Page 387 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fracturing fluid flow back 359

imbibition. During the flow back, both the pressure and the saturation are
reduced, especially the saturation can be easily decreased to lower than the
immobile saturation, thus water cannot flow back when the saturation reaches
this level.

12.4 Effect of shut-in time on flow back

From the preceding discussions, the flow back is closely related to the
shut-in. There are opposite opinions about the effect of shut-in on flow
back. One is that the fracture filtrate (water) near the fracture-matrix
interface will dissipate into the deep formation, so that the water blockage
is mitigated. The other one is that the fracture filtrate that dissipates into
the deep formation will block the gas flow out, and immediate flow back
will mitigate the water blockage. Field observations and research results
are mixed.
Cheng (2012) presented the performance of a Marcellus shale gas well, as
shown in Fig. 12.16. The well was a horizontal well completed with a multi-
stage fracturing treatment. After the treatment, it had a short time for flow
back, followed by a shut-in of almost a half year. When the well was re-op-
ened, the gas rate was significantly increased, while the water rate was much
lower than that before the shut in. Although her simulation work showed
the similar trends in gas rate and water rate, the cumulative gas production
was almost the same, and the cumulative water production was significantly
lower from her simulation from an extended shut-in of 3 or 6 months,
compared with the case without an extended shut in. However, the dis-
counted gas production will be lower in the case of extended shut-in, if
the accelerated production is considered.

Figure 12.16 Field production data from a well in Marcellus shale, (A) water production
and (B) gas production (Cheng, 2012).