1.
n. [Formation Evaluation]
A
model of
shaly formations that considers there to be two waters in the
pore space: far water, which is the
normal formation water; and near water (or
clay-
bound water) in the
electrical double layer near the clay surface. The clay-bound water consists of clay counter-ions and the associated water of
hydration. The volume of this layer is determined by its thickness, which is constant at high salinities, and its area, which is proportional to the counter-ion concentration per unit pore volume (Q
_{v}). The volume of clay-bound water per unit pore volume, S
_{wb}, can therefore be written as:
S
_{wb} = alpha * v
_{q} * Q
_{v}
where v
_{q} = 0.28 cm
^{3}/meq at 25
^{o}C is the factor relating volume to counter-ion concentration at high salinity and is a function only of temperature, and alpha = 1 above a certain salinity, below which it increases with temperature and with decreasing salinity. The fractional volume of the far water is then (1 ? alpha?* v
_{q} * Q
_{v}).
The dual-water concept was developed for the
interpretation of
resistivity in shaly sands, but is also useful in the interpretation of nuclear and nuclear
magnetic resonance logs. In these cases, the
parameter most used is the total volume of clay-bound water in the
rock, equal to S
_{wb} multiplied by the total
porosity.