Sound velocity measurement methods for porous sandstone. Measurements, finite element modelling, and diffraction correction

📝 Abstract

Acoustic material parameters of gas hydrate bearing porous rocks are important for evaluation of methods to exploit the vast methane gas resources present in the earth’s subsurface, potentially combined with CO2 injection. A solid buffer method for measuring changes of the compressional wave velocity in porous rocks with changing methane hydrate contents under high-pressure hydrate-forming conditions, is tested and evaluated with respect to effects influencing on the measurement accuracy. The limited space available in the pressure chamber represents a challenge for the measurement method. Several effects affect the measured compressional wave velocity, such as interference from sidewall reflections, diffraction effects, the amount of torque (force) used to achieve acoustic coupling, and water draining of the watersaturated rock specimen. Test measurements using the solid buffer method in the pressure chamber at atmospheric conditions are compared to independent measurements using a water-bath immersion measurement method. Compressional wave velocity measurements have been done in the steady state region at frequency 500 kHz for various specimen made of plexiglas and Bentheim sandstone. Finite element simulations of the solid buffer measurement method with plexiglas specimen have been used for comparison with the measurements, and to aid in the design, control, and evaluation of the measurement method and results. Highly favorable agreement between the two measurement methods has been obtained, also with respect to repeatability and reproducibility. The results indicate that the solid buffer method may be suitable for use in the pressure chamber with Bentheim sandstone and changing methane hydrate contents under high-pressure hydrate-forming conditions, for quantitative measurements of the compressional wave velocity in such rock core samples at these frequencies.

💡 Analysis

Acoustic material parameters of gas hydrate bearing porous rocks are important for evaluation of methods to exploit the vast methane gas resources present in the earth’s subsurface, potentially combined with CO2 injection. A solid buffer method for measuring changes of the compressional wave velocity in porous rocks with changing methane hydrate contents under high-pressure hydrate-forming conditions, is tested and evaluated with respect to effects influencing on the measurement accuracy. The limited space available in the pressure chamber represents a challenge for the measurement method. Several effects affect the measured compressional wave velocity, such as interference from sidewall reflections, diffraction effects, the amount of torque (force) used to achieve acoustic coupling, and water draining of the watersaturated rock specimen. Test measurements using the solid buffer method in the pressure chamber at atmospheric conditions are compared to independent measurements using a water-bath immersion measurement method. Compressional wave velocity measurements have been done in the steady state region at frequency 500 kHz for various specimen made of plexiglas and Bentheim sandstone. Finite element simulations of the solid buffer measurement method with plexiglas specimen have been used for comparison with the measurements, and to aid in the design, control, and evaluation of the measurement method and results. Highly favorable agreement between the two measurement methods has been obtained, also with respect to repeatability and reproducibility. The results indicate that the solid buffer method may be suitable for use in the pressure chamber with Bentheim sandstone and changing methane hydrate contents under high-pressure hydrate-forming conditions, for quantitative measurements of the compressional wave velocity in such rock core samples at these frequencies.

📄 Content

Proceedings of the 39th Scandinavian Symposium on Physical Acoustics, Geilo, Norway, Jan. 31 – Feb. 3, 2016 Sound velocity measurement methods for porous sandstone. Measurements, finite element modelling, and diffraction correction Mathias Sæther 1, Per Lunde 1,3, Geir Ersland2 1 Acoustics group, Department of Physics and Technology, University of Bergen, Postboks 7803, N-5020 BERGEN, Norway 2 Petroleum and process technology group, Department of Physics and Technology, University of Bergen, Postboks 7803, N-5020 BERGEN, Norway 3 Christian Michelsen Research AS (CMR), P.O. Box 6031 Postterminalen, N-5892 BERGEN, Norway Contact email: mathias.sather@uib.no Abstract Acoustic material parameters of gas hydrate bearing porous rocks are important for evaluation of methods to exploit the vast methane gas resources present in the earth’s subsurface, potentially combined with CO2 injection. A solid buffer method for mea- suring changes of the compressional wave velocity in porous rocks with changing methane hydrate contents under high-pressure hydrate-forming conditions, is tested and evaluated with respect to effects influencing on the measurement accuracy. The limited space available in the pressure chamber represents a challenge for the mea- surement method. Several effects affect the measured compressional wave veloc- ity, such as interference from sidewall reflections, diffraction effects, the amount of torque (force) used to achieve acoustic coupling, and water draining of the water- saturated rock specimen. Test measurements using the solid buffer method in the pressure chamber at atmospheric conditions are compared to independent measure- ments using a water-bath immersion measurement method. Compressional wave ve- locity measurements have been done in the steady state region at frequency 500 kHz for various specimen made of plexiglas and Bentheim sandstone. Finite element simulations of the solid buffer measurement method with plexiglas specimen have been used for comparison with the measurements, and to aid in the design, control, and evaluation of the measurement method and results. Highly favorable agreement between the two measurement methods has been obtained, also with respect to re- peatability and reproducibility. The results indicate that the solid buffer method may be suitable for use in the pressure chamber with Bentheim sandstone and changing methane hydrate contents under high-pressure hydrate-forming conditions, for quan- titative measurements of the compressional wave velocity in such rock core samples at these frequencies. 1 Introduction There is an increasing interest in exploiting the methane gas resources present in natural gas in the earth’s subsurface. Even if conservative estimates are considered, the consensus is that hydrocarbon gas hydrate resources are vast [1–4]. Given the more stable nature of CO2 - containing hydrates, safe storage of CO2 might be accomplished in porous rock resorvairs. This has already been tested by injecting CO2 ISBN 978-82-8123-016-3 1 Proceedings of the 39th Scandinavian Symposium on Physical Acoustics, Geilo, Norway, Jan. 31 – Feb. 3, 2016 from the Sleipner oil and gas field into the Utsira formation. Potential leakage and other safety concerns have been studied in detail by [5–7] Gas hydrate or ice in sediment pores will significantly affect the acoustic parameters such as compressional and shear wave velocities, cL, cS, and compressional and shear wave absorption coefficients, αL, αS) [8, 9]. This can potentially be used to monitor the amount of gas hydrates in porous rocks, and has been indicated to be the most promising method to detect hydrate deposits remotely [1]. Acoustic parameters also play and impor- tant role in reservoir simulations [10] and as input parameters in the Biot model [11, 12]. These models can give insight in physical and geological properties of the porous rock and the hydrates trapped in its pores. There are numerous publications on wave velocity measurements in rocks. In [13–17] pulse methods were used to measure compressional and shear wave velocities at different pressures and temperatures. In these works, transducers were attached directly to the specimen, and the setups were immersed in pressurized oil cells inducing a hydrostatic pressure. It has been pointed out that the oil may penetrate some of the pores in the rocks that may influence on the results [18]. Biot [11, 12] developed a theory for propagation of elastic waves in a fluid-saturated porous solid. Dispersion curves for for phase velocities, group velocities and attenuation factors were presented. Input parameters in the model are bulk modulus and mass density of the sand grains, permeability, turtuosity, and porosity of the sediment, viscosity, bulk modulus and mass density of the fluid and shear and bulk moduli of the sediment frame. In [18] McSkimin categorizes and discusses strengths and weaknesses of different acoustic measurement methods for various media ranging from low and hi

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