Page 83 - Enhanced Oil Recovery in Shale and Tight Reservoirs

70 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 3.11 History of differential pressure between the two ends of core sample dur-
ing the n-heptane reverse flooding.

The stabilized differential pressure was 2.69 MPa (390 psia). Given that the
viscosity of n-heptane at 2.69 MPa (390 psia) at room temperature is
0.399 cP (Sagdeev et al., 2013; Zhang and Liu, 1991), the permeability
was calculated to be 198.8 nD using Darcy’s Law. This permeability is
considered the permeability with adsorption but without entrainment
because the entrainment of asphaltene particles only occurs when the inter-
stitial velocity is higher than a critical velocity (Behbahani et al., 2012; Beh-
bahani et al., 2015; Bolouri et al., 2013; Wang et al., 1999), and n-heptane
cannot dissolve asphaltene. This rate of 0.01 cc/min was considered below
the critical velocity, as the differential pressure was not significantly reduced
as the flood continued. The higher pressure drop in the early time was
caused by the existence of residual oil of higher viscosity.
To quantify the effect of entrainment (mechanical plugging) of asphaltene
particles, the flow rate of n-heptane injection was increased from 0.01 to
0.05 cc/min. The differential pressure increased fast first owing to the
increased rate, but it decreased later indicating the removal of asphaltene plug-
ging. The flow rate of 0.05 cc/min was approximately equivalent to
0.0008 cm/s for this core plug. This velocity was considered above the critical
velocity, as the plugging was removed at this velocity. Using the stable differ-
ential pressure at around 7000 min and the viscosity of n-heptane of 0.427 cP
at 9.24 MPa (1340 psia) at room temperature (Sagdeev et al., 2013; Zhang