Page 175 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Water injection 159

Figure 7.4 Oil recovery from water huff-n-puff at different huff and soaking times.

time from 1 to 12 h, there was an obvious improvement in cumulative
oil recovery factor from 7.67% to 14.04%, respectively, after 12 cycles.
But when the huff and soaking time was increased from 12 to 24 h, the
oil recovery was increased less than 1%. It indicates that the oil recovery
might not effectively benefit from a further longer soaking time. Since
the pressure buildup in the experimental setup was very fast, a soaking
period was necessary to transfer the surrounding pressure into the inside
of the core. In a reservoir, there might be an optimum soaking time which
was not studied. As the injection fluid might cause the change in the rock
properties, and cores could not be totally cleaned to resume the original con-
ditions before operating the tests, four cores from the same batch of cores
which has similar rock properties were used, instead of using one core to
repeat the four tests.
Fig. 7.5 shows the effect of injection (huff) pressure on huff-n-puff water
injection. The huff and soaking time together was 12 h, and the puff time
was 3 h. This figure shows that injection pressure significantly affected oil
recovery.
Altawati (2016) did huff-n-puff experiments when cores initially had
some water saturation. He observed that the liquid recovery factor (defined
as the total produced water and oil divided by the total initial water and oil)
was lower than the oil recovery under huff-n-puff when no water was