Page 90 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Asphaltene precipitation and deposition in a huff-n-puff process 77

oil recovery, a maximum allowable huff pressure should be used and a min-
imum allowable puff pressure should be used. Shen and Sheng (2019) stud-
ied how asphaltene deposition affects this optimization principle by
simulation, as briefly presented below.
The basic reservoir grid model from Sheng (2017) is used. The PVT and
compositional data of a live oil sample reported by Ashoori and Balavi (2014)
are used. They measured the amount of asphaltene precipitation during pri-
mary depletion and CO 2 injection process. Shen and Sheng (2019) used a
CMG PVT simulator, Winprop, to match the asphaltene precipitation
data. The asphaltene deposition model described in the preceding section
is used. For the base model, during the primary depletion of 1800 days,
the bottom hole pressure is 1000 psi. During the huff-n-puff CO 2 injection,
the puff pressure is 8000 psi, and the puff pressure is 1000 psi. The huff time
and puff time are set to 100 days without a soaking time. The oil recovery
factors from this base case with and without including asphaltene deposition
are shown in Fig. 3.16. During the primary depletion, the oil recovery fac-
tors from the two cases are very close, no CO 2 is injected, and asphaltene
deposition is insignificant. At the end of huff-n-puff CO 2 injection of
5600 days, the asphaltene deposition reduces 3.25% oil recovery. The simu-
lation is repeated and the reductions in oil recovery factors caused by asphal-
tene deposition are shown in Fig. 3.17, when the huff pressures are 7000,

Figure 3.16 Oil recovery factors with and without including asphaltene deposition.