Page 407 - Enhanced Oil Recovery in Shale and Tight Reservoirs

378 Enhanced Oil Recovery in Shale and Tight Reservoirs

cleanup is ineffective and formation damage occurs; and there is a critical rate
above which severe and permanent damage occurs to the effective fracture
length and conductivity.
During the fracture operation, the region near the fracture is compressed
and high energy is stored. This energy can help fluid flow back or cleanup.
However, this energy will quickly disperse after shutdown. Therefore, it is
general to make use of the energy by flowing back as soon as possible after
the fracture treatment (Martin and Rylance, 2007). In tight formations,
there is a significant elapse time to fracture closure and there is potential
for proppant convection away from the pay zone and wellbore; consider-
ation should be given to immediate flow back or forced closure (preclosure)
(Ely et al., 1990), or reverse gravel packing (Ely, 1996). However, it is gener-
ally accepted that forced closure will lead to reduced fracture width, and
therefore reduced conductivity, immediately at the wellbore, and conse-
quently this approach has a limited range of application and should be
considered carefully. After the fracture closure is confirmed, the flow back
process (postclosure flow back) is begun as soon as the treatment has been
shut down. After the fracturing fluids have been produced back toward
the wellbore, it is very important to keep the fluids moving and not to
halt the process until the well is fully cleaned up. Each time a well is
shut-in during fracture fluid recovery, it can become increasingly difficult
to reinitiate fluid recovery. This is particularly true if gas is also being
recovered with the fracturing fluid, as the multiphase flow effects will
tend to decrease the effective permeabilities of both phases. Also, additives
in fracturing fluids to prevent formation damage such as demulsifiers, clay
stabilizers, and low-tension additives have a limited life, generally in hours
not in days or weeks. The longer the fracturing fluids are left in the forma-
tion, the less effective these additives will be and the greater the potential for
permanent formation or fracture damage (Martin and Rylance, 2007). Haw-
kins (1988) found that both final permeability and permeability recovery
decreased dramatically with increasing shut-in time in his experiments,
because of gelling agent becoming more concentrated which reduces
permeability.
The flow back process should also be designed to minimize proppant
flow back. One method is to control the drawdown or flow rate. A sudden
change should be avoided. The choke sizes should be staged slowly up or
down as required (Martin and Rylance, 2007).
Bilden et al. (1995) measured fracture conductivity in laboratory and
evaluated well skins from well pressure transient analysis. The results did