Page 187 - Enhanced Oil Recovery in Shale and Tight Reservoirs

170 Enhanced Oil Recovery in Shale and Tight Reservoirs

Around the year of 2015, eight wells were put in huff-n-puff water in-
jection in the An 83 zone (this zone is named huff-n-puff 3 in Table 7.4).
Among these eight wells, 6 wells had one cycle, while the other two wells
had two cycles. Seven wells benefitted from the water injection. The incre-
mental oil from the huff-n-puff wells was 456 tons, but 1127 tons of oil from
the neighbor wells. Among the 6 wells with one cycle injection, two wells,
Well AP 53 and Well An 120, did not see pressure increase, but the neighbor
wells had 497 tons of incremental oil. The rest of the four wells had injection
pressure raised to 7.25 MPa and benefitted. For example, Well AP 83 had
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5100 m of water injection and 45 days of soaking. Injected water reached
its two neighbor wells AP 48 and AP 84. By June 2016, Well AP 83 had
daily incremental oil rate 2.71 tons, and cumulative incremental oil 361
tons, while its neighbor wells AP 48 and AP 84 had daily incremental oil
4.52 tons, and cumulative incremental oil 596 tons. A second cycle was
put on the two wells AP 20 and AP 21 who performed well in 2014.
Compared with their first cycle, the performance was not good.
Huff-n-puff water injection was also performed in the Chang 6 reservoir,
Yanchang oil field near Qieli Village, Ordos Basin, China (Wei, 2016)
(Huff-n-puff 4 in Table 7.4). The average porosity was 8%, and the average

permeability was 0.54 mD in the reservoir. The oil viscosity at 50 C was
4.67 cP. The formation rock was weakly water-wet with the wettability
index 0.17. The reservoir pressure was 4.25 MPa, and the bubble-point
pressure was 1.12 MPa. The initial well oil daily rate was 0.74 tons. The
rate declined very fast. The water flooding sweep efficiency and injectivity
were low. Therefore, huff-n-puff water injection was proposed. Some
surfactant was added in the water. But no details about the surfactant
were reported. Nine wells in Block G and 20 wells in Block Z were selected
in November 2015. These wells had produced for more than 10 years. The
daily oil rate by March 2016 was less than 0.2 tons on average. The eco-
nomic oil daily rate was 0.3 tons.
For 29 wells, the soaking time was 7 days for most of wells and 3 days for
the rest. Test results showed that 7-day soaking outperformed 3-day soaking
in terms of incremental oil rate. A longer soaking time helped water imbi-
bition and pressure propagation from higher-permeability zones to low-
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permeability zones. Water injection volume was 50 m for most of wells
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and 70 m for the rest. Test results showed that higher volume injection
was better. After water injection, the incremental daily oil rate was
0.1e0.4 tons on average.