Pressure-varying CO2 distribution affects the ultrasonic velocities of synthetic sandstones

Giorgos Papageorgiou,Ismael Falcon-Suarez,Mark Chapman,Angus Best

doi:10.1016/j.ijggc.2018.03.022

Abstract

We performed a novel experiment in which three synthetic sandstones – manufactured using a common method but having different porosities – were saturated with brine and progressively flooded with CO2 under constant confining pressure. The fluid pressure was varied around the critical pressure of CO2 and repeated measurements were made of resistivity, in order to assess the saturation, and elastic wave velocity during the flood. The measured saturated bulk moduli were higher than those predicted by the Gassmann–Wood theory, but were consistent with behaviour described by a recently derived poroelastic model which combines “patch” and “squirt” effects. Measurements on two of the samples followed a patch-based model while those on the highest porosity sample showed evidence of squirt-flow behaviour. Our analysis suggests that the appropriate fluid mixing law is pressure dependent, which is consistent with the notion that the effective patch size decreases as fluid pressure is increased. We derive simple empirical models for the patch dependence from fluid pressure which may be used in seismic modelling and interpretation exercises relevant to monitoring of CO2 injection.

Highlights

Estimation of CO2 saturation through seismic methods is a central component of seismic monitoring geological carbon storage projects (Chadwick et al, 2006)
The rig has been designed to simultaneously measure geophysical and hydromechanical properties of rock samples exposed to the co-injection of up to two fluid phases, at controlled pressure and temperature conditions matching those of realistic shallow North Sea-like CO2 storage reservoirs
To the contrary, when sample 3 is modelled on the frequency dependent approach (Fig. 8c) the fitted model performs significantly better when the frequency-dependent squirt flow mechanism is taken into account

Summary

Introduction

Estimation of CO2 saturation through seismic methods is a central component of seismic monitoring geological carbon storage projects (Chadwick et al, 2006). Our ability to perform this task reliably and effectively depends on an understanding of the fundamental physics of wave propagation through rocks saturated with multiple fluids. This area is not yet completely understood. Even though difficult to measure in the field, patchy saturation can be assessed in the controlled environment of the laboratory. In this regard, Lebedev et al (2009) showed direct experimental evidence of fluid patches and assessed their influence in measured seismic velocities and Caspari et al (2011) inverted patch-size from sonic logs

Methods

Findings

Discussion

Conclusion