Borehole velocity-prediction models and estimation of fluid saturation effects

Zandong Sun, Stephen R. Stretch, R. James Brown

Three types of elastic-wave velocity-prediction models were analyzed by means of well logs and seismic section ties by considering fluid saturation in five wells which include two different reservoir types, clastic and carbonate, in two different areas.

The bulk and shear moduli of a formation are functions of the bulk and shear moduli of the matrix and fluid components. By using the volumes of different minerals and fluids obtained from log analysis, bulk and shear moduli of the matrix and fluid were calculated. The nature of the matrix, the fluid, and the pore geometry are the critical factors in computing the velocities of a porous rock formation. The time-average equation, the Gassmann equation, and the Toksz-Kuster model were all applied in order to find a best velocity-prediction model. Fluid saturation parameters for both invaded and uninvaded zones can be obtained from log analysis; and there are significant changes in velocities between invaded and uninvaded zones.

A comparison of different models has been carried out, and the Toksz-Kuster model is found to be the best theoretical model, enabling reliable predictions of pore geometry. The Gassmann equation is found to be a good approximation; however, the time-average equation is not recommended for velocity prediction.

Comprehensive research encompassing laboratory study of core, well-log analysis, and study of seismic response is critical in setting up the criteria for identifying fluid saturation and fluid type in seismic sections.