Combined acoustical-electrical modeling for tight sandstones verified by laboratory measurements

Tight sandstone reservoirs basically have low porosity and permeability, a complex pore structure and a heterogeneous distribution of immiscible fluids. With the development of theoretical models, it is common to characterize rock properties, i.e., pore structure, microfractures, fluid type and saturation, etc., based on acoustic and electrical properties. We have taken four tight sandstone samples and performed X-ray diffraction and cast thin section analyses. We measure porosity and permeability as well as ultrasonic properties and electrical conductivity at different confining pressures and fluid saturations. These measurements show that the P-wave velocity, P-wave attenuation and conductivity strongly depend on the type and saturation of the fluid and the microstructure of the rock. We propose a combined acoustic-electrical model based on the concept of equivalent medium and on the double porosity, patchy saturation and squirt flow models. We then create rock physics templates calibrated with wellbore log data to estimate fluid saturation and equant and soft porosities, which are well corroborated by gas production reports. This work demonstrates the link between combined acoustic-electrical responses and rock properties and provides an effective approach for applications in reservoirs.