Page 186 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Water injection 169

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75.7 m /ton. Pulsed water injection was performed. It was found that after
water injection was stopped, the reservoir pressure declined fast, and the wa-
ter cut was not significantly reduced. It was not clear what optimal cycle
time should be. Once water broke, the water cut sharply rose up (Wang
et al., 2015a).

7.8.2 Asynchronous water injection
Asynchronous water injection is when injectors are open and producers are
shut in, and vice versa. While the water is injected, injected water is pre-
vented from breaking through producers. While the producers are shut-
in, the water is promoted to enter matrix from fractures by high pressure
difference and capillary pressure. During a short period of shut-in for both
injectors and producers, the pressure is equilibrated among matrix and
fractures. When the producers are open, oil is produced from the matrix
to fractures. Such operation was practiced in the An 83 zone in the Chang
7 formation. The daily rate of oil from the producer An 18 increased from
3.6 to 5.4 tons/day, and the water cut decreased from 100% to 37.2% after
five cycles of operation (Wang et al., 2015a). This method was also practiced
in a metamorphic reservoir where the oil rate was increased from 21.8 to
42.5 tons/day after asynchronous water injection (Li, 2011).

7.8.3 Huff-n-puff water injection
Huff-n-puff water injection injects water and produces fluid at the same
well. Two different huff-n-puff patterns were conducted in two An 83
zones in Chang 7 Field in 2014. In the first zone (Huff-n-puff 1), totally
four wells were tested with one huff-n-puff well in the middle, while the
other two wells in the two sides of the huff-n-puff well were continuous
production wells. In the second zone (Huff-n-puff 2), all three wells were
huff-n-puff wells.
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In Huff-n-puff 1, the average daily water injection was 109 m and cu-
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mulative water injection was 2177 m . The pressure was raised to 4.7 MPa.
The incremental oil from the huff-n-puff well itself was 419 tons, and the
decline rate decreased from 17.9% to 10.8%. The incremental oil from
the neighboring continuous production wells was 2358 tons, and the decline
rate decreased from 16.6% to 31%. These data show that oil production
benefitted from the huff-n-puff water injection, and the benefit was better
for the two continuous production wells than the huff-n-puff itself. In the
second zone of all four huff-n-puff wells, the wells were interfered, and
the performance was not as good as in the first zone (Lin et al., 2016).