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156 Enhanced Oil Recovery in Shale and Tight Reservoirs

injectors were found more economical than vertical injectors (Karpov et al.,
2016).

7.2.5 Waterflooding in Pembina Cardium in Alberta
By July 2014, only 15 out of 1500 horizontal wells with multistage
fracturing were switched to water injectors. The distances from injectors
to producers were 200e450 m. An obvious increase in oil rate was not
observed from waterflooding (Karpov et al., 2016).

7.2.6 Waterflooding in Vinogradova field in Russia
The field was produced by horizontal wells of 1000 m with multistage frac-
turing, and water was injected through vertical deviated wells. The well
spacing from an injector to a producer was about 800 m. Low waterflood
efficiency from vertical wells was observed (no significant oil rate increase
but decreased decline rate). Horizontal injectors were better and planned
for future development (Karpov et al., 2016).

7.2.7 Summary of waterflooding performance
The above three waterflooding projects were all conducted in Bakken for-
mation. Their performance is summarized in Table 7.1. It can be seen that
low sweep efficiency was a problem. However, there are a number of other
fields where direct water breakthrough channels occurred, but higher oil
recovery factors were still able to be reached (Baker et al., 2016).

7.3 Water huff-n-puff injection
Li (2015) summarized several favorable conditions for water huff-n-
puff injection: (1) water-wet reservoirs, (2) well controlled volume (contain-
ment, low permeability reservoirs), (3) high natural fracture density, (4) lack
of reservoir energy (low pressure). Yu and Sheng (2017) conducted water
huff-n-puff experiments. The experimental setup is shown in Fig. 7.3.
The Quizix pump (QX-6000) was used for water injection to provide
huff pressure in the vessel through the accumulator. An oil-saturated core
was put in the vessel. There was 1.0 cm space between the inside wall of
the vessel and the core outer boundary; such an annular space represented
fractures surrounding the matrix in a reservoir. The huff and puff pressures
were achieved through the three-way valve. The pump was operated in the
mode of constant pressure delivery. With the solution injected into the