An empirical method for estimation of anisotropic parameters in clastic rocks

Abstract

Clastic sediments, particularly shale, exhibit transverse isotropic properties with the symmetric axis perpendicular to bedding (VTI). These anisotropic rock properties are important in seismic imaging, prestack seismic analysis, and reservoir characterization. Laboratory tests are the main techniques used in measuring rock anisotropic properties. Vertical seismic profiling (VSP) and seismic refraction are the in-situ techniques used to determine anisotropic parameters. All of these methods provide significant insight to the anisotropic properties of the subsurface. However, limitations exist in the applications of these methods because of cost issues and restrictions on the number of measurements that can be conducted. Therefore, new techniques that are cost effective, easy to use, and able to measure continuous anisotropic profiles are desirable. This paper describes a methodology which involves the use of conventional rock properties to derive anisotropic rock properties.The commonly accepted cause of anisotropy in clastic rocks is the alignment of minerals. Hornby et al. (1994) confirmed this theory with evidence of the alignment of platy clay minerals using the SEM (scanning electron microscope) image technique. Johnston and Christensen (1995) performed a quantitative investigation of the alignment of clay minerals using the orientation index determined by X-ray diffraction patterns. They found a linear relationship between the orientation index (a measure of the degree of alignment of platy clay minerals in a given direction) and clay content. They also found that variation in seismic velocity due to anisotropy is proportional to the orientation index or clay content. These observations and findings provide the motivation to explore the physical linkage and quantitative relationship between conventional and anisotropic rock properties (Li, 2002). Because conventional rock properties are determined directly or indirectly from well logging, lab measurements, and insitu seismic measurements, anisotropic rock properties can be estimated after a quantitative relationship between conventional and anisotropic rock properties has been established. In this paper an empirical method is devised to establish this relationship and thus estimate anisotropy. The method is validated by comparing the estimated anisotropy values with those derived from VSP and seismic refraction data. In addition, anisotropy values inverted from prestack depth migration (PSDM) are compared. The potential for application of this method to anisotropic synthetic seismograms, and anisotropic parameter inversion from prestack seismic data is also explored.Physical background. Clay and quartz are two principal mineral constituents of clastic rocks. In general, two basic factors-the volume fraction of clay and the degree of compaction-determine the anisotropic properties of rocks. The first factor constitutes the physical basis for anisotropy, i.e., the platy clay minerals. The second factor forces clay minerals to align in a preferred direction wh...

Attempts to calculate the pseudo-anisotropy of elastic parameters of shales gas formations based on well logging data and their geostatistical analysis

“…It was noted that the ε and γ values are positive, while the δ values are negative or have low positive values. Li (2006) presents the method of calculating the anisotropy parameters ε and γ based on recorded velocities V P(0) , Vs (0) and clay volume, using laboratory data published by Thomsen (1986), Vernik & Nur (1992), Johnston & Christensen (1995) and Vernik & Liu (1997).…”

“…(www.mapa-polski.org/kujawsko-pomorskie,rw.html, access: June 2018) The coefficients a and b in equations 3and 4can be defined as follows. The slope for any line was determined (Li 2006):…”

“…The velocity parameters can be determined by measuring with the Compact Cross Dipole Tool (Weatherford Company), Cross Dipole Wave Sonic Tool (WSTT) (Halliburton Company) or the Dipole Shear Sonic Imager (DSI) (Schlumberger Company). Figure 3 shows the V P , V S velocities and the V P /V S ratio measured by the use of the acoustic dipole tool compared with the theoretically calculated V PEQ , V SEQ and V PEQ /V SEQ values using the Biot-Gassmann model and the ESTYMACJA TP program (Bała & Cichy 2005, 2006 as the example for well BX located on the shelf. The V P , V PEQ curves coincide, the determination coefficient R 2 = 0.858.…”

“…An interesting method to assess the parameters of anisotropy in sandstone and clay rocks was presented by Li (2004Li ( , 2006.…”

A novel evaluation on fracture pressure in depleted shale gas reservoir

“…The results indicate that the clay mineral is obviously arranged along the fixed orientation. Some studies have shown that the directionally arranged clay mineral is an important factor that causes rock anisotropy, for shale stone, the degree of anisotropy is depend on the clay mineral content, and for gas shale with high carbon, the degree of anisotropy is depend on the clay mineral and organic matter content, and the higher the content of these two components, the greater the anisotropy . Furthermore, the clay minerals often have flaky structures, so the microcracks were developed along the fixed orientation in the layers and interlayer , and the width of the microcracks observed in the core samples ranges from 1 to 25 um, which also causes rock anisotropy.…”