Page 156 - Enhanced Oil Recovery in Shale and Tight Reservoirs

140 Enhanced Oil Recovery in Shale and Tight Reservoirs

Table 6.1 Core plug properties.
Average
Diameter Length Dry weight Saturation permeability
Core no. (mm) (mm) (g) Porosity (nD)
CEF_1 38.5 50.9 152.099 4.4% 85
CEF_2 38.1 101.8 249.697 13.1% 400

Table 6.2 Test schedules.
Gas injection
Core no. mode Test no. Test schedule
CEF_1 Flooding 1 Flooding period: 48 hrs
Huff-N-Puff 2 Huff and soaking 0.5 h, puff 1 h (32 cycles)
3 Huff and soaking 2 h, puff 1 h (16 cycles)
4 Huff and soaking 5 h, puff 1 h (8 cycles)
CEF_2 Flooding 5 Flooding period: 72 h
Huff-N-Puff 6 Huff and soaking 1 h, puff 3 h (18 cycles)

schedules are presented in Table 6.2. Note that the total test times for flood-
ing and huff-n-puff for each core plug were the same; for the huff-n-puff
tests, huff time and soaking time were added together.
The recovery factors of tests #1 to #4 within 48 h using the core CEF_1
are shown in Fig. 6.8. The recovery factor of the flooding test was 17.9% at
the end of 48 h of flooding. The recovery factors of the huff-n-puff tests
depended on the huff and soaking time. When the huff and soaking time
was shorter, more cycles could be performed within the same total test
time, and the recovery factors at the end of test were higher. When it was
0.5 h, the recovery factor was the highest (22.5%). However, Yu et al.
(2017) observed that little oil was produced when the huff and soaking
time was too short (e.g., a few minutes). They thought soaking time was
necessary for the pressure (or energy) to be transmitted to the inner area
of the core and for gas to diffuse into the matrix, which is not consistent
with simulation results presented in the preceding chapters. Note that in
these experiments discussed here, the huff time and soaking time were added
together. If this add time was a few minutes, the injected gas could not reach
or even the pressure could not transmit to the inner area of a core. There-
fore, litter could be produced. When the soaking time was 2 h, the recovery
was almost the same as that from the flooding test. These results show two
important points: (1) huff-n-puff performance strongly depends on