Static and Dynamic Mechanical Properties of Organic-Rich Gas Shale

Yan Li,Hui Li,Tao Chen,Ming Qu,Lele Cao,Xin Zhao,Chi Dong,Hongwei Yu

doi:10.1155/2021/6695975

Abstract

Rock mechanical properties are critical for drilling, wellbore stability, and well stimulation. There are usually two laboratory methods to determine rock mechanical properties: static compression tests and acoustic velocity measurements. Rocks are heterogeneous, so there are significant differences between static elastic constants and the corresponding dynamic ones. Usually, static test results are more representative than dynamic methods but the static tests are time consuming and costly. Dynamic methods are nondestructive and less expensive, which are practical in the laboratory and field. In this paper, we compare the static and dynamic elastic properties of Eagle Ford Shale by triaxial compressive tests and ultrasonic velocity tests. Correlations between static and dynamic elastic properties are developed. Conversion from dynamic mechanical properties to static mechanical properties is established for better estimating reservoir mechanical properties. To better understand the relationship of static and dynamic mechanical properties, 30 Eagle Ford Shale samples were tested. According to the test results, the dynamic properties are considerably different from the static counterparts. For all tested samples, static Young’s modulus is lower than dynamic Young’s modulus, ranging from 55% to 90%. The difference of the static and dynamic Young’s moduli decreases with the increasing of confining pressure. The reason may be because the microcracks closed in high confining pressure. Correlations between static and dynamic Young’s modulus are developed by regression analysis, which are crucial to understand the rock mechanical properties and forecast reservoir performance when direct measurement of static mechanical properties is not available or expensive. There are no strong correlations between static and dynamic Poisson’s ratios observed for the tested samples. Two potentially major reasons for the discrepancy of the static and dynamic properties of Eagle Ford Shale are discussed. Lithology and heterogeneity may be the inherent reasons, and external causes are probably the difference in strain amplitude and frequency.

Highlights

In recent decades, shale gas is more and more important in the global energy and petroleum industry
Triaxial compression tests and acoustic velocity tests were conducted on 30 Eagle Ford Shale samples
CP: 20 MPa CP: 27 MPa CP: 35 MPa dynamic Young’s moduli which decreases with the increasing of confining pressure, which is consistent with previous studies [17, 25]

Summary

Introduction

Shale gas is more and more important in the global energy and petroleum industry. Mechanical properties can be obtained either from static compression tests or from dynamic acoustic velocity measurements [5]. Holt et al [20] conducted an experiment on three different shales and concluded that dynamic moduli are higher than the static counterparts. The difference was related to inclination angle He et al [9] proposed that the dynamic moduli of Bakken samples were considerably different from the static moduli measured by triaxial compression tests. Correlations are developed based on the static and dynamic parameters of 30 Eagle Ford Shale samples to adjust the uncertainty of elastic moduli calculated from the acoustic measurements for Eagle Ford Shale formation and to lower the cost and test time

Theory and Experimental Procedures

Experimental Results and Analysis

Discussions

Conclusions

Conflicts of Interest