave velocity was rather insensitive to the compliant pores. Thus, the large phase-velocity effect on the Stoneley wave mode was mainly due to changes in effective formation P- and S-wave velocities and not to additional fluid mobility.

Description: Modeling the elastic properties of clay-bearing rocks (shales) requires thorough knowledge of the mineral constituents, their elastic properties, pore space microstructure, and orientations of clay platelets. Information about these variables and their complex interrelationships is rarely available for real rocks. We theoretically modeled the elastic properties of synthetic clay-water composites compacted in the laboratory, including estimates of pore space topology and percolation behavior. The mineralogy of the samples was known exactly, and the focus was on two monomineralic samples comprised of kaolinite and smectite. We used differential effective medium theory (DEM) and analysis of scanning electron microscope (SEM) images of the compacted kaolinite and smectite samples. Percolation behavior was included through calculations of critical porosities from measurements of the liquid limits of the individual clay powders. Quantitative analysis of the SEM images showed that the large scale ( $$ 〉 0.1\hbox{ \hspace{0.17em} }\hbox{ \hspace{0.17em} }\mathrm{\mu m}$$ ) pore space of the smectite composite had more rounded pores (mean aspect ratio $$\alpha =0.55$$ ) than the kaolinite composite (mean pore’s aspect ratio $$\alpha =0.44$$ ). However, models that used only these large-scale pore shapes could not explain the compressional and shear velocity measurements. DEM simulations with a single pore aspect ratio showed that bulk and shear moduli are controlled by different pore shapes. Conversely, modeling results that combined critical porosity and dual porosity models into DEM theory compared well with the measured bulk and shear moduli of compacting kaolinite and smectite composites. The methods and results we used could be used to model unconsolidated clay-bearing rocks of more complex mineralogy.

Description: Ultrashallow seismic-reflection data were collected at a test site in Great Bend, Kansas. The purpose of the experiment was to image seasonal submeter-scale fluctuations in the water table over a period of one year to identify the factors important in monitoring the water table when using seismic-reflection techniques. The study indicates that detailed velocity information must be used when interpreting water-table levels. Using detailed velocity information as a control when depth-converting the seismic profiles yielded correct positioning of the water table within + or -12 cm at the test site.