Estimation of uniaxial compressive strength of shale using indentation testing (original) (raw)
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Estimating elasticity modulus and uniaxial compressive strength of sandstone using indentation test
Journal of Petroleum Science and Engineering, 2018
Elastic modulus (E) and uniaxial compressive strength (UCS) are very important engineering parameters that is required for many engineering projects. In some projects such as well drilling for oil production, deep underground tunnels in crushed zone, due to the depth of the well and fractures of rocks, there are some problems for obtained standard core samples. In this situations various methods have been developed for determining UCS and E using drilling cutting. Indentation test is one of these methods, in this method, an indenter penetrates into the stabilized samples in the surrounding material and then Critical Transition Force (CTF) and Indentation modulus (IM) determine. In the present study in order to developing the application of indentation testing for sandstone, and investigate the effect of particles size, shape and thickness, 14 sandstone blocks were collected from different formations in Iran and their physical characteristics, UCS and, E were measured using standard methods. After crushing the blocks and preparing 600 particles with dimensions of 3×3×3 mm 3 , 5×5×3 mm 3 , 5×5×5 mm 3 , 5×5×8 mm 3 , and 8×8×8 mm 3 and irregular shape particles with a thickness of 5 mm, indentation tests were performed on each particle and CTF and IM were calculated for each particle separately. According to the tests carried out some empirical relationships were proposed between UCS-CTF (with R 2 ≥ 0.86) and E-IM (with R 2 ≥ 0.73) for different shapes and dimensions. Due to the differences in the dimensions of the produced particles, investigating the effect of surface and thickness of particle on the CTF and the IM revealed that the particle's surface had a considerable effect on the CTF and IM; however, the effect of thickness was negligible. Using the dimensionless surface parameter (S and S i), some empirical relationships between UCS
Empirical estimation of uniaxial compressive strength of shale formations
The uniaxial compressive strength of rock (σc) is an important parameter for petroleum engineers, drilling operations, and all related activities from exploration through to production and abandonment. A thorough understanding of the parameters affecting σc is a basic prerequisite for accurate geomechanical modeling of the reservoir and overburden properties. Uniaxial compressive strength plays a significant role in mud weight determination while drilling, especially for a troublesome lithology such as shale. However, standard geomechanical practice requires well-preserved core samples for measurement of σc in the lab. Because core samples are not often available, there is a need for alternative methods to obtain fit-for-purpose values of σc, based on other related rock parameters. Our primary objective was to identify a minimum set of related rock properties that could be used to predict σc. From a review of existing data in the literature, supplemented by laboratory measurements on Iranian samples, we established a database and accomplished extensive statistic analysis. Also, a normality test was executed to make sure a statistically acceptable set of data was collected. We suggested that two parameters of Young’s modulus (E) and porosity (ϕ), which might be estimated from geophysical log data, were sufficient for a reliable prediction of σc in shale formations, and the overall contribution of E was more than ϕ. We obtained a prediction equation with improved accuracy compared to previous investigations. Furthermore, we determined that the relative sensitivity of shale strength to porosity and Young’s modulus very much depended on the range of porosity.
Engineering Geology, 2013
A number of methods have been proposed to indirectly assess the uniaxial compressive strength (UCS) of intact rock in the drilling of oil wells and underground drilling. Indentation testing is a method in which an indentor of a specific diameter penetrates a particle of rock and the force-displacement curve is plotted to determine the critical transition force (CTF). In the present study, 10 shale block samples were collected from a cretaceous shale formation in Iran from which standard cores were prepared and subjected to UCS testing. Cubic particles 4, 5 and 7 mm 3 in size were cut and entrenched in disks containing resin and a total of 300 indentation tests were conducted on them. Empirical relations for the relation between UCS and CTF were developed for each size. The highest correlation coefficient was recorded for the 7 mm 3 particles and the lowest for the 4 mm 3 particles. A simple method is proposed to determine the empirical relationship independent of particle dimensions between UCS and CTF that has a correlation coefficient of 0.78. Verification of the proposed equations show that they predicted UCS with 85% accuracy. A comparison of the proposed relationships and those from previous studies indicates that the empirical relationship between these two variables is influenced by variation in the uniaxial compressive strength and lithology of the different samples.
Rock Mechanics and Rock Engineering, 2014
Indentation testing has been developed as an unconventional method to determine intact rock strength using small rock specimens within the size of drill cuttings. In previous investigations involving indentation testing, researchers have used different indenter stylus geometries, penetration rate (PR) and specimen sizes. These dissimilarities can restrict applications of this method for strength measurement and lead to non-comparable results. This paper investigates the influence of indenter diameter (ID) and PR on indentation indices for carbonate rocks to provide objective comparison and application of the existing correlations. As part of this research, several indentation tests were conducted using different IDs and PRs. The laboratory test results showed that indentation indices can be affected by ID while PR has only minor effect on the indentation indices. Thus, a normalizing function was presented to reduce the dependency of test results to ID. Verification of the findings with independent data confirms the suitability of the suggested normalizing function in determining the rock uniaxial compressive strength using testing data obtained from various IDs and PRs.
Empirical Approach for Evaluation of Compressive Strength of Shale
EAGE Shale Workshop 2010, 2010
A sound knowledge regarding the strength and stiffness properties of rock material could significantly improve engineering geological assessments. Shales are known as the most problematic rock material worldwide. Uniaxial compressive strength UCS is an essential input parameter for development of almost any engineering design. However, appropriate core specimen for measurement of UCS in the lab is often a real dilemma. Accordingly, extensive attempts have been made for strength estimation based on other parameters. In this research, an empirical equation is suggested for estimating UCS of shale based on Young s modulus and porosity. This equation was achieved based on statistical analysis of lab experiments obtained over a wide range of geographical locations. A further attempt was made to describe mathematical meaning of the statistical results based on theory poroelasticity. Accordingly, at low porosity values both Young s modulus and porosity significantly contribute in prediction of the UCS. This was attributed to poroelastic behavior of shale under these conditions. At high porosity values, however, E was the dominant parameter. One of the advantages of this research is that suggested equation is independent of the geographical location, while it is based on two input parameters Young s modulus, and porosity.
Correlation study between indentation indices and physical-mechanical properties of rocks
CRC Press eBooks, 2023
Indentation test is one of the laboratory approaches that is used to predict the performance of Raise Boring Machines (RBMs) and Tunnel Boring Machines (TBMs). This test is a nonstandard laboratory experiment that was originally developed to provide a method to predict the normal force acting on the cutters used on mechanical miners. However, in some cases due to difficulties in obtaining rock samples as well as insufficient laboratory equipment it is not possible to perform indentation test. In this study, based on the indentation tests on different rock samples obtained from raise-boring projects in Turkey, three indentation indices (brittleness indices and force index) are obtained; then, the multi-variable correlations between these indices and physical-mechanical properties of rocks are investigated. The study indicates that all three indentation indices can be estimated by using the velocity of P-wave, static elasticity modulus, Cerchar abrasivity index, and Schmidt hammer hardness of rocks.
Rock Mechanics and Rock Engineering, 2013
Shale mechanical properties are evaluated from laboratory tests after a complex workflow that covers tasks from sampling to testing. Due to the heterogeneous nature of shale, it is common to obtain inconsistent test results when evaluating the mechanical properties. In practice, this variation creates errors in numerical modeling when test results differ significantly, even when samples are from a similar core specimen. This is because the fundamental models are based on the supplied test data and a gap is, therefore, always observed during calibration. Thus, the overall goal of this study was to provide additional insight regarding the organization of the non-linear model input parameters in borehole simulations and to assist other researchers involved in the rock physics-related research fields. To achieve this goal, the following parallel activities were carried out: (1) perform triaxial testing with different sample orientations, i.e., 0°, 45°, 60°, and 90°, including the Brazilian test and CT scans, to obtain a reasonably accurate description of the anisotropic properties of shale; (2) apply an accurate interpretative method to evaluate the elastic moduli of shale; (3) evaluate and quantify the mechanical properties of shale by accounting for the beddings plane, variable confinement pressures, drained and undrained test mechanisms, and cyclic versus monotonic test effects. The experimental results indicate that shale has a significant level of heterogeneity. Postfailure analysis confirmed that the failure plane coincides nicely with the weak bedding plane. The drained Poisson's ratios were, on average, 40 % or lower than the undrained rates. The drained Young's modulus was approximately 48 % that of the undrained value. These mechanical properties were significantly impacted by the bedding plane orientation. Based on the Brazilian test, the predicted tensile strength perpendicular to the bedding plane was 12 % lower than the value obtained using the standard isotropic correlation test. The cyclic tests provided approximately 6 % higher rock strength than those predicted by the monotonic tests. Keywords Elastic moduli Á Shale mechanical properties Á Shale anisotropy Á Strength anisotropy Á Weak bedding plane Á Drained Á Undrained Á Cyclic triaxial test
Arabian Journal of Geosciences, 2019
A number of methods have been proposed to indirectly assess the uniaxial compressive strength (UCS) of intact rock in the drilling of oil wells and underground drilling. Indentation testing is a method in which an indentor of a specific diameter penetrates a particle of rock and the force-displacement curve is plotted to determine the critical transition force (CTF). In the present study, 10 shale block samples were collected from a cretaceous shale formation in Iran from which standard cores were prepared and subjected to UCS testing. Cubic particles 4, 5 and 7 mm 3 in size were cut and entrenched in disks containing resin and a total of 300 indentation tests were conducted on them. Empirical relations for the relation between UCS and CTF were developed for each size. The highest correlation coefficient was recorded for the 7 mm 3 particles and the lowest for the 4 mm 3 particles. A simple method is proposed to determine the empirical relationship independent of particle dimensions between UCS and CTF that has a correlation coefficient of 0.78. Verification of the proposed equations show that they predicted UCS with 85% accuracy. A comparison of the proposed relationships and those from previous studies indicates that the empirical relationship between these two variables is influenced by variation in the uniaxial compressive strength and lithology of the different samples.