Page 183 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Water injection 167

concentrations are added in the first cycle, and the next two cycles are water
only. Each cycle has 6 months of injection and 12 months of production.
Their results show that water injection of three cycles improves oil recovery
by 43% and surfactant injection doubles the improvement.

7.8 Water injection in China

Because water injection modes in China are not simply huff-n-puff or
flooding, and the permeability is generally higher than those cases presented
in the previous sections, this section is allocated to discuss water injection in
China. Typical tight formations in China are distributed in Ordos Basin
(Chang 7, Chang 6, and Chang 8), Zhengger Basin, and Songliao Basin.
The typical technologies to develop tight oil are long horizontal wells
with multistage fracturing, primary depletion followed by water injection,
production above the bubble point pressure, and other means to reduce
production costs (Li et al., 2015a,b). Sometimes, water injection proceeded
primary depletion. Different modes of water injection and their perfor-
mance are summarized in Table 7.4 and discussed separately below. Some
of them are further briefed next.

7.8.1 Pulsed water injection
Pulsed water injection is to change water injection rate at some cycles of
time. Sometimes water injection is completely stopped for some time at
the injectors, but the oil producers are kept flowing. The idea is to generate
pulsed elastic energy. It is a flooding process. The mechanisms of pulsed
water injection are believed to be as follows. (1) Oil is displaced out of
lower-permeability zones by water imbibition. (2) During the high-rate in-
jection period, high pressure drives water from high-permeability zones to
lower-permeability zones to displace oil out. (3) Pulsed pressure pulses
enhance elastic energy. Thus, the conditions to apply pulsed water injection
are heterogeneous, water-wet, and low oil viscosity reservoirs, short normal
injection history, and the facility can enable increase in water injection rate
(Guo et al., 2004). A typical cycle is 30 days injection and 30 days shut in
(Xie et al., 2016). Here is a field case of pulsed water injection.
The porosity and permeability of the An 83 zone in the Chang 7 forma-
tion were 8.9% and 0.17 mD. The rock was weakly water-wet to water-
wet. The formation water salinity was 51 g/L and the water was CaCl 2
type. It was a thick formation but there were many separation layers. The
oil viscosity was 1.01 cP in situ and 6.5 cP at the surface. The GOR was