Summary

In this study, we used nanoindentation technique to map Young’s modulus of microporous limestones at the microscale (map of 300*300 µm2, with a step measurement of 10 µm). Results show a broad distribution of moduli, from very low values to values consistent with calcite mineral. Because the length scale of heterogeneities of the probed areas was larger than the indent size, the samples can be seen as composites of different phases and upscaling rock physics models such as self-consistent approximation can be used to get the effective elastic properties. Scanning Electron Microscopy (SEM) imaging of the same probed areas allowed to correlate micrite microstructure to elastic properties: the more microporous micrites displayed Young’s moduli as low as 9 GPa and the tighter ones as high as 64 GPa.

Introduction

Rocks are heterogeneous at all scales, from the pore scale upwards. Compared to siliclastic rocks, carbonate rocks exhibit a far more complex microstructure, which is the result of both deposition and transport processes of fossiliferous materials containing intraporosity, as well as of diagenetic alterations (dissolution/precipitation) that leads to complex changes in the initial microstructure. This complexity in the microstructure will translate in complex relationship between geophysical observables and rock parameters (e.g. elastic moduli vs. porosity, permeability vs. porosity) and makes thus reservoir quality prediction difficult. Many factors have been recognized to control seismic parameters: lithology, pore size and pore geometry, cement, grain size, pore connectivity, etc.

Among carbonate rocks, microporous limestones account for many carbonate reservoirs, especially in the Middle East (Cantrell et al., 1999; Alsharhan and Nairn, 2003; Volery et al., 2009). They exhibit a microcrystalline matrix of calcite, or micrite, whose grain size is typically less than 4 µm (Moshier, 2009). The variety of microstructure in micrite matrix has been attributed to be the result of sedimentalogic and diagenetic controls (Lambert et al., 2006 and Deville de Periere et al., 2011) and it was shown that there is a correlation between microstructure and reservoir properties (namely porosity and permeability). Recent laboratories studies (e.g. Vanorio and Mavko, 2011; Regnet et al., 2014) have shown that these microporous limestones display also characteristic responses in term of acoustic velocities.

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