Exploring Trends in Microcrack Properties of Sedimentary Rocks: An Audit of Dry and Water Saturated Sandstone Core Velocity–Stress Measurements

Doug A Angus,Quentin J Fisher,James P Verdon

doi:10.4236/ijg.2012.324083

Abstract

Stress dependent rock physics models are being used more routinely to link mechanical deformation and stress perturbations to changes in seismic velocities and seismic anisotropy. In this paper, we invert for the effective non-linear microstructural parameters of 69 dry and saturated sandstone core samples. We evaluate the results in terms of the model input parameters of two non-linear rock physics models: A discrete and an analytic microstructural stress-dependent formulation. The results for the analytic model suggest that the global trend of the initial crack density is lower and initial aspect ratio is larger for the saturated samples compared to the corresponding dry samples. The initial aspect ratios for both the dry and saturated samples are tightly clustered between 0.0002 and 0.001, whereas the initial crack densities show more scatter. The results for the discrete model show higher crack densities for the saturated samples when compared to the corresponding dry samples. With increasing confining stress the crack densities decreases to almost identical values for both the dry and saturated samples. A key result of this paper is that there appears to be a stress dependence of the compliance ratio BN/BT within many of the samples, possibly related to changing microcrack geometry with increasing confining stress. Furthermore, although the compliance ratio BN/BT for dry samples shows a diffuse distribution between 0.4 and 2.0, for saturated samples the distribution is very tightly clustered around 0.5. As confining stresses increase the compliance ratio distributions for the dry and saturated samples become more diffuse but still noticeably different. This result is significant because it reaffirms previous observations that the compliance ratio can be used as an indicator of fluid content within cracks and fractures. From a practical perspective, an overarching purpose of this paper is to investigate the range of input parameters of the microstructural models under both dry and saturated conditions to improve prediction of stress dependent seismic velocity and anisotropy observed in time-lapse seismic data due to hydro-mechanical effects related to fluid production and injection.

Highlights

Non-linear or stress dependent rock physics models are being applied increasingly to model the influence of stress perturbations due to reservoir production and injection activities on seismic velocities
We examine the microcrack properties of two non-linear rock physics models: A discrete microcrack model defined by a second- and a fourth-rank crack density tensor [1,2,3] and an analytic microcrack model defined by an initial crack density and initial aspect ratio [3,9]
The initial crack density is lower for the saturated sample, there is no change in the initial aspect ratio. [Note, the model parameters are assumed to be isotropic only because the data contain only one P- and one S-wave measurement for each dry and saturated sample

Summary

Introduction

Non-linear or stress dependent rock physics models are being applied increasingly to model the influence of stress perturbations due to reservoir production and injection activities on seismic velocities. Laboratory measurement of non-linear rock physical properties of dry core samples can provide valuable information on the stress dependent elastic properties of reservoir rocks [13] and have the potential for up scaling to seismic frequencies [4] as well as relating to static elasticity [5]. Core measurements can be used to calibrate rock physics models [6] for the forward prediction of the stress dependence of seismic velocities. We compare the microcrack parameters of the discrete and analytic microstructural stress-dependent model described in [3] for dry and water saturated core. The data used in this study come from the sandstone ultrasonic velocity-stress measurements of [7] and so allow a direct comparison between dry and water saturated microcrack parameters.

Results

Discussion

Conclusion