Page 361 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Forced imbibition 333

had 12-hour huff (soaking) and 12-hour puff periods. The soaking pressure
was controlled to 3000 psi and the cell was depleted to atmospheric pressure.

The cores were initially oil-wet (contact angles about 150 ) altered by

surfactants to 35e40 . Note that during the huff and soaking period,
because the pressure could be quickly increased, a huff and soaking period
was actually a soaking period.
The oil recovery factors of huff-n-puff for different surfactant solutions
and brine are shown in Fig. 11.20. The oil recovery factor was higher
when the IFT was higher, the same trend as the spontaneous imbibition.
Note that the last two points of high oil recovery factor for the intermediate
IFT were caused by fractures created in the core. It implies that the sponta-
neous imbibition during huff and soaking period is very important. This
observation becomes more obvious when the simulation data of pressure
and recovery factor are presented in the same plot in which the continuous
pressure and oil recovery data are presented. For example, the pressure and
oil recovery data for the high-IFT case is presented in Fig. 11.21. Look at a
single cycle, the high-pressure step (3000 psig) indicates the huff and soaking
period, and the low-pressure step (0 psig) indicates the depletion period.
From a close look at the oil recovery data during a single cycle, we can
see that the oil coming out of the core during the huff and soaking period
is higher than that during the puff (depletion) period. It becomes clear

Figure 11.20 Oil recovery factors of huff-n-puff for different solutions with different
IFTs and wettability alteration.