ABSTRACT:

New modifications of the optical scanning technique provide non-contact measurements of rock thermal conductivity and volumetric heat capacity on full-size core samples, core plugs, broken cores and other types of rock samples. Data on formation heterogeneity and anisotropy and rock structure and texture can be obtained from continuous non-contact thermal core logging developed that allows to characterize heterogeneity and structural-textural peculiarities of rocks with a spatial resolution of 0.2–1 mm. Principal axes of thermal anisotropy can be established from several non-parallel optical scannings. Thermal scanning presents information for a reasonable selection of rock samples for following measurements of geomechanical characteristics. The implementation of the technique opens wide possibilities in rock mechanics and rock engineering, because continuously measured thermal properties, and data on heterogeneity and anisotropy reflect variations in rock fabric and composition that also drive the variability of geomechanical properties. Observed correlations between the rock thermal and geomechanical properties confirm that.

1 INTRODUCTION

Reliable data on rock thermal properties are required when the heat transport is considered jointly with other phenomena in mining, geotechnical, civil and underground engineering, in environmental sensitive projects such as disposal of high-level radioactive waste in deep underground sites and repositories, various engineering projects such as the design and installation of buried high-voltage power cables, oil and gas pipe lines, and other ground modification techniques employing heating and freezing.

The optical scanning (OS) approach and first version of corresponding measuring instrument were developed in 1980s and 1990s for simultaneous determination of thermal conductivity (TC) and thermal diffusivity (TD) (hence volumetric heat capacity (VHC) also) within one experiment (Popov 1997). The noncontact character of the OS measurements removed the influence of thermal resistance between the rock sample and the heater and sensors that results in (1) absence of problems with uncertain quality of the measurements, and (2) absence of requirement for careful mechanical treatment of rock samples under study. The OS technique has also provided continuous profiling thermal properties and thermal heterogeneity of rock samples with flat or cylindrical surface and along the entire scanning line length that allows investigating thermal heterogeneity related to the structural and textural characteristics of rocks. Other important characteristics of the OS method are as follows:

  1. absence of contact between the instrument components and the rock sample,

  2. ability to measure on full size core, split core, broken core with one smooth surface and core plugs without any additional mechanical treatment,

  3. ability to determine TC and TD anisotropy tensor components for every rock sample studied,

  4. high speed of operation and short measuring time,

  5. flexibility in spatial resolution and penetration depth of measurements by changing the scanning velocity and heater-sensor separation,

  6. wide range of sample lengths accommodated. At the same time, necessity to coat the surfaces of the rock samples with a paint restricted the OS technique application as penetration of the paint into rock fractures and pores changing rock properties. Additionally, the OS technique should be adapted to continuous profiling on full size cores for development of the thermal core logging that did not exist until now. It was established also that spatial resolution of thermal heterogeneity profiling should be improved for some OS applications and minimum dimensions of rock samples under study should be reduced to provide the measurements on standard core plugs (1 × 1") and small pieces of broken cores with a length of 7–8 mm and more. Unique possibilities of the OS technique should be used to help in study of other rock physical properties.

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