Page 354 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Forced imbibition 327

through a fracture, the pressure gradient between the wells are the pressure
gradient in the fracture (dp/dl) f . Since the pressure gradient varies with time,
the values presented in Table 11.7 and the subsequent tables are the average
pressure gradient during the injection. It is shown that as the injection rate or
pressure gradient (dp/dl) f is increased, the oil recovery factor is higher, both
from sand and shale models. The higher pressure gradient provides a higher
force to drive the injected alkaline solution into matrix, resulting in higher
oil recovery. Such result has been presented for surfactant injection in
laboratory by Parra et al. (2016).
Now we look at the surfactant injection. The oil recovery factors from
the sand models and shale models at different injection rates (pressure gradi-
ents) are presented in Table 11.8. From the sand models, it is clear that as the
injection rate or pressure gradient is increased, the oil recovery factor
increases. If the first (very low rate) and last (very high rate) pair of data
are removed, oil recovery factor versus pressure gradient is shown in
Fig. 11.17. An approximately linear relationship can be observed. For the
shale models, the oil recovery factors are very low with the maximum
0.0261. When the rates are low, as the pressure gradient is increased, the
oil recovery factor increases. However, in the higher rate range, as
the pressure gradient is increased, the oil recovery factor even decreases.
The relationship between oil recovery factor and pressure gradient is not
clear. This may be caused by numerical errors in simulation because of
too low oil recovery factors. These results also show that even if a high pres-
sure gradient is imposed, the ultralow IFT surfactant solution cannot enter
the ultralow permeability matrix. It indicates that a surfactant solution of ul-
tralow IFT with oil will not be beneficial to oil recovery, if there is the

Table 11.8 Effect of pressure gradient in surfactant injection.
Sand Shale
3
Injection rate, ft /day (dp/dl) f , psi/ft RF, fraction (dp/dl) f , psi/ft RF, fraction
0.0000033 0.012295868 0.040 0.001733818 0.0000
0.00000165 0.044261351 0.126 0.012411145 0.0055
0.000033 0.061503113 0.166 0.02336886 0.0261
0.00011 0.115751967 0.198 0.049332963 0.0201
0.00033 0.260944796 0.267 0.132130724 0.0132
0.001 0.693085916 0.619 0.389119313 0.0107
0.0033 1.427742839 0.971 1.271664234 0.0096
0.033 8.71952236 1.000 12.67878541 0.0097