Page 285 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

256 Rouzbeh G. Moghanloo et al.

The mechanisms through which asphaltene damage can occur are sur-
face deposition, entrainment, and pore throat plugging. As asphaltene
deposits accumulate on the pore surface, the pore throat for the oil phase
decreases, with some deposits being swept away and entrained by the liq-
uid phase when increasing shear rate overcomes the critical value. When
asphaltene deposits accumulate in front of a pore, they can plug them
(Seifried, 2016).
Wang modified Civan’s model for near wellbore asphaltene deposition,
assuming negligible capillary pressure and one dimensional horizontal
flow. As shown in Eq. (6.3) below, the overall deposition process is mod-
eled by momentum and mass balance, with concepts from precipitation,
deposition models, and porosity and permeability reduction equations
(Wang, 2000,pp 53 58). The first term is the rate of asphaltene deposi-
tion on the pore surface, with coefficient α. The second is the entrain-
ment of asphaltene deposits in the fluid, with the last term being the pore
throat plugging rate. Rate of net deposition is therefore:

@E A
5 αC A φ 2 βE A v L 2 v cr;L 1 γu L C A (6.3)
@t
where, E A is volume fraction of asphaltenes deposition; v L is interstitial
velocity (5u L =φÞ; v cr;L is critical interstitial velocity; u L is superficial
velocity; α is surface deposition coefficient; β is entrainment rate coeffi-
cient; γ is pore throat plugging coefficient.
According to Boek et al. (2008) and Wang (2000), the surface deposi-
tion rate is directly proportional to the concentration of suspended parti-
cles concentration in the flowing fluid. For the second term it’s been
identified that entrainment becomes dominant once the critical interstitial
velocity is exceeded. The magnitude of the difference between the aver-
age and critical velocities influences the transitional location of the depos-
ited particles (Dabir et al., 2016). The value of the entrainment
coefficient is set as β, when v L . v cr;L and otherwise to zero. Finally, the
last term describes the plugging contribution to the net deposition. Here,
asphaltene precipitates in suspension, where the plugging rate is directly
proportional to the superficial velocity. Wang (2000) defined the pore
plugging coefficient, γ:

ð
γ 5 γ 1 1 σE a Þ; if 0 . R . R c (6.4)
i
γ 5 0; otherwise