Page 209 - Geology of Carbonate Reservoirs

190 FRACTURED RESERVOIRS

where m = Archie cementation exponent

φ s = Matrix porosity calculated from sonic log
φ t = Total porosity from neutron or density logs
Asquith (1985) noted that it is necessary to confirm that the reservoir has fracture

porosity in order to use the Rasmus equation successfully, otherwise the calculated
m value will be too low. When calculated porosity from the sonic log is less than
total porosity, the Rasmus equation always gives an m value less than 2, and cal-
culated S w will be lowered. Like fractured rocks, those with vuggy porosity can
mimic the reservoir behavior of fractures and they have φ s values less than φ t .
Using the wrong equations to calculate m in nonfractured reservoirs will result in
serious errors in calculated S w . Asquith (1985) notes that m values for rocks with
vuggy porosity should be greater than 2.0 — not less than 2.0 as when the Rasmus
equation is used. He presents an example of the errors that result from misapply-
ing the Rasmus equation to reservoirs with vuggy porosity. He gives the following
values for an example calculation: R w = 0.04, R t = 20, φ s = 0.05, and φ t = 0.15. It
is assumed that the reservoir is vuggy and that the Nugent (1984) equation, m ≥
(2 log φ s )/(log φ t ) is used to compute m . The results are m = 3.16 and S w = 89.6%.
If the Rasmus equation is used instead, assuming that the reservoir has both
fracture and vuggy porosity, the results are m = 1.20 and S w = 13.9%. If the S w =
89.6% value is used, the reservoir is water - wet and nonproductive. If the 13.9%
value is used, the reservoir is productive. This dramatic difference in outcomes
illustrates the importance of recognizing pore type and rock properties from direct
observation of core samples before undertaking evaluation of carbonate
reservoirs.

7.3 CLASSIFICATION OF FRACTURED RESERVOIRS

Fractured reservoirs can be divided into four types, following Nelson (2001) :

Type I . Fractures provide essential reservoir porosity and permeability.

Type II. Fractures provide essential permeability.
Type III. Fractures assist permeability in an already producible reservoir.
Type IV. Fractures provide no additional porosity or permeability but they do
impose significant anisotropies such as barriers to flow.

Examples of type I reservoirs include Amal Field in Libya, with reserves of about

1700 MMbbls and several Ellenburger dolomite fields in Texas. Normally reservoirs

of this type do not have large reserves because matrix porosity, the principal storage

volume in reservoirs, is insignificant. The large reserves at Amal Field exist because
the reservoir is 800 feet thick and extends over 100,000 acres. Fracture porosity in
Amal Field is only about 1.7%, assuming no contribution from the Cambrian quartz-
ite matrix (Nelson, 2001 ).
Type II reservoirs have greater storage capacity because matrix porosity is impor-
tant. Because of this, type II reservoirs typically have greater reserve volumes than