Page 416 - Enhanced Oil Recovery in Shale and Tight Reservoirs

386 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 12.44 Flow back efficiency versus invasion depth in a water-wet chip for
deionized water.

Figure 12.45 Flow back efficiency versus invasion depth in a water-wet chip for two
surfactant solutions.

were added, the capillary pressure became lower, and the effect of the capil-
lary pressure gradient became less sensitive to the invasion depth. Note that
the surfactants cannot change the chip wettability.
However, the stabilized oil rate during the late-time flow back decreased
with the invasion depth, both for water and surfactant solution cases, as
shown in Figs. 12.46 and 12.47. Therefore, we wish to have a shallow in-
vasion depth to have higher oil rate. Note this deep invasion was caused
by the fluid leak off, not caused by the imbibition during shut-in.
However, when the chip is oil-wet, the flow back efficiency decreased
with invasion depth, as shown in Fig. 12.48 (Tangirala and Sheng, 2018).
During the flow back, the capillary pressure was a driving force. As the in-
vasion depth was increased, the driving pressure gradient was decreased, and
the flow back efficiency was decreased. Consequently, the stabilized oil rates