An experimental study of solid matrix weakening in water‐saturated Savonnières limestone

Abstract

A B S T R A C TPetrophysical properties of carbonate reservoirs are less predictable than that of siliciclastic reservoirs. One of the main reasons for this is the physical and chemical interactions of carbonate rocks with pore fluids. Such interactions can significantly change the elastic properties of the rock matrix and grains, making the applicability of Gassmann's fluid substitution procedure debatable. This study is an attempt to understand the mechanisms of fluid-rock interactions and the influence of these interactions on elastic parameters of carbonates. We performed precise indentation tests on Savonnières limestone at a microscale level under dry, distilled water, and n-Decane saturated conditions. Our experiments display softening of the rock matrix after water saturation. We have found that mainly the ooid cortices, peloid nuclei and prismatic intergranular cement are affected by water flooding. We also observed a shear modulus reduction in Savonnières limestone in an experiment performed at ultrasonic frequencies. One of the most important results obtained in our experimental study is that the Gassmann fluid substitution theory might not always be applicable to predict the elastic moduli of fluid-saturated limestones.

Digital carbonate rock physics

“…However, if nanoindentation technique is a well-established technique in material sciences, which deals with homogeneous, purely elastic materials, this is, as of today, not the case for rocks, which are heterogeneous materials with both elastic and nonelastic behaviour (creep). Though nanoindentation tests provide significant insights into elastic properties of heterogeneous rocks such as carbonates (Lebedev et al, 2014;Vialle and Lebedev, 2015) or shale (Ulm and Abousleiman, 2006;Abousleiman et al, 2007), there are still some points to be looked at before using the derived values of Young's (or shear and bulk) moduli in a quantitative way for DRP: the value of Poisson's ratio to be used, effect of surface roughness, local mechanical damage induced on the sample's surface by polishing techniques, etc. Nonetheless, the histograms of the indentation moduli of both samples show a broad distribution of moduli values ranging from very low values (a few GPa, where the indenter tip measures stiffness of an area mostly made of a pore) to values consistent with calcite.…”

“…The IBIS nanoindentation system (Model B, Fisher-Cripps Laboratories Pty.Ltd.) is equipped with a Berkovich-type diamond indenter (Lebedev et al, 2014) and was used in static mode: the tests consist of continuously recording the load, P , and the displacement, h, of the indenter as it pushes into and withdraws from the surface of the sample. A constant maximum loading force of 10 mN and an initial contact force of 0.15 mN were used.…”

“…In total, 961 (31 × 31) measurements were performed on a 300 × 300 µm surface with a spacing of 10 µm between measurement points. Typically, the extraction of the mechanical properties is achieved by using the P -h curves and by applying a continuum-scale mechanical model to obtain the indentation modulus M (Fischer-Cripps, 2004;Lebedev et al, 2014):…”

Effect of temperature and pore fluid on the strength of porous limestone

“…In highly porous chalks, yield strength appears to negatively correlate with activity of water in the pore space [ Risnes and Flaageng , ; Risnes et al , ]. A similar trend has been observed in moderate and low porosity limestones [ Rutter , , ; Lebedev et al , ]. Observations from the field, such as the Ekofisk oil field in the North Sea, suggest that water weakening may be pervasive in depleting reservoirs and can lead to field‐scale fracturing [ Teufel et al , ].…”

Experimental evidence that ooid size reflects a dynamic equilibrium between rapid precipitation and abrasion rates

“…We set the volumetric abrasion rate of a given particle that makes successive impacts with the bed to = (2) where Vi is the volume eroded per impact, Ir is the number of particle-bed impacts per unit time. The volume eroded per impact is defined as (Sklar and Dietrich, 2004) where Vp is particle volume, ρs is particle density (~2.8 g/cm 3 , see Table S3), wi is impact velocity normal to the bed, and εv is the kinetic energy per unit volume eroded, which is defined as = 2 2 (4) where σT is tensile strength (we chose 1 MPa following Torok, 2007), Y is Young's modulus of elasticity (we chose 20 GPa following Lebedev et al, 2014), and kv is a dimensionless coefficient that accounts for differences in material properties between the particles and the bed surface (e.g. Scheingross et al, 2014).…”