Page 292 - Fundamentals of Enhanced Oil and Gas Recovery

280 Mohammad Ali Ahmadi

on reactivation of natural fractures to induce permanent shear-induced dilation, which
enhances reservoir permeability [50,51]. Hydraulic fracturing is performed where
shear failure is anticipated to dominate [52] in shale under anisotropic stress.
Hydration swelling due to water imbibition can weaken the mechanical strength of
shale [53 55], and it can reduce the shear-induced fracture conductivity [1,56,57].

9.3.2.2 Huff-n-Puff Water Injection
One general mechanism for water huff-n-puff is that water preferentially invades in
large pores and then imbibes into small pores to displace oil. Another important
mechanism is the invaded water and imbibed water increase reservoir pressure and
local pressure so that the drive energy is boosted. From the imbibition point of view,
water-wet formation is preferred.
Yu and Sheng [57] carried out water flooding tests in a huff-n-puff mode. Their
experimental results reveal that the oil recovery factor highly depends on the injection
pressure. In their experiment increasing the soaking period resulted in increasing the
oil recovery factor. However, the oil recovery factor from such a method is much
lower than that the case of CO 2 huff-n-puff scenario. Altawati [58] conducted core
displacement experiments in a huff-n-puff setting; he considered the effect of initial
water saturation in their experiments. According to his experimental work, the pres-
ence of initial water saturation affects the oil recovery performance drastically; in a
case of no initial water saturation, the amount of recovered oil was much higher than
that the case with initial water saturation. Sheng and Chen [11] performed numerical
simulations to compare the performance of the water and gas injection in a huff-n-
puff mode. They concluded that the oil recovery factor of the CO 2 huff-n-puff
method is 2 3 times higher than the case of water injection in the huff-n-puff set-
ting. Some field tests are discussed below and the performances are reported in
Table 9.1.

Table 9.1 Performance of Water Injection in Huff-n-puff Mode [1]
Field Huff Time Soak Time Puff Time Performance
(Day) (Day) (Day)
Bakken, ND 30 15 90 120 No considerable increase in oil
production
Parshall 30 10 No considerable increase in oil
production
Parshall At first, 439,000 barrels injected through the No considerable increase in oil
reservoir and then WAG applied production
WAG, water alternating gas.