Page 152 - Enhanced Oil Recovery in Shale and Tight Reservoirs

136 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 6.4 Gas saturation profiles during a nitrogen flooding test (GF_4) at different
flood times.
gas flooding. Fig. 6.4 shows the gas saturation profiles for test GF_4 at
different times which shows that gas broke through at about 0.625 days.
By this time, the oil recovery was about half the total recovery by 5 days.
Cross-checking the recovery factor in the previous figure, it can be seen after
0.625 days that the recovery factor curve decreased the increasing trend. In
other words, after gas breakthrough, it will be more difficult to produce oil.
Early experimental data show that the oil recovery was very sensitive to
the injection pressure in early time. However, extended simulation results
from a model based on GF_6 core experiments show that the ultimate
RF at the injection pressures of 1000, 3000, and 5000 psi are 80.5%, 82%,
and 85%, respectively. The corresponding times that the ultimate RF is
achieved are about 60, 140, and 220 days. The recovery factor is increased
by 4.5% for the unrealistically long flood time in the small core scale, when
the injection pressure is increased from 1000 psi to 5000 psi. The benefitofa
high injection pressure is to increase the oil recovery rate at a higher injec-
tion volume or cost within the same time interval. The process should be
optimized based on economic benefits.
Zhu et al. (2015) proposed injection of gas from a fracture and produc-
tion of fluids from another adjacent fracture, with the injection fracture
and the producing fracture alternating in the same horizontal well. They
called it the fracture-to-fracture gas-flooding scheme. Such a scheme can
mitigate the need of high pressure gradient or overcome a minimum
threshold pressure gradient (Wang and Sheng, 2017a; 2017b) in a shale
and tight formation. Their simulation results show that injection pressure
is an important operation parameter with higher pressure leading to higher
incremental oil recovery; formation heterogeneity reduces oil recovery;
reducing the hydraulic fracture spacing increases early-time rates because