Measuring ultrasonic characterisation to determine the impact of TOC and the stress field on shale gas anisotropy

Abstract

While the majority of natural gas is produced from conventional sources, there is significant growth from unconventional sources, including shale-gas reservoirs. To produce gas economically, candidate shale typically requires a range of characteristics, including a relatively high total organic carbon (TOC) content, and it must be gas mature. Mechanical and dynamic elastic properties are also important shale characteristics that are not well understood as there have been a limited number of investigations of well-preserved samples. In this study, the elastic properties of shale samples are determined by measuring wave velocities. An array of ultrasonic transducers are used to measure five independent wave velocities, which are used to calculate the elastic properties of the shale. The results indicated that for the shale examined in this research, P- and S-wave velocities vary depending on the isotropic stress conditions with respect to the fabric and TOC content. It was shown that the isotropic stress significantly impacts velocity. In addition, S-wave anisotropy was significantly affected by increasing stress anisotropy. Stress orientation, with respect to fabric orientation, was found to be an important influence on the degree of anisotropy of the dynamic elastic properties in the shale. Furthermore, the relationship between acoustic impedance (AI) and TOC was established for all the samples.

Shale elastic property relationships as a function of total organic carbon content using synthetic samples

“…In the literature, core measurements have stated that the presence of high TOC content on the organic shale reduces both velocity and density, while increasing velocity anisotropy. Organic shale also affects the effective Vp/Vs of the rock (Altowairqi et al, 2015;Altowairqi et al, 2013;Vernik and Milovac, 2011). Table 1 shows measured ultrasonic wave velocities with different TOC content.…”

“…The Poisson's Ratios calculated from these synthetic shale samples decreased from approximately 0.20 to 0.165 as the clay mineral percentage increased from 30% to 60%. Core measurements have reported previously that the presence of clay minerals in organic shale reduces both velocity and density (Altowairqi et al, 2015;Altowairqi et al, 2013).…”

“…The results showed that the velocities increased with the stress levels and the velocities decreased as the TOC percentage increased. In addition, other factors including TOC, clay minerals, porosity and density have an influence on the elastic wave velocities of organic shale (Altowairqi et al, 2013). Such experiments in testing real shale samples are difficult to control in terms of all the factors that affect the elastic and mechanical properties of real shale samples and are dependant on the sample quality.…”

“…Results of limited experiments that include stress control have reported that fabrics, the orientation of stress anisotropy with different fabric and fraction orientation influence the elastic properties of shale (Altowairqi et al, 2013;Delle-Piane et al, 2011;Dewhurst et al, 2011). Therefore, it is difficult to account for all the factors affecting the velocity and anisotropy of a real shale sample.…”

“…Presently our understanding of their dynamic and elastic behaviour is very limited, due to the low volume of variable organic shale samples available as well as the amount of time taken to test low-permeability samples. Experiments involving organic shale core samples can be found in a limited number of publications (Altowairqi et al, 2015;Altowairqi et al, 2013;Dewhurst et al, 1998;Dewhurst and Siggins, 2006;Dewhurst et al, 2011;Delle-Piane et al, 2011). One of the difficulties is that organic rich shales are chemically and mechanically unstable, due to the organised distribution of platy clay minerals (Zhu et al, 2011) and compliant organic materials (Vernik and Nur, 1992;Vernik and Liu, 1997;Sondergeld et al, 2000;Vernik and Milovac, 2011).…”