Cation Exchange Capacity
Active sites exist on clay surfaces where cations may be exchanged between non-equilibrium coring, drilling, completion or injection fluids. This ion exchange may alter porosity, reduce permeability, reduce formation resistivity and result in erroneously high calculated water saturation from downhole logs.
Clay conductivity is normally referred to as excess conductivity and depends upon the type and quantity of clay. Cation exchange on the surface of the clays generate a negative charge which attracts a layer of counterions from the brine. This layer of concentrated counterions contributes significantly to the total conductivity.
The most representative way to measure excess conductivity is to determine the rock conductivity (Co) at a variety of different brine conductivities (Cw) in a brine saturated sample.
This yields a relationship between Co and Cw. The straight line portion of which can be extrapolated to Cw=0 to yield the excess conductivity.
Then, using a suitable model (Waxman-Smits, dual water) it is possible to determine the intrinsic formation factor F* and porosity exponent m*, n* and the cation-exchange capacity.
The excess conductivity is termed BQv. Qv is a function of CEC and B is related to the mobility of the clay cations.
B = 4.6 * (1 - 0.6 exp (-0.77 / RW@77F))
Qv = CEC * DENS / PHIe / 100
Co = (1 / F*) * (B * Qv + Cw)
Co: Conductivity of rock fully saturated with brine solution (mho/m)
F* = formation factor for shaly sandstone
Qv = cation exchange capacity per unit pore volume (meq/cc)
Cw = conductivity of the brine (mho/m)
B = equivalent conductance of clay exchange cations at room temperature (mho cm2/meq)
Rw@77F = formation water resistivity converted to 77 degrees F