An Overview: Special Issue on 'The Slow Subsurface Fluid Flow Processes (Part I): Theory, Model, and Physical Properties Estimation'

Subsurface fluid flow processes have been studied for over 150 years, starting from securing water resources for drinking and agricultural purposes, salt production from brines, and exploration and production of energy resources such as petroleum and natural gas. Infiltration theory based on Darcy’s law (Darcy, 1856) has been used in analyses. Hubbert (1956) conducted a thorough review of this equation and showed that it can be understood as a microscopic momentum equation averaged over macroscopic scales, even though the equation was originally developed as an empirical one. Hubbert (1956) also stated that Darcy’s law can be used under the condition that viscous resistance is dominant compared to inertial resistance. The laminar flow condition is met with a Reynolds number of 600 to 700 in porous media (Dudgeon, 1966; Kyle and Perrine, 1971); however, according to Hubbert (1956)’s discussion, Darcy’s law does not hold when the Reynolds number is larger than unity in porous media with the reference length set to be the mean diameter of solid grains. The Darcy flux of 1 (cm /s /cm) corresponds to this condition when the mean grain size of the medium is 10-2 cm. Bear (1972) also presented similar results and showed that Darcy’s law is applicable when the Reynolds number is smaller than 1 or 10. As explained above, subsurface fluid flow is known to be slow. Recently problems related to subsurface fluid flow processes have been increasing, therefore, slow subsurface processes have become more and more important. Expectations are high today for the appropriate use of stable subsurface environments to solve waste disposal problems. For example, concepts for the geological disposal of radioactive wastes and subsurface sequestration of carbon dioxide (CO2) have been developed based on the fact that there are places where subsurface fluid flow is extremely slow and that we can expect possible very slow mass transport conditions in the subsurface if we design procedures for disposal/sequestration properly. Also, studies on the petroleum system include migration, accumulation, preservation, and destruction of petroleum and natural gas reservoirs over geological time scales. Here, an extremely slow subsurface fluid flow is the issue. Land subsidence can also be understood as a