Measurements of Seismic Anisotropy in Synthetic Rocks with Controlled Crack Geometry and Different Crack Densities

Abstract

Seismic anisotropy can help to extract azimuthal information for predicting crack alignment, but the accurate evaluation of cracked reservoir requires knowledge of degree of crack development, which is achieved through determining the crack density from seismic or VSP data. In this research we study the dependence of seismic anisotropy on crack density, using synthetic rocks with controlled crack geometries. A set of four synthetic rocks containing different crack densities are used in laboratory measurements. The crack thickness is 0.06 2 / 32 mm and the crack diameter is 3 mm in all the cracked rocks, while the crack densities are 0.00, 0.0243, 0.0486 and 0.0729. P and S wave velocities are measured by an ultrasonic investigation system at 0.5 MHz while the rocks are saturated with water. The measurements show the impact of crack density on the P and S wave velocities. Our results are compared to the theoretical prediction of (Chapman, 2003) and (Hudson, 1981). The comparison shows that measured velocities and theoretical results are in good quantitative agreement in all three cracked rocks, although Chapman's model fits the experimental results better. The measured anisotropy of the P and S wave in the four synthetic rocks shows that seismic anisotropy is directly proportional to increasing crack density, as predicted by several theoretical models. The laboratory measurements indicate that it would be effective to use seismic anisotropy to determine the crack density and estimate the intensity of crack density in seismology and seismic exploration.

Frequency‐dependent PP and PS reflection coefficients in fractured media

“…Chapman (2003) developed a model to predict frequency-dependent anisotropy due to mesoscale fractures in porous rocks. The results of this model have a good correspondence with measured data in laboratory (Maultzch et al 2003;Tillotson et al 2014;Ding et al 2017).…”

Joint Elastic‐Electrical Properties of Artificial Porous Sandstone With Aligned Fractures

“…Two synthetic sandstone samples-one fractured (with fractures) and the other intact (without fractures)made from a mixture of different sized sand, feldspar, and kaolinite and aqueous sodium silicate gel were provided by the Chinese National Petroleum Corporation's Key Laboratory of Geophysical Exploration using a manufacturing procedure similar to that employed by Ding et al (2017). A predetermined number of 3-mm diameter polymeric material discs of 0.045 mm thickness were distributed on successive layers of sand mixture for the creation of penny-shaped fractures.…”

“…When studying the physical properties of anisotropic rocks, instead of using natural fractured samples (Sarout & Guéguen, 2008;Valcke et al, 2006), where the fracture parameters (e.g., fracture density and geometry) cannot be controlled, experimentalists usually employ synthetic materials to construct physical models that contain aligned penny-shaped fractures (Ass'ad et al, 1992;Ding et al, 2017;Rathore et al, 1995;Tillotson et al, 2012;Wang et al, 2015), although such geometries are not readily available in natural rocks. The idealization of fractures as penny-shape spheroids is recognized as a good assumption, because they (1) can capture some essential properties of the subsurface voids, (2) can provide intuitively simple parameterization of enormous complexity of the real fracture space, and (3) are relatively easily amenable to theoretical analysis (Gurevich et al, 2009).…”

Combined Effects of Pressure and Water Saturation on the Seismic Anisotropy in Artificial Porous Sandstone With Aligned Fractures

“…In geological conditions, fractures are often aligned along a certain preferential direction due to stress orientation and/or tectonic movement (Wang & Tang, ; Xu et al, ), making the physical properties of the rocks anisotropic. Seismic anisotropy caused by the aligned fractures and influenced by the fracture geometry provides one of the most effective methods for determining the fracture amount, distribution, and orientation in rocks (Ding et al, ). However, the geological materials filling the fractures (usually fluids but can sometimes also be solid minerals) are usually softer than the rock matrix, making the seismic anisotropy of the rocks rather sensitive to pressure changes.…”