BAKER HUGHES - Drilling Fluids Reference Manual

Formation Mechanics
• Calcareous sandstone, composed of limestone fragments
Clean, well-sorted sandstones can be tightly or loosely cemented and can, therefore, have a wide range
of porosity and permeability. Many Gulf Coast sandstones, because they are clean, well-rounded, and
weakly cemented, provide good hydrocarbon reservoirs. Certain Ordovician sandstones of the Mid-
Continent area are some of the best-sorted, best-rounded, clean sandstones developed anywhere in the
world. Again, being loosely cemented, they are excellent reservoirs for hydrocarbon deposits.
Porosity and Permeability
Porosity represents the amount of void space in a rock and is measured as a percentage of the rock
volume. In sandstones, porosity is controlled primarily by sorting, cementation, and to a lesser extent,
by the way the grains are packed together. Porosity is maximized when grains are all spherical and of
equivalent size, but becomes progressively less as the gains are more angular since such grains pack
together more closely.
Grains pack together in one of two ways – cubic or rhombohedral as shown below.
Figure 2-7 Structure of Montmorillonite Clay
The figure on the left is open (cubic) packing where the porosity is about 48 percent. The close
(rhombohedral) packing on the right has only 26 percent porosity because the grains are packed into a
smaller space. Artificially mixed clean sand has measured porosity of about 43 percent for extremely
well sorted sands, irrespective of grain size, decreasing to about 25 percent for poorly-sorted, medium
to coarse sand, while very fine grained sands still have over 30 percent porosity.
The ease with which a fluid moves through the interconnected pore spaces of a rock denotes the
degree of permeability. Permeability of highly porous, well-sorted sand varies from 475 millidarcies
(mD) for a coarse-grained sand to about 5 mD for a very fine-grained sand. Permeability may
decrease for a coarse-grained sand to about 10 mD if it is poorly sorted.
Compaction and Sedimentation
Compaction by weight of the overlying sediments squeezes the sand grains closer together. At greater
depths, it may crush and fracture the grains. The results are smaller pores and, therefore, lower
porosity and a decrease in permeability. Thus, a sandstone reservoir which could produce petroleum
at 10,000 feet might become too impermeable to be of any economic value at 20,000 feet.
Reference Manual
Baker Hughes Drilling Fluids
2-10 Revised 2006