Empirical rock physics relationships on carbonate dry-frame elastic properties

Gautier Njiekak,Zi-Zhen Wang,Douglas R. Schmitt,Rui-He Wang

doi:10.1007/s12182-021-00565-y

Abstract

Laboratory tests were conducted to analyze the ultrasonic velocity response to the pressure change in dry carbonate rocks from the Weyburn oilfield, Canada. Twenty-four samples are from seven wells with helium porosities ranging from 1% to 29%. Thin-section images, SEM and mercury intrusion porosimetry were performed to show their inner structures and pore throat size distributions. P- and S-wave velocities (Vp and Vs) measurements were first done under hydrostatic loading and then while unloading, with confining pressures varying between 3 and 35 MPa. The results indicate that Vp and Vs in these samples follow a linear relation independent of the pressure change. The ratio Vp/Vs is more responsive to pressure change irrespective of the pore volume. One-third of the carbonate samples show abnormal Vp/Vs reduction with the increase in the effective pressure. The pressure dependence of velocities (PDV) of Weyburn carbonate rocks varies widely even for samples from the same formation with similar sedimentary history. Samples with loosely packed crystals and/or relatively large dominant pore diameter have higher PDV. The exponential empirical model V = A-Ctext{e}^{DP_{text{e}}} was tested; therein, V is the elastic wave velocity, Pe is the effective confining pressure, and A, C and D are the best fitting coefficients determined by curve fitting. The model gives good fits for most of the Weyburn carbonate samples. From a statistical point of view, there is no difference between the Vp- and Vs-derived exponential coefficient D.

Highlights

The change in stress condition influences the elastic frame moduli of naturally occurring rocks and results in the change in their elastic wave velocities
The pressure dependence of velocities (PDV) of upper crust rocks is of vital importance for a variety of applications, such as pore
This study aims to provide a new set of experimentally measured P- and S-wave speeds in a suite of carbonates, to assess the PDV of these carbonates and the factors controlling this and to test some existing empirical velocity–pressure relations that are often employed in the literature

Summary

Introduction

The change in stress condition influences the elastic frame moduli of naturally occurring rocks and results in the change in their elastic wave velocities. Nonlinear response of elastic wave velocities to the effective pressure change has been observed for a long time in a wide variety of rocks, such as mudstones and sandstones (e.g., Han et al 1986; Freund 1992; Khaksar et al 1999; Sayers 1999, 2002; Ong et al 2016; Gao et al 2019; Li et al 2019a). From these studies, it is clear that with increasing effective pressure, the elastic wave velocities of rocks first quickly increase and get slower before reaching a plateau (Fig. 1). The rate of change depends on the distribution of the crack-like porosity

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Discussion

Conclusion