Page 345 - Reservoir Formation Damage

Inorganic Scaling and Geochemical Formation Damage 325

is in abundance, its concentration can be taken constant. Then, the
chemical equilibrium constant for Eq. 13-1 reads as:

[CaC0 3][CQ 2]
=
(13-2)

Therefore, applying Le Chatelier's cause-and-effect principle, when CO 2
is liberated and removed by pressure reduction, the CO 2 concentration
will reduce. To compensate this effect, more CaCO 3 will be produced to
maintain the constant K eq.
Amaefule et al. (1988) explain that induced scaling occurs by mixing
the formation brine with extraneous incompatible fluids invading the
reservoir formation during drilling, cementing, completion, and workover
operations. The same may occur by injection of fluids for enhanced
+2
recovery purposes. Any increase of the dissolved calcium \Ca ) ion
concentration caused by these operations is compensated by cal-
cium carbonate (CaCO 3) precipitation according to Eq. 13-1, following
Le Chatelier's principle of the cause-and-effect.
Dewers et al. (2000) draw attention to some important phenomena that
have been overlooked for the most part. First, the presence of oil and
gas phases may effect the thermodynamics and chemistry of aqueous
phases. Second, scale formation in the near wellbore formation is more
kinetically controlled than thermodynamically controlled because the
rapid flow that occurs around wellbores compared to the rest of the
reservoir does not allow for sufficient time to attain equilibrium. This may
result with an incomplete release of the light dissolved gases, such as
carbon dioxide, from the aqueous phase. Hence, the saturation conditions
at the actual near-wellbore fluid pressure may not be attained and the
partitioning of various light gases between the liquid and gas phases may
not reach the equilibrium condition. Dewers et al. (2000) caution that
these phenomena should be considered for accurate scale prediction.
Geochemical interactions of the aqueous phase and the solid porous
matrix result in alterations of minerals and the texture, porosity and
permeability of porous formation. As stated by Lichtner (1985), geo-
chemical systems involve various reversible and irreversible chemical
interactions, such as oxidation-reduction reactions, ion complexing,
mineral dissolution/precipitation, and adsorption. Dissolution of solid
minerals is a slow process and complete dissolution cannot occur within
the convection time scale of the flow in the near wellbore (Nordstrom
and Munoz, 1994). However, alteration of the composition and saturation
of the aqueous phase and the fluid shear can induce the entrainment,