Dynamic elastic properties of coal

Geophysics ◽  
2010 ◽  
Vol 75 (6) ◽  
pp. E227-E234 ◽  
Author(s):  
Anyela Morcote ◽  
Gary Mavko ◽  
Manika Prasad
Keyword(s):  
Elastic Properties ◽  
High Pressures ◽  
Confining Pressure ◽  
Acoustic Properties ◽  
Coal Rank ◽  
S Wave ◽  
Wide Range ◽  
Confining Pressures ◽  
Intrinsic Anisotropy ◽  
Dynamic Elastic Properties

Laboratory ultrasonic velocity measurements of different types of coal demonstrate that their dynamic elastic properties depend on coal rank and applied effective pressure. In spite of the growing interest in coal beds as targets for methane production, the high abundance in sedimentary sequences and the strong influence that they have on seismic response, little data are available on the acoustic properties of coal. Velocities were measured in core plugs parallel and perpendicular to lamination surfaces as a function of confining pressure up to [Formula: see text] in loading and unloading cycles. P- and S-wave velocities and dry bulk and dry shear moduli increase as coal rank increases. Thus, bituminous coal and cannel show lower velocities and moduli than higher ranked coals such as semianthracite and anthracite. The [Formula: see text] relationship for dry samples is linear and covers a relatively wide range of effective pressures and coal ranks. However, there is a pressure dependence on the elastic properties of coal for confining pressures below [Formula: see text]. This pressure sensitivity is related to the presence of microcracks. Finally, the data show that coal has an intrinsic anisotropy at confining pressures above [Formula: see text], the closing pressure for most of the microcracks. This intrinsic anisotropy at high pressures might be due to fine lamination and preferred orientation of the macerals.

The influence of confining pressure and water saturation on dynamic elastic properties of some Permian coals

Geophysics ◽  
1993 ◽  
Vol 58 (1) ◽  
pp. 30-38 ◽  
Author(s):  
Gang Yu ◽  
Keeva Vozoff ◽  
David W. Durney
Keyword(s):  
Elastic Properties ◽  
Water Saturation ◽  
Elastic Anisotropy ◽  
Confining Pressure ◽  
S Wave ◽  
Bedding Planes ◽  
Confining Pressures ◽  
Mine Development ◽  
Water Saturated ◽  
Dynamic Elastic Properties

Laboratory measurements are described on Permian coals from Wollongong, New South Wales, Australia related to the dependence of ultrasonic P‐ and S‐wave velocities, attenuation, anisotropy and the dynamic elastic moduli on confining pressure, water saturation, and pore pressure. Five independent stiffness constants are used to represent the elastic anisotropy of the specimens as a function of confining pressure and water saturation. The anisotropy is believed to be controlled mainly by the internal structure of the coals, while the pressure dependence of the constants is controlled mainly by randomly oriented cracks. P‐ and S‐wave dispersions were measured on water‐saturated specimens as confining pressures increased from 2 MPa to 40 MPa. The samples represented cores taken both parallel and perpendicular to bedding planes. Velocities along bedding planes are marginally higher than those across bedding planes. This anisotropy is insensitive to confining pressure. Attenuation was also measured, both normal and parallel to bedding planes, on dry and water‐saturated specimens from 2 MPa to 40 MPa confining pressures. The experimental results show that dynamic elastic properties are potential indicators of the states of stress and saturation in coal seams, and provide necessary information for computer modeling and interpreting seismic surveys carried out to assist mine development.

scholarly journals An experimental study to investigate the physical and dynamic elastic properties of Eagle Ford shale rock samples

2021 ◽  
Author(s):  
Faisal Altawati ◽  
Hossein Emadi ◽  
Rayan Khalil
Keyword(s):  
Elastic Properties ◽  
Injection Pressure ◽  
Confining Pressure ◽  
Xrd Analysis ◽  
Low Permeability ◽  
S Wave ◽  
Eagle Ford ◽  
Core Samples ◽  
Wave Velocities ◽  
Dynamic Elastic Properties

AbstractUnconventional resources, such as Eagle Ford formation, are commonly classified for their ultra-low permeability, where pore sizes are in nano-scale and pore-conductivity is low, causing several challenges in evaluating unconventional-rock properties. Several experimental parameters (e.g., diffusion time of gas, gas injection pressure, method of permeability measurement, and confining pressure cycling) must be considered when evaluating the ultra-low permeability rock's physical and dynamic elastic properties measurements, where erroneous evaluations could be avoided. Characterizing ultra-low permeability samples' physical and elastic properties helps researchers obtain more reliable information leading to successful evaluations. In this study, 24 Eagle Ford core samples' physical and dynamic elastic properties were evaluated. Utilizing longer diffusion time and higher helium injection pressure, applying complex transient method, and cycling confining pressure were considered for porosity, permeability, and velocities measurements. Computerized tomography (CT) scan, porosity, permeability, and ultrasonic wave velocities were conducted on the core samples. Additionally, X-ray Diffraction (XRD) analysis was conducted to determine the mineralogical compositions. Porosity was measured at 2.07 MPa injection pressure for 24 h, and the permeability was measured using a complex transient method. P- and S-wave velocities were measured at two cycles of five confining pressures (up to 68.95 MPa). The XRD analysis results showed that the tested core samples had an average of 81.44% and 11.68% calcite and quartz, respectively, with a minor amount of clay minerals. The high content of calcite and quartz in shale yields higher velocities, higher Young's modulus, and lower Poisson's ratio, which enhances the brittleness that is an important parameter for well stimulation design (e.g., hydraulic fracturing). The results of porosity and permeability showed that porosity and permeability vary between 5.3–9.79% and 0.006–12 µD, respectively. The Permeability–porosity relation of samples shows a very weak correlation. P- and S-wave velocities results display a range of velocity up to 6206 m/s and 3285 m/s at 68.95 MPa confining pressure, respectively. Additionally, S-wave velocity is approximately 55% of P-wave velocity. A correlation between both velocities is established at each confining pressure, indicating a strong correlation. Results illustrated that applying two cycles of confining pressure impacts both velocities and dynamic elastic moduli. Ramping up the confining pressure increases both velocities owing to compaction of the samples and, in turn, increases dynamic Young's modulus and Poisson's ratio while decreasing bulk compressibility. Moreover, the results demonstrated that the above-mentioned parameters' values (after decreasing the confining pressure to 13.79 MPa) differ from the initial values due to the hysteresis loop, where the loop is slightly opened, indicating that the alteration is non-elastic. The findings of this study provide detailed information about the rock physical and dynamic elastic properties of one of the largest unconventional resources in the U.S.A, the Eagle Ford formation, where direct measurements may not be cost-effective or feasible.

On: “The influence of pore pressure and confining pressure on dynamic elastic properties of Berea sandstone,” by N. I. Christensen and H. F. Wang (GEOPHYSICS, 50, 207–213, February 1985).

Geophysics ◽  
1986 ◽  
Vol 51 (4) ◽  
pp. 1016-1016
Author(s):  
G. H. F. Gardner
Keyword(s):  
Pore Pressure ◽  
Confining Pressure ◽  
Hysteresis Curve ◽  
Hysteresis Cycle ◽  
Berea Sandstone ◽  
Experimental Accuracy ◽  
Confining Pressures ◽  
History Of ◽  
Pressure Changes ◽  
Dynamic Elastic Properties

The authors present their results as if Berea sandstone were an elastic material; that is, velocities are given as functions of confining and pore pressure. In fact, most rocks are inelastic and velocities depend on the history of the confining and pore pressure, and not just on the present values. Some measurements of hysteresis were reported by Gardner et al. (1965). The confining pressure was cycled between two pressures [Formula: see text] and [Formula: see text] for a fixed pore pressure [Formula: see text], following a fixed schedule of pressure changes, until repeatable values of velocity were obtained. (At any intermediate pressure the velocity measured for increasing pressure was different from the value for decreasing pressure, giving rise to a hysteresis cycle). When the same schedule of pressure changes for the differential pressure [Formula: see text] was followed by holding [Formula: see text] fixed and varying [Formula: see text], the measured velocities followed the same hysteresis curve within the limits of experimental accuracy. In brief, when hysteresis was taken into account, changes in pore and confining pressures were equally effective in changing velocity. In their article, Christensen and Wang do not refer to hysteresis; perhaps they would like to comment on its relevance.

The Effect of Contact Generation on the Elastic Properties of a Granular Medium

1990 ◽  
Vol 57 (2) ◽  
pp. 330-336 ◽  
Author(s):  
Anthony L. Endres
Keyword(s):  
Elastic Properties ◽  
Granular Medium ◽  
Confining Pressure ◽  
Hydrostatic Compression ◽  
Experimental Measurements ◽  
Contact Points ◽  
Confining Pressures ◽  
Sphere Radius ◽  
General Statistical ◽  
Possible Cause

Previous models for the elastic properties of a granular medium have assumed that all grain contacts are established in its undeformed configuration. Experimental data for the change in elastic properties as a function of confining pressure cannot be explained by these models. Contact creation is cited as one possible cause for this discrepancy. In this paper a model for a granular material is derived that allows for the creation of grain contacts during hydrostatic compression. This formulation allows for the use of general contact microphysics and a general statistical distribution of gap widths at the near-contact points. Numerical results show that for very small values of the average near-contact gaps (approximately 1/1000 of a sphere radius), there can be significant effects occurring in the range of confining pressures between 106 to 107 Pa. The results of this contact generating model are consistent with published experimental measurements.

Origin of the temporal evolution of elastic properties during laboratory seismic cycle.

2020 ◽  
Author(s):  
Federica Paglialunga ◽  
François X. Passelègue ◽  
Mateo Acosta ◽  
Marie Violay
Keyword(s):  
Elastic Properties ◽  
Wave Velocity ◽  
Confining Pressure ◽  
Seismic Damage ◽  
P Wave ◽  
Seismic Cycle ◽  
Stick Slip ◽  
P Wave Velocity ◽  
Confining Pressures ◽  
Thermally Treated

<p>Recent seismological observations highlighted that earthquakes are associated to drops in elastic properties around the fault zone (Brenguier et al., 2008). This drop is often attributed to co-seismic damage produced at the rupture tip, and can mostly be observed at shallow depths. However, it is known that in the upper crust, faults are surrounded by a zone of damage (Caine, Evans, & Forster, 1996). Because of this, the origin of the velocity change associated to earthquakes, as well as its recovery in the months following the rupture remains highly debated.</p><p>We conducted stick-slip experiments to explore the evolution of elastic waves velocities during the entire seismic cycle. The tests were run on saw-cut La Peyratte granite samples presenting different initial degrees of damage, obtained through thermal treatment. Three types of samples were studied: not thermally treated, thermally treated at 650 °C and thermally treated at 950 °C. Seismic events were induced in a triaxial configuration apparatus at different confining pressures ranging from 15 MPa to 120 MPa. Active acoustic measurements were carried through the whole duration of the tests and P-wave velocities were measured.</p><p> </p><p>The evolution of P-wave velocity follows the evolution of the shear stress acting on the fault, showing velocity drops during dynamic slip events. The evolution of the P-wave velocity drops with increasing confining pressure shows two different trends; the largest drops can be observed for low confining pressure (15 MPa) and decrease for intermediate confining pressures (up to 45 MPa), while for confining pressures of 60 MPa to 120 MPa, drops in velocity slightly increase with confining pressure.</p><p>Our results highlight that at low confining pressures (15-45 MPa), the change in elastic velocity is controlled by the sample bulk properites (damage of the medium surrounding the fault), while for higher confining pressures (60-120 MPa), it might be the result of co-seismic damage.</p><p>These preliminary results bring a different interpretation to the seismic velocity drops observed in nature, attributed to co-seismic damage. In our experiments co-seismic damage is not observed, except for high confining pressures (laboratory equivalent for large depths), while the change in P-wave velocity seems to be highly related to combined stress conditions and initial damage around the fault for low confining pressures (laboratory equivalent for shallow depths).</p>

Mechanical properties of shale-gas reservoir rocks — Part 1: Static and dynamic elastic properties and anisotropy

Geophysics ◽  
2013 ◽  
Vol 78 (5) ◽  
pp. D381-D392 ◽  
Author(s):  
Hiroki Sone ◽  
Mark D. Zoback
Keyword(s):  
Elastic Properties ◽  
Shale Gas ◽  
Organic Content ◽  
S Wave ◽  
Gas Reservoir ◽  
Reservoir Rocks ◽  
Shale Gas Reservoir ◽  
Static Modulus ◽  
Static Young’S Modulus ◽  
Dynamic Elastic Properties

Understanding the controls on the elastic properties of reservoir rocks is crucial for exploration and successful production from hydrocarbon reservoirs. We studied the static and dynamic elastic properties of shale gas reservoir rocks from Barnett, Haynesville, Eagle Ford, and Fort St. John shales through laboratory experiments. The elastic properties of these rocks vary significantly between reservoirs (and within a reservoir) due to the wide variety of material composition and microstructures exhibited by these organic-rich shales. The static (Young’s modulus) and dynamic (P- and S-wave moduli) elastic parameters generally decrease monotonically with the clay plus kerogen content. The variation of the elastic moduli can be explained in terms of the Voigt and Reuss limits predicted by end-member components. However, the elastic properties of the shales are strongly anisotropic and the degree of anisotropy was found to correlate with the amount of clay and organic content as well as the shale fabric. We also found that the first-loading static modulus was, on average, approximately 20% lower than the unloading/reloading static modulus. Because the unloading/reloading static modulus compares quite well to the dynamic modulus in the rocks studied, comparing static and dynamic moduli can vary considerably depending on which static modulus is used.

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

2013 ◽  
Vol 53 (1) ◽  
pp. 245 ◽  
Author(s):  
Yazeed Altowairqi ◽  
Reza Rezaee ◽  
Milovan Urosevic ◽  
Claudio Delle Piane
Keyword(s):  
Elastic Properties ◽  
Shale Gas ◽  
Ultrasonic Transducers ◽  
S Wave ◽  
Gas Reservoirs ◽  
Wave Velocities ◽  
Degree Of Anisotropy ◽  
The Impact ◽  
The Relationship ◽  
Dynamic Elastic Properties

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.

A modified approach for determination of nonlinear properties in seismic response analyses for 200 m high core rock-fill dams

2008 ◽  
Vol 45 (8) ◽  
pp. 1064-1072 ◽  
Author(s):  
Hongjun Li ◽  
Shichun Chi ◽  
Gao Lin ◽  
Hong Zhong
Keyword(s):  
Site Response ◽  
Confining Pressure ◽  
In Situ Stress ◽  
Nonlinear Properties ◽  
Rock Fill ◽  
Equivalent Linear ◽  
Wide Range ◽  
Confining Pressures ◽  
The Impact

Equivalent linear analyses are widely used for estimation of site response and safety status of rock-fill dams subjected to strong earthquakes. However, the average normalized curves incorporated in the equivalent-linear iterative process cannot precisely depict the variations of dynamic parameters with shearing strain for one type of soil material under a wide range of confining pressures. Thus, a modified approach for the determination of nonlinear properties for soil elements confined under a broad range of effective pressures in site response analyses for high rock-fill dams (>200 m) is proposed. In this approach, the normalized confining-pressure-interpolating curves (CPI curves) of each soil element under different effective stress are obtained by linearly interpolating or extrapolating by its in situ stress. By comparing the results obtained by equivalent linear analyses incorporating the average curves and the CPI curves, respectively, the impact of utilizing the recommended curves when determining nonlinear soil properties on site response analyses of high rock-fill dams (>200 m) is discussed. It is shown that the refinement in the determination of nonlinear properties during site response can be utilized in the near future by incorporating the results of this study in practice.

The relationship between acoustic properties and the petrographic character of carbonate rocks

Geophysics ◽  
1984 ◽  
Vol 49 (10) ◽  
pp. 1622-1636 ◽  
Author(s):  
F. Rafavich ◽  
C. H. St. C. Kendall ◽  
T. P. Todd
Keyword(s):  
Wave Velocity ◽  
Carbonate Rocks ◽  
Wave Impedance ◽  
Acoustic Properties ◽  
P Wave ◽  
S Wave ◽  
Seismic Section ◽  
Small Influence ◽  
In Situ Conditions ◽  
Wide Range

Laboratory studies of the detailed relationships between acoustic properties and the petrographic character of brine‐ and air‐saturated carbonate rocks with a wide range of facies, porosities, lithologies, and rock fabrics indicate that porosity is the major factor influencing both P- and S-wave impedance and velocity. Primary lithology and secondary mineralogy have only a small influence on impedance and velocity. Combined use of P- and S-wave velocity data discriminates porosity changes from lithologic changes. All other variables, including pore‐fluid type and petrographic fabric, have no significant influence on velocities. Laboratory measurements of P‐wave velocity under simulated in‐situ conditions reproduce well‐log velocity values reliably. Laboratory porosity‐velocity trends agree with the time‐average equation when the correct matrix velocities are used. Rock property results were used to interpret porosity/lithology variations for an inverted seismic section from the Williston basin. Where well control was available, the porosity/lithology interpretation was found to be in agreement with the subsurface control.

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