Page 91 - Geology of Carbonate Reservoirs

72 PETROPHYSICAL PROPERTIES OF CARBONATE RESERVOIRS

100.0 Withdrawal Curve
Injection Curve

Pressure (kg/cm²) 10.0

1.0

U S
MIN MAX
S R
0.1
100 50 0
Percent Pore Volume Saturated by Hg

Figure 3.11 Capillary pressure curves showing the drainage (injection) and imbibition
(withdrawal) curves. S max and S min are volumes of mercury at maximum injection pressure
and minimum withdrawal pressure, respectively. U min is the minimum unsaturated pore
volume that can be compared to irreducible water saturation in oil – brine systems. Withdrawal

efficiency [100 ( S max − S min )/ S max ] can be used as a qualitative indicator of how rock properties
influence reservoir recovery efficiency. For more accurate estimates of how petrophysical

rock properties predict reservoir recovery effi ciency, the reservoir fl uid properties and drive
mechanisms must also be considered. (Adapted from an illustration in Wardlaw and Taylor
(1976) .)
For constant fl uid properties, recovery effi ciency depends primarily on (1) pore/
pore throat size ratio, (2) pore - to - pore accessibility, (3) heterogeneity of pore dis-
tribution, and (4) pore throat roughness. The pore/pore throat size ratio is a very
important factor in estimating recovery efficiency because large pores connected by

small pore throats are difficult to drain. As nonwetting fluids are withdrawn from a

system of large pores and small pore throats, the nonwetting fluid column breaks

apart in the narrow throats, leaving a large amount of fluid isolated in the large

pores. This phenomenon is called snap - off (Yu and Wardlaw, 1986a,b ). Pore - to - pore
accessibility is determined by coordination number, or the number of pore throats
that connect with a pore. Heterogeneity of pore distribution is an index of how
uniformly pore and pore throat sizes are distributed throughout the reservoir rock.
A group of large pores and pore throats (a low coordination number geometry)
surrounded by uniformly distributed small pores and pore throats (higher coordina-
tion number) will have lower recovery efficiency than the surrounding small pores

(Figure 3.12 ). Heterogeneity depends to a large extent on the origin of the pore
system. In a pore system created by depositional processes, pores and pore throats
are distributed according to the depositional texture and fabric of the rock. For
example, a rudstone lens surrounded by bioclastic grainstones around a reef would