Page 46 - Geology and Geochemistry of Oil and Gas

RESERVOIR ROCKS 23
3
where Q is the flow rate (m /s), m the dynamic viscosity (Pa s), Dp the pressure
2
gradient along the length L (in m, Pa/m), A the cross-sectional area (m ), and k the
2
permeability (m ).
Permeability is a function of the pore size and shape, pore throat and/or channel
diameter, grain size and shape, grain packing density, tortuosity, sorting, cementing,
fracturing, and residual fluid saturation. The above definition of permeability in-
dicates that its value should not be affected by the nature of a liquid moving through
the porous medium. Actually, however, permeability changes depend on the type of
flowing fluid. These changes are sometimes greater than 100%. According to def-
inition, permeability also should not change with time. Experiments, however, often
demonstrate permeability drop of up to 50% within 1 h. There are different expla-
nations of the reasons causing permeability change in time, and the effect of fluid
properties on permeability. If fluids flow through loose reservoir rocks that include
some fine sands, rock grains may change their positions (a phenomenon called
‘‘suffusion’’), and the pore channels may become plugged with fine material. Col-
loidal particles suspended in oil may precipitate and plug the pores. Resins and
asphaltenes present in crude oil may also precipitate, and result in a decrease of the
cross-sectional area of pore channels, throats, and canals. Wettability of rocks (oil-
wet versus water-wet) also changes the relative permeability to water and to oil (see
Chilingarian et al., 1992, 1996).
When water flows through reservoir rocks that include clay minerals, many of the
clay minerals swell, which also results in a decrease in the cross-sectional area of pore
channels. Water in contact with silica may give rise to colloidal silica in porous
space, which may lead to plugging of the pore channels. When CO 2 is released from
water, CaCO 3 precipitates within reservoirs according to the following reaction:

ðHCO 3 Þ Ca ! CO 2 " þ CaCO 3 # þ H 2 O
2
decreasing the pore throat and canal diameters.
Based on experiments, Khanin (1976) recorded a substantial decline in gas per-
meability of sandy rocks with the cement content of 4–10% (see also Fig. 2.2).
Dutton and Diggs (1992) also reported drastic permeability changes in the Upper
Cretaceous sandstones of eastern Texas having similar cementation ranges. Bury-
akovsky (1985) provided quantitative relationships between permeability and ce-
mentation (both due to argillaceous cement and calcareous cement) based on
laboratory analyses of cores recovered from the Pliocene Productive Series of the
onshore and offshore Azerbaijan.
It is established that the Darcy’s law may be applied for the description of a steady
fluid flow only within certain limits. The upper applicability boundary is associated
with inertia forces at high velocity flow and is defined by the critical value of the
Reynolds number (Re cr ¼ 7:5–9). The lower applicability boundary, typical for low-
velocity flow (for instance, in clays) is associated with fluid interactions with the
porous medium (non-Newtonian rheological properties of liquids). Some liquids (oil,
water) can form colloidal films on the surface of a porous medium that may totally
obstruct the liquid movements through the pores. In this case, in order for the
movement to begin, an additional force should be applied (the initial pressure