Groundwater pressure

Calculation of the dow in the saturated portion of the subsurface is generally much easier than that in the unsaturated zone. However, calculation of dow in either requires a fundamental understanding of groundwater pressure and energy. 

Forces applied to a water-saturated porous medium will cause stresses which result in strain (deformation). The stress, strain and groundwater pressure in a water-saturated porous medium are coupled, as first recognized by Biot (1941). Under the assumed stress conditions, the vertical normal component of total stress (o ) that acts downwards on a horizontal plane at any depth is caused by the weight of the overlying water-saturated rock. This stress is born by the solid matrix of the porous medium (o ) and by the pressure of the groundwater in the pores (p ) (e.g. Hubbert andRubey, 1959)... 

The groundwater pressure in a representative elementary volume of the porous medium fixed in space, changes directly with the hydraulic head, i.e. dp = p, gdh and Equation 1.24 can be written as... 

On a geological time scale the conservation of mass for the solid phase is determined by the deformation of the solid matrix and the accompanying time-changes of porosity, and the dislocation of the solid matrix in space. The deformation and dislocation of the solid matrix depend on the changing stress, groundwater pressure, temperature and chemical conditions. 

Figure 2.8 Change of groundwater pressure with depth in a sand-shale sequence (modified after Hunt, 1979. Reprinted by permission of W.H. Freeman and Company).

Figure 2.19 Change of groundwater pressure with depth in different parts of a flow system of a hypothetical homogeneous drainage basin with simple ground surface geometry (modified after T6th, 1980. Reprinted by permission of the American Association of Petroleum Geologists).

Groundwater overpressure = observed groundwater pressure -hydrostatic groundwater pressure (for Pw = 1020 kg/m3)... 

Figure 3.10 Observed groundwater pressures in Jurassic and Triassic carrier-reservoir rocks in the Viking Graben, North Sea (based on data presented by Buhrig, 1989, in Fig. 6, p, 38, Marine and Petroleum Geology, Vol. 6. Reproduced by permission of the publishers, Butterworth Heinemann Ltd. ).

Tectonic processes may influence both the groundwater pressure condition and the hydrogeological framework in a basin (Section 2.2), and as a consequence may affect the driving force for hydrocarbon migration and the... 

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