Page 92 - Geology of Carbonate Reservoirs

FLUID WITHDRAWAL EFFICIENCY 73

Low High

Pore Throat Pore Throat

Pore / Throat = Large Pore / Throat = Small

Coordination Number 3 Coordination Number 6

Figure 3.12 Rock properties that infl uence fluid recovery efficiency in reservoirs: pore/pore
throat size ratio and coordination number. Large pore throats and large pores provide high

recovery efficiency. Large pores will not drain efficiently through small pore throats. Coordi-
nation number refers to the number of pore throats that connect each pore. The more pore
throats there are to connect each pore, the greater the recovery efficiency, all else being equal.

(Adapted from an illustration in Wardlaw (1979) .)
present large intergranular pores with low coordination numbers surrounded by
smaller intergranular pores with higher coordination numbers. Although the rud-
stone has large pores, it will have low recovery efficiency because snap - off will

prevent the large pores from draining through the surrounding small pore throats.
Finally, pore throat roughness is calculated as an empirical parameter derived from
a three - stage graphical procedure described by Kopaska - Merkel (1991) , who found
that the roughness value, designated as the a factor, did not correlate with median
throat size, mercury recovery efficiency, or permeability. The “ a factor ” was devel-

oped by Shively (1991) in his efforts to find a descriptor of pore throat shape;
however, as the a factor has not been shown to correlate strongly with ordinary
reservoir rock and pore properties, it will not be considered further here. Pore throat
characteristics are important. Examining resin pore casts with scanning electron
microscopy is one way that provides direct observational data to compare with
drainage and imbibition capillary pressure curve characteristics.
In summary, pores are connected by pore throats that vary in size and shape
depending on their geological origin. The number of pore throats that connect a
single pore is the coordination number for that pore - to - pore throat system. Inter-
granular pores consist of curvilinear surfaces connected by pore throats that may
be modeled as cylinders, while intercrystalline pores tend to be tetrahedral and
exhibit sheet - like pore throats (Wardlaw, 1976 ). Large, disconnected pores such as
separate vugs, shelter pores, constructed voids in reef rocks, and widely spaced
fractures may contribute greatly to total porosity but they depend on fl uid transmis-
sion through the small connected pores in the host rock matrix to remain fi lled. Such
a rock type exhibits moderate to high porosity but comparatively low permeability