Gravitational and Matric/Capillary Potential Controls on Distributing Unsaturated Flow of Water in Fracture Rocks and Talus
Abstract
Unsaturated flow of water through rocks occurs in diverse settings including in naturally fractured rock formations, talus deposits along mountain slopes, and hydraulically fractured shale gas reservoirs. When the matrix permeability is very low, these different environments share characteristics that are important in determining how water percolates down fractures and pores between effectively impermeable rock blocks. Transmissivities of fractures and rock surfaces decline sharply when subjected to desaturation at low magnitude matric (capillary) potentials, and local hydraulic potential gradients are largely influenced by gravity. Over lengths associated with characteristic sizes of talus rocks and larger distances separating fracture intersections in rock masses, the dominance of gravitational potential gradients relative to capillary potential gradients forces drainage to follow distinct paths. Water draining from any arbitrary starting point within a fractured mass or talus deposit follows paths that progressively diverge with increased depth. In addition to this divergence of percolating pathways, a large fraction of deeper region fractures and pores are hydraulically inactive because they do not receive drainage from overlying rocks. In talus deposits, unsaturated flow evolves into distinct pathways such that flux densities become geometrically distributed with about half of the system becomes devoid of flow. In deep, hydraulically fractured, unconventional shale gas reservoirs, gravity facilitates water drainage from vertical fractures above horizontal wells while impeding flow from below the well axis.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H34F..01T
- Keywords:
-
- 1838 Infiltration;
- HYDROLOGYDE: 1847 Modeling;
- HYDROLOGYDE: 1865 Soils;
- HYDROLOGYDE: 1875 Vadose zone;
- HYDROLOGY