SUMMARY Few data are available on shales in terms of seismic to ultrasonic properties and anisotropy, although all are important with regards to imaging problems often encountered in such lithologies. Additionally, mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions. The results presented here are derived from a set of experiments run on Muderong Shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response. This shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy, both likely the result of the presence of fluid-filled cracks in a low- permeability porous medium. Changes in velocity and Vp/Vs ratios are dependent on both stress and smectite content. S-wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress (<20 MPa) is less sensitive to stress change than P-wave velocity. Vp/Vs ratios are noted to increase in this shale up to 20 MPa effective stress, then decrease slightly due to stress-induced loss of interlayer water in smectite. Intrinsic anisotropy comes from composition, a strong compaction fabric and the presence of microfractures; changes to ultrasonic anisotropy are the result of the magnitude of the stresses, their orientation with respect to the fractures and the degree of stress anisotropy.

/pdf/impact-of-fabric-microcracks-and-stress-field-on-shale-298juteoxr.pdf

Impact of fabric, microcracks and stress field on shale anisotropy

Stress anisotropy and velocity anisotropy in low porosity shale

Anisotropic elastic, strength, and fracture properties of Marcellus shale

Shale gas is becoming an important energy source worldwide. The geomechanical properties of shale rocks can have a major impact on the efficiency of shale gas exploration. This paper studied the mineralogical and mechanical characteristics of a typical gas shale in Ohio, USA. Scanning electron microscope (SEM) with energy dispersive X-ray (EDX) analyses was employed to measure the microstructure and material composition of the shale rock. The anisotropic behaviors of shale rock, including compressive and tensile strengths, were experimentally measured. The characteristics of shale rock were also studied by nondestructive wave speed measurements. The shale demonstrated strong anisotropic behaviors with the tensile strengths perpendicular to the bedding plane around 300–360 times of that parallel to bedding plane. Results of ultrasonic tests indicated that both compression and shear wave velocities show strong anisotropic patterns. The compression wave speed was the smallest in the direction perpendicular to the bedding plane; while the shear wave speed was the smallest in the direction parallel to the bedding plane. The ratio of wave speed anisotropy is around 1.3–1.4 for compression wave; the ratio of shear wave speed anisotropy is larger and more diverse compared with the compression wave anisotropy. This might be related to the larger variability in the frictional adhesive strength along bedding plane than the compressive adhesive strength.

https://cyberleninka.org/article/n/223371.pdf

Laboratory study on the mechanical behaviors of an anisotropic shale rock

This paper summarizes some developments and applications of discrete particle modelling based on the commercial code PFC (Particle Flow Code). The following examples with relevance to geomechanical reservoir behaviour are described: - Numerical and experimental results are shown illustrating nonlinear evolution of depletion-induced compaction and stress path for simulated reservoir rock, i. e. materials that are formed under stress. This nonlinearity is often lost in core measurements, as a result of stress release during coring (core damage). - Angular and breakable superparticles have been implemented in the particle model and applied to simulate the formation of a compaction band at high stress level. The model is also used to investigate the formation of a shear band at lower confinement. - Numerical scheme of fluid coupling has been developed in both 2D and 3D models and applied tostudy stress-dependent permeability. - Stress-dependency of the acoustic properties of a bonded particle assemblage has been studied. The preliminary results of the simulations with various developments show qualitative and also quantitative consistence with experiments. It proves the feasibility of applying a discrete particle model as a numerical laboratory to study particle scale reservoir mechanics.

https://ogst.ifpenergiesnouvelles.fr/articles/ogst/pdf/2002/05/li_v57n5.pdf

Particle Scale Reservoir Mechanics

Effects of currently and previously applied stresses on P- and S-wave velocities in sedimentary rocks (sandstones, chalks, shales) have been studied. Acoustic velocities have been measured on core samples in two orthogonal directions simultaneously under varying (isotropic, anisotropie) conditions of applied stress. Also, a series of model experiments has been performed, where synthetic sandstones were formed under stressed conditions, and where acoustic velocities were measured during and after unloading. The experiments show that the sensitivity to stress generally is largest at low stress levels, in particular below the maximum stress experienced previously. The symmetry of the stress field is to a large extent reflected in the wave propagation. Anisotropic unloading leads to an acoustic anisotropy which remains after the unloading is completed. The results underline that laboratory acoustic measurements should be performed under proper stress conditions. The unloading effect may help to quantify thein situstress field.

Influence of Stress State and Stress History on Acoustic Wave Propagation in Sedimentary Rocks

Summary Svelvik CO2 Field Lab is a small-scale field laboratory designed for the development and testing of geophysical monitoring technologies required for large scale CO2 storage applications. The site consists of an injection well surrounded by four 100m deep monitoring wells for cross-well monitoring and allows injection at 64 – 65m depth. High-resolution seismic data sets were acquired before, during and after CO2 injection. We present results investigating the distribution and migration of the injected CO2, including observed leakage at the injection well. These are based on seismic travel time analysis showing the CO2 plume during and after the injection. We also present data showing various effects of small amounts of CO2 on amplitudes and frequency spectra. 

C. Ringstad

Papers

Influence of Stress State and Stress History on Acoustic Wave Propagation in Sedimentary Rocks

CO2 Detection and Monitoring at the Svelvik CO2 Field Lab