Page 37 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 27

into the system quickly until the system pressure reaches 1000 psi, followed
by 12 h of soaking. Then the system pressure is blown down to the atmo-
sphere followed by 3 h of production. So, the total operation time for one
cycle is 15 h. During the soaking phase, the system pressure declines rapidly
in the first 3 h, and then the decline rate decreases gradually until the pres-
sure levels off. The pressure drop (DP) in the first cycle is about 10 psi. This
pressure decreases more in the subsequent cycles. This is because in the early
cycles, oil saturation is high; it is difficult for gas to diffuse into the oil. At
later cycles, some oil is produced leaving more gas channels for gas to enter
the matrix and dissolve in the oil, the pressure decreases more (about 16 psi).
It is 18 psi in the 6th cycle.
Fig. 2.19 further shows the pressure distribution in the system. After gas
injection (huff phase), the pressure in fracture area builds up to 1000 psi
quickly (in 30 s). With the gas diffusing into the shale matrix, the matrix
pressure increases with soaking time from the outer to the inner sections.
After about 8.5 h of soaking, the whole system reaches almost 1000 psi.
Therefore, a soaking time longer than 8.5 h may not effectively help to
improve oil recovery.

Figure 2.19 System pressure profile in 1 cycle of huff-n-puff process.