Page 434 - Petrophysics

402 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

TABLE 6.5
($)
CHANGE OF POROSITY AND FORMATION RESISTIVIN FACTORS (FR) FOR
A
SANDSTONE AND LIMESTONE CORES ACCOMPANYING CHANGE OF
PRESSURE FROM 3 MPA [71]
Sandstone Cores
1 2 3 4
0 at 3 MPa 0.144 0.150 0.195 0.184
41 at 20 MPa 0.137 0.143 0.189 0.18
% change 4.9 4.7 3.1 1.6
FR at 3 MPa 33.7 33.1 21.3 22.1
FR at 20 MPa 40.4 41.4 23.2 23.6
% change 19.9 25.1 8.9 6.8
~
Limestone Cores
1 2 3 4 5 6
0 at 3 MPa 0.204 0.231 0.246 0.241 0.260 0.261
$ at 20 MPa 0.197 0.225 0.236 0.231 0.249 0.249
% change 3.4 2.6 4.1 4.1 4.2 4.6
FR at 3 MPa 17.4 13.3 11.4 11.3 13.3 10.4
FR at 20 MPa 19.1 14.6 12.4 13.6 15.8 11.7
% change 9.8 9.8 8.1 10.6 18.8 12.5

where the cylinder containing the displacing fluid is driven by a separate
metering pump, is required to maintain close control of the injection and
withdrawal of fluids.
The changes of porosity and formation resistivity factor obtained by
Longeron et al. for sandstone and limestone cores, when stressed to
a moderate pressure of 20 MPa, are listed in Table 6.5. At this total
overburden pressure, the deformation was found to be completely
elastic-that is, the cores returned to their original porosities when the
overburden stress was removed. If greater stress is applied, however,
inelastic deformation will take place and a reduction in porosity will
result.
Although the relative changes of porosity and formation resistivity
factors were approximately the same for the stress change, their
responses to the step increases of pressure were much different. The
sandstone cores deformed immediately in response to the applied stress,
whereas the limestone cores exhibited gradual deformation at each step
increase of stress.
A 15% underestimation of FR will lead to underestimation of Sw by
7.5% for a clean sand with a saturation exponent of 2.0. At greater
depths, the error will be more serious because expanded reservoir cores