Abstract

Understanding acoustic velocity variations in reservoir rocks is key to reservoir quality assessment using sonic tools and seismic data. We conducted experiments aimed at investigating the effects of varying stress on the acoustic properties of 19 limestone core plugs ("Core A") with starting porosity σ0 = 23–30%, and 11 dolostone-dominated core plugs ("Core W") with σ0 = 6–18%. Core A plugs are composed of calcite and show a wide range of pore types (e.g., vuggy, mouldic, micropores). Samples have been grouped as ‘micro-', ‘meso-', or ‘macroporous' based on pore type and size. By contrast, Core W plugs show compositional variation, but are texturally relatively uniform. Each experiment employed isotropic loading cycles at constant pore pressure (Pp), axial stress cycling at constant mean stress, triaxial compression to reservoir conditions, and Pp-cycles. Comparison between compressional and shear wave velocities (vp and vs) at representative, key states of stress showed that under reservoir conditions versus under near-isotropic conditions, for Core A plugs Δvp ≈ +2.5–4% whereas for Core W plugs Δvp ≈ 0. The stress-sensitivity of vp is larger for macro- and mesoporous samples compared with for microporous samples, suggestive of a dependence of stress-induced changes in acoustic velocity on carbonate pore type.

Introduction

It is estimated that carbonate rocks constitute more than 50% of the world's hydrocarbon reservoirs (e.g., Sing & Joshi, 2020), including recent major discoveries in the Pri-Caspian Depression and offshore Brazil. Elsewhere, carbonates are targeted for production of geothermal energy (Montanari et al., 2017), or considered as sites for injection of CO2 (Siqueira et al., 2017). Thus, exploration, production, and monitoring of carbonate reservoirs has a central role in the energy industry. This relies on data from sonic tools and seismic survey, which in turn requires calibrated rock physics models for prediction of petrophysical properties such as porosity and mineralogy.

By comparison with siliciclastic reservoirs, core-to-log-to-seismic calibration in carbonate rocks is particularly challenging (e.g., Sun, 2004; Bailly et al., 2019). Carbonates are frequently characterized by a complex, biologically controlled depositional environments and are especially prone to major diagenetic changes (dissolution, cementation, dolomitization). Therefore, the pore shapes and the composition of carbonates may vary on the μm-/cm-scales up to the reservoir scale (fracturing, karstification) (Archie, 1952; Lucia, 1995). Such multiscale textural and compositional heterogeneity yields acoustic wave velocity properties that are difficult to capture using classical empirical sonic velocity-porosity transforms such as Wyllie's Time Average model or the Raymer-Hunt-Gardner model (Wyllie et al., 1956; Raymer et al., 1980; see also e.g., Anselmetti & Eberli, 1993).

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