Page 90 - Geology of Carbonate Reservoirs

FLUID WITHDRAWAL EFFICIENCY 71

where h max is the maximum height of hydrocarbon column that can be sustained by
the sealing facies, P dS is the brine – hydrocarbon displacement pressure in psi of the
sealing rock, P dR is the brine – hydrocarbon displacement pressure in psi of the res-

ervoir rock, and ρ w and ρ hc are the respective densities of brine and hydrocarbons.

3.3 FLUID WITHDRAWAL EFFICIENCY

Withdrawal efficiency is a measure of the ease with which a rock will give up

nonwetting fluids, usually mercury, after high pressure injection followed by pres-

sure release. Mercury withdrawal efficiency in laboratory experiments is at least
phenomenologically related to hydrocarbon recovery efficiency in carbonate reser-

voirs. In fact, for water - wet reservoirs, the same factors of pore geometry that con-
tribute to increased mercury withdrawal efficiency also contribute to increased

hydrocarbon recovery efficiency in subsurface reservoirs (Vavra et al., 1992 ).
Visual examination of pores and pore throat sizes, shapes, and coordination
numbers provides information with which to evaluate withdrawal effi ciency and
predict hydrocarbon recovery effi ciency. Pore and pore throat geometry are exam-
ined on resin casts made from carbonate reservoir rocks impregnated with resin and
then dissolved in acid. The remaining pore casts in the insoluble resin are studied
under a microscope or with the scanning electron microscope (SEM) in the manner

described by Wardlaw (1976) . Estimates of withdrawal efficiency require injection
at increasing pressure increments followed by pressure release and withdrawal of
the nonwetting fluid. Ordinarily, this procedure involves mercury injection and

withdrawal from cleaned rock samples. The procedure for obtaining MICP curves
has developed largely from the pioneering work of Purcell (1949) and is described
in detail in Amyx et al. (1960) . Studies by Pickell et al. (1966) , Wardlaw and Taylor
(1976) , and Wardlaw and Cassan (1978) applied mercury injection and withdrawal

techniques to investigate withdrawal efficiency. Injection curves are known to petro-
leum engineers as drainage curves because the wetting fl uid is being drained as the

nonwetting fluid is injected. Withdrawal curves are known as imbibition curves

because the wetting fluid is imbibing pore space as the nonwetting fluid is withdrawn

(Figure 3.11 ). On injection, the wetting fluid is displaced (drained) from the rock.

On withdrawal, the wetting fluid displaces some of the nonwetting fluid and some

of the nonwetting fluid remains trapped in the pore and pore throat network. Effi -
cient withdrawal equates to low imbibition; that is, if the rock returns a high percent-
age of the nonwetting fluid as pressure is decreased, it has high withdrawal effi ciency

or high recovery effi ciency . Recovery efficiency is defined by Wardlaw and Taylor
(1976) as “ the ratio of the volume of mercury withdrawn from a sample at minimum
pressure to the volume injected at maximum pressure before the pressure was
reduced. ” This can be expressed as
−
( SS R )
W E = × 100
S
to convert to percent. Here, W E is withdrawal effi ciency, S is the volume of mercury
at maximum pressure before pressure was reduced, and S R is the volume of mercury
withdrawn from the sample at minimum pressure.