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 cm³/meq at 25^oC 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 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.

See: clay-bound water,  far water,  formation water,  nuclear magnetic resonance,  total porosity,  wet-clay porosity