Page 97 - Enhanced Oil Recovery in Shale and Tight Reservoirs

84 Enhanced Oil Recovery in Shale and Tight Reservoirs

liquid oil saturation in the core is calculated using CT numbers following
Eq. 2.11, and the liquid condensate oil recovery factor can be calculated
by Eq. 2.12.
For a flooding mode, first a condensate saturation is established. Valve C
is closed. Valves A and B are open, but Valves E, F, and H are closed initially.
The core is saturated by a gas condensate at a set pressure above a dew point
pressure by opening Valve G. Then open Valves E and H at a pressure lower
than the dew point pressure, some condensate forms in the core. Then
increase the syringe pump rate at a set pressure higher than the earlier
pressure, making the pressure in the core higher but the pressure at the
exit end near Valve A lower than the dew point pressure. By doing so,
the liquid condensate in the core is reduced. The average liquid oil
saturation and the oil recovery in the core are calculated using CT numbers
following the method for huff-n-puff.

4.3 Huff-n-puff gas injection
Meng et al. (2017) did huff-n-puff experiments. An Eagle Ford
outcrop core of 1.5 inches in diameter and 4 inches in length was
used. The porosity was 6.8% and the permeability was 100 nD. To make
experiment easier to run and analyze, a synthetic gas condensate of
0.85 mol fraction of methane and 0.15 mol fraction of n-butane was
used. Its phase diagram is shown in Fig. 4.4. The experiment was conducted

at the room temperature of 68 F. The liquid drop of this mixture at this
temperature is shown in Fig. 4.5. The dew point pressure of this gas

condensate mixture at 68 F is 1860 psi. Thus, the initial pressure of the
mixture was set 2200 psi at the Accumulator 2 but the injection pressure
was set 1900 psi. The injection and soaking time was 30 min. After that,
the back-pressure regulator was set 1460 psi (below the dew point pressure
of 1860 psi). Valves B, A, E, and H were open while other valves were
closed. The core was depleted at 1460 psi from both ends of the core for
30 min. This was one cycle. After that, gas mixture was injected from
Accumulator 2 again followed by depletion to Accumulator 3 again. This
process is repeated for 5 cycles. The liquid condensate saturation was
measured by CT numbers which were obtained the end of every puff
period. The condensate recovery was calculated by CT numbers.
As shown in Fig. 4.6, the condensate saturation after the primary
depletion was 10%, and it was 9.1% at the end of the first puff process.
The saturations at the end of remaining cycles are shown in the figure.