Freshwater Two Miles Down
Conventional wisdom tells us that the salinity of pore water increases with depth, becoming fresh to brackish to multiple times the salinity of seawater. The lack of flow-inducing hydraulic gradient and the generally low permeability, all leading to long residence times, conspire to enhance rock water interactions and increase the total dissolved solids.
There are major exceptions to this rule. For example, when particularly high permeability pathways associated with favorable hydraulic gradients push meteoric water deep into the surface. Such examples have been observed in the Central Valley of California and in the Gulf Coast. However, the recent development of oil and gas production from unconventional reservoirs has brought to the fore a different type of deep brackish water with practical implications.
The source rocks targeted for unconventional production contain a significant fraction of fine-grained material, some of which is made of clay minerals. Clay minerals, like all minerals, have a range of stability in the subsurface. Smectite clays, which are often the result of weathering and other shallow subsurface processes, contain a significant amount of structural water. This water is released as the source rock is buried and the smectite converted into more stable illite clays. This mechanism has been known to exist in the Gulf Coast, but took on new relevance when oil and gas operators started production in the Eagle Ford Shale in South Texas and the Wolfcamp Shale in West Texas, where barely brackish water has been brought to the surface in large quantities.
This brackish produced water can be recycled directly for new hydraulic fracturing operations or can help in diluting other higher-salinity source water.
Read the publications
Nicot, J.-P., Darvari, R., Eichhubl, P., Scanlon, B. R., Elliott, B. A., Bryndzia, L. T., Gale, J. F. W., and Fall, A., 2020, Origin of low salinity, high volume produced waters in the Wolfcamp Shale (Permian), Delaware Basin, USA, Applied Geochemistry, v. 122, p. 104771, doi:10.1016/j.apgeochem.2020.104771.
Nicot, J.-P., Gherabati, A., Darvari, R., and Mickler, P., 2018, Salinity reversal and water freshening in the Eagle Ford Shale, Texas, USA: ACS Earth and Space Chemistry, v. 2, no. 11, p. 1087–1094, doi:10.1021/acsearthspacechem.8b00095.