Page 368 - Enhanced Oil Recovery in Shale and Tight Reservoirs

340 Enhanced Oil Recovery in Shale and Tight Reservoirs

of a fractured well can therefore be very expensive. Significant cost savings
can be realized after the flow back of fracturing fluid after the last stage is
fractured so that the rig is moved quickly. We also need to know whether
a long shut-in time is not harmful to the subsequent hydrocarbon produc-
tion. Therefore, study of shut-in time effect is very important.
There does not appear to be significant impairment of stimulation effec-
tiveness, if delays of up to 2 h occur prior to the start of flow back. There is
conclusive evidence that whenever any shut-in occurs during flow back,
especially prior to gas breakthrough, damage occurs. Delay in starting
flow back is less damaging than shut-ins during flow back. The well should
not be shut-in prior to hydrocarbon breakthrough, and even afterward.
Delayering breakthrough of hydrocarbon production by maintaining lower
flow back rates, not by shutting-in, appears to result in better effective
stimulation (Crafton, 1998).
In low-permeability formations, a large amount of proppant is pumped
at lower proppant concentrations, requiring an excessive volume of
fracturing fluid to transport the proppant. This extends the closure time,
allowing proppant to transport away from the wellbore into the fracture
after the end of treatment. In such a situation, an early flow back procedure
to force proppant bridging at the wellbore in a reverse screen-out mode is
essential to restore the proppant pack conductivity near the wellbore (Barree
and Mukherjee, 1995). This process can be immensely accelerated by
increasing proppant concentration toward the end of the treatment as
suggested by Ely et al. (1990), Coulter and Wells (1972), and Cleary et al.
(1994). Or small pad volume is used (Cleary et al., 1994).
One of the controversial issues in hydraulic fracturing is the flow back of
fracturing fluids. It is intuitive that higher percentage of fracturing fluid flow
back should lead to higher hydrocarbon recovery, because the remaining
fracturing fluid may block the paths for hydrocarbon to flow to the well
during production. However, that has not been always the case. In the
case of low-permeability reservoirs, wells with good permeability and
flow capacity may often start recovering significant hydrocarbon after very
little production of the fracturing fluid (Malone and Ely, 2007).
In this chapter, the flow back and production performance from common
shale reservoirs and experimental results are summarized. Mechanisms of low
flow back are proposed. The effects of shut-in, initial rock wettability, inva-
sion depth, and surfactant additives on flow back are discussed. Finally, some
solutions to deal with flow back are summarized.