Petroleum reservoirs can have primary permeability, which is also known as the matrix permeability and secondary permeability. Matrix permeability originated at the time of deposition and lithification (hardening) of sedimentary rocks. As with secondary (induced) porosity, secondary permeability resulted from the alteration of the rock matrix by: compaction, cementation, fracturing and solution. Whereas, compaction and cementation generally reduce the primary permeability; fracturing and solution tend to increase. In some reservoir rocks, particularly low-porosity carbonates, secondary permeability provides the main conduit for fluid migration.
Fig. 4. Effects of clay cementing material on porosity and permeability
Factors affecting the magnitude of permeability
Permeability of petroleum reservoir rocks may range from 0.1 to 1000 or more millidarcies. The quality of a reservoir as determined by permeability in mD, may be judged as:
· K < 1 = poor
· 1 < K = fair
· 10 < K < = moderate
· 50 < K < 250 = good
· K > 250 = very good
Reservoirs having permeability below 1 mD are considered“tight”. Such low permeability values are generally found in limestone matrices and also in tight gas sands of western United States.
The factors affecting the magnitude of permeability in sediments are:
1. shape and size of sand grains: if the rock is composed of large and flat grains uniformly arranged with the longest dimension horizontally - its horizontal permeability (kH) will be very high, whereas, the vertical permeability (kv) will be medium-to-large. If the rock is composed mostly of large and uniformly rounded grains, its permeability will be considerably high and of the same magnitude in both directions. Permeability of reservoir rocks is generally lower, especially in the vertical direction, if the sand grains are small and of irregular shape. Most petroleum reservoirs are in this category. Reservoirs with directional permeability are called anisotropic. Anisotrophy greatly affects fluid flow characteristics. The difference in permeability measured parallel and vertical to the bedding plane is a consequence of the origin of that sediment. Subsequent compaction of the sediment increases the ordering of the sand grains so that they generally lie in the same direction;
2. cementation: of both permeability and porosity sedimentary rocks are influenced by the extent of cementation and the location of the cementing material within the pore space;
3. fracturing and solution: in sandstones, fracturing is not important cause of secondary permeability, except where sandstones are interbedded with shales, limestones and dolomites.
Capillary pressure is the difference in pressure between two immiscible fluids across a curved interface at equilibrium. Curvature of the interface is the consequence of preferential wetting of the capillary walls by one of the phases.