Digital Rock Physics in Four Dimensions: Simulating Geological Processes and Estimating the Response of Geophysical Properties
Abstract
Understanding the relationship between geophysical properties (e.g., seismic velocity and electrical resistivity) and porosity is fundamental to many rock physics models. However, the geological processes that dominate the formation of porosity, such as cementation and dissolution, will often occur over very long timescales, making the experimental calibration of velocity-porosity trends challenging. Simulating such geological processes in 3D digital rocks and estimating elastic properties from the 3D volumes allows for velocity-porosity trends to be characterized without the long times required for laboratory experiments.
Here we simulate deposition of two carbonate clastic rocks, grainstone (near spherical grains) and coquina (shelly fragments), then simulate both cementation and dissolution. These simulations output a set of 3D volumes representing rocks of varying porosity with known mineral and grain phases. Using the spatial phase information, combined with known velocity and densities of the relevant phase properties (we assume all mineral grains to be calcite, and porosity is fully saturated with fresh water) we create velocity and density models corresponding to each stage of cementation and dissolution. We then estimate seismic velocity from simulated wavefield propagation through each medium using the 3D staggered-grid finite difference method. We use these estimated velocity-porosity trends to test the elastic model of Cilli & Chapman (2018), which extends differential effective medium theory with the claim that a rock's effective pore aspect ratio changes by power law with porosity. Inverting our digital elastic measurements for effective pore aspect ratio, we find our modelled rocks do follow this power-law relationship. This validates the new rock physics model. Moreover, we see different effective pore aspect ratio-porosity trends for different rock types. This discovery paves the way to use the new rock physics model to link observed changes in effective pore aspect ratio to changes in porosity due to a wider range of geological processes, for example fracturing or compaction. Cilli, P., & Chapman, M. (2018, June). Modelling the Elastic and Electrical Properties of Rocks with Complex Pore Geometries. Paper presented at 80th EAGE Conference and Exhibition, Copenhagen, Denmark.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR33B0100C
- Keywords:
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- 3909 Elasticity and anelasticity;
- MINERAL PHYSICSDE: 3919 Equations of state;
- MINERAL PHYSICSDE: 7299 General or miscellaneous;
- SEISMOLOGYDE: 8124 Earth's interior: composition and state;
- TECTONOPHYSICS