Page 388 - Enhanced Oil Recovery in Shale and Tight Reservoirs

360 Enhanced Oil Recovery in Shale and Tight Reservoirs

Yaich et al. (2015) evaluated the impact of soaking on well performance
in the Marcellus shale gas. Their results showed that soaking was possibly
beneficial in three out of the four areas investigated. The 30-day average
gas rates could be several times those at the end of flow back. The improve-
ment was better with longer soaking time. But most of the improvement in
well performance happened during the first 100 days of soaking. The rates
were normalized by pressure drawdown. Reduced water production was
observed in soaking wells. However, the time loss due to soaking was not
included in the evaluation.
Fakcharoenphol et al. (2016) simulated the effect of shut-in time on flow
back. The gas rates and cumulative gas production at different shut-in times
are shown in Fig. 12.17. They used triple-porosity models to simulate flow
in fractures, organic pores, and inorganic pores. The simulator used was
TOUGH-REACT (Xu et al., 2012). A shut-in period allows the shale ma-
trix adjacent to the fractures to imbibe the filtrate. Thus the water saturation
inside the fractures is reduced. The initial gas rate is increased with a longer
shut-in period. However, this effect does not last long. The rates at different
shut-in times merge to almost the same curve after about 1 month
(Fig. 12.17A); and the cumulative gas production curves almost overlap
each other (Fig. 12.17B), indicating that shut-in times do not affect cumu-
lative production. Because water has imbibed deep into the formation, it
will be more difficult for water flow out of formation and thus the water
production (rate and cumulative) is reduced with longer shut-in times, as
shown in Fig. 12.18. It can be expected that the imbibed water may have
a long-term impairment on gas production, because the water may block
gas flow paths deep inside the formation, and may cause shale swelling.
Here, the loss of production time is not included in the comparison. If it is
included, the effect of shut-in time on gas production will be worse. But
delayed water production may keep the energy inside the reservoir.
Fakcharoenphol et al. (2016) also did a sensitivity study of the effects of
other parameters like osmosis and wettability. All their results show the ef-
fects are in the early flow back period of less than 100 days. This implies that
those effects may not play significant roles in improving gas production.
Bertoncello et al. (2014) simulated the BHPs when the well is shut in 1, 7,
and 14 days before the first flow back (or called the first shut-in immediately
after well stimulation) at the same gas rate, as shown in Fig. 12.19.Itshows
that to produce in the well at the gas rate of about 8000 Mscf/d, the BHP
required for 1-day shut-in is about 5000 psig, while the required BHP is
2300 psig for 14-day shut-in. These data indicate that earlier flow back