Applied Geothermics

The Earth is about 4.6 billion years old. In terms of its thermal regime, the planet is in the process of cooling. However, to have reached its current state, the Earth and the other objects making up the Solar System went through a number of stages such as the accretion of the planet from dust of the solar nebula, the formation of the magma-ocean, stratification of matter by density, solidification of the magma-ocean, formation of the lithosphere which is taking place today, periods of increased volcanic and metamorphic activity, numerous tectonic processes with global and regional significance (obduction, subduction, orogeny, etc.), heat production by short-lived and long-lived radioisotopes, and numerous other features and processes related to thermodynamic and temperature conditions. In this Chapter are analyzed such fundamental phenomena as sources of the thermal energy in the Earth's interior, geothermal gradient, density of heat flow, heat flow and geological age, mantle heat flow, temperature distribution inside the Earth and other Earth-thermal interactions.

This Chapter describes the most important thermal parameters such as conductivity, capacity, duffusivity and interrelation of these parameters between themselves. Effect of thermal anisotropy in some cases may significantly change the studied geothermal pattern. Investigation of melting points of rocks and minerals, and temperature and pressure influence to thermal properties of rocks and minerals and fluid density play an important role in development of deep geothermal models. It is shown that analysis of the early Earth atmosphere is significant for detecting some peculiarities of the modern geothermal regime of the Earth.

In the early days of geothermics, temperature measurements were made mainly in soil, underground water, mines, tunnels, draw-wells, shafts and caves. Temperature was measured with various air, water and alcohol thermometers with inaccurate scales. The measurements were often inaccurate and not compatible. Relatively precise graduated mercury and alcohol thermometers with conventional scales have only been used since the middle of the 18th century. Today, geophysical temperature devices can register temperature values with an accuracy of 0.001 °C and higher. This Chapter contains a scheme of different types of thermal observations in applied geophysics and their short description.

Thermal waters (hot springs, geysers) and volcanoes were among the first manifestations of the Earth’s internal heat that people encountered in ancient times. This helps explain the long history of scientific analysis of such events as volcano and geyser eruptions, hot spring regions, etc. Mud volcanoes origin is not completely recognized yet, but it is very important signature of thermal regime in many regions of the world. A separate attention was paid to such reliable indicator of tectonic processes as formation of overpressure and ultra-high pressure within the lithosphere and asthenosphere.

The permafrost base is defined as 0 °C isotherm, and thickness is defined as the depth from the surface to that isotherm. Permanent ice is found on or beneath approximately 20 % of dry land: 75 % of Alaska, 63 % of Canada, and 47 % of the Russian Federation are covered by permafrost. Wherever permanent ice is found, a necessary condition is satisfied for the existence of permafrost. Thermal data analysis could be successfully applied to analysis of mechanical properties of permafrost, its temperature regime, thickness and dynamics, as well as its influence to Earth's climate changing.

During the formation of the Earth as a planet heat energy released by accretion and certain other processes was sufficient to heat the entire Earth beyond the melting point of composing it rocks. This definitely led to formation of hundreds kilometers deep magma-ocean or melting the entire planet. That molten stage could have existed for quite some time. Such processes as possible collision of Earth with Mars-size body (Moon forming event) and bombardment of surface by huge cosmic objects [e.g., “late heavy bombardment” at about 3.85–3.9 Ga] could have significantly slowed down process of solidification of magma-ocean and/or create local magma-oceans. Following solidification of magma-ocean led to formation of the lithosphere through complex processes formation and recycling rocks and minerals, interaction surface rocks with the atmosphere, formation crust, formation water-ocean, and many others. All these processes took place during general process of Earth cooling. In this Chapter such processes as the formation and evolution of magma-ocean, evolution of early Earth’s atmosphere, the formation of water-ocean, thermal regime during early lithosphere formation, dynamic interactions of the asthenosphere and the lithosphere and many other problems are discussed.

Measurements of a number of thermal parameters [e.g., temperature (T), the geothermal gradient (G or Γ), heat flow (Q or q), heat generation (A or H), heat conductivity (λ or κ), heat capacity (c) and heat diffusivity (a)], interpretation of thermal measurements and analysis of thermal regime of different layers of Earth (e.g., crust and lithosphere) are among the main tasks of geothermics. Since temperature (T) is one of the key parameters used in thermodynamics, correct determination of temperature at any depth within the Earth where it cannot be measured is an extremely important problem. Thermodynamic regime is also important for analysis of conditions for generation and preservation of oil and gas fields, as well as such events as metamorphism and volcanism as well as many other processes. The development of a geothermal model of the medium, methods of geothermal regime analysis, problems of heat absorption in the Earth's strata and theory of heat absorption are discussed. It is shown that advanced methods developed in magnetic prospecting may be applied for quantitative interpretation of thermal anomalies. The Chapter is finalized by considering the models of strongly nonlinear thermal phenomena and thermal precursors of earthquakes.

The knowledge of downhole and surrounding the wellbore formations temperature is an essential factor during drilling operations, shut-in and cementing of casing periods. The downhole temperatures while drilling affects the viscosity of the drilling mud and, subsequently, the frictional pressure losses; the performance of drilling bits in hot wells; the density of drilling fluids a.o. In deep and hot wells, the densities of water/oil muds and brines can be significantly different from those measured at surface conditions. For this reason determining the density of drilling mud under downhole conditions is needed for calculating the actual hydrostatic pressure in a well. The borehole temperature recovery process (disturbed by drilling operations) affects the technology of the casing cementing operations. Temperature surveys following the cementing operation are used for locating the top of the cement column behind casing and are very important to predict the temperature increase during the cement setting. This will enable to determine the optimal time lapse between cementing and temperature survey. During the shut-in period in the wellbore are conducted transient downhole and bottomhole temperature surveys and geophysical logging. In this are presented methods of determination of the drilling mud circulation temperatures, borehole temperatures during cementing of casing and temperature in surrounding wellbore formations during drilling and shut-in periods. Also several techniques of calculation of the static formation temperatures are considered.

Groundwater is one of the most valuable and widely available natural resources, because it is present practically everywhere. Groundwater is also an essential component of the water cycle and underground hydrological systems. Groundwater plays an extremely important role in the supply of drinking water, water for agriculture, water for industry, etc. In this Chapter are analyzed: effect of vertical and horizontal water movements on temperature profiles, application of Horner method, determination of the formation temperature, permeability and skin-factor, temperature profiles in water injection and production wells, monitoring water reserves, and some other problems.

Near-surface thermal prospecting is based on temperature measurements in shallow (up to several meters in depth) drill holes. Since geological objects differ in terms of their thermal properties, these temperature measurements provide valuable information about features of the geological targets in the areas under investigation. The developed interpreting system includes: (1) elimination of seasonal variations by using repeated observations with following linear filtering, (2) terrain relief corrections by a correlation technique which facilitates the identification of anomalies associated with ceratin buried features, (3) application for quantitative analysis of thermal anomalies the advanced methodologies developed for magnetic prospecting for conditions of inclined relief, arbitrary magnetization (polarization), and an unknown level of the normal field. Several examples illustrate effective application of quantitative examination of thermal anomalies in ore and oil & gas geophysics, in archaeological and environmental investigations.

Climate can be defined by the long-term statistics that describe the behavior and structure of the Earth’s atmosphere, hydrosphere, cryosphere, and according to Vernadsky (1945) the noosphere (“mind-sphere” or the Earth’s mental sheathe). Generally speaking, all life on the Earth depends on the hospitality of its climate. Any change in the Earth’s climate will have an immediate impact on humankind, biodiversity, and on the health. Climate of the past may be studied by the processing of thermal data observed in wells. The selection of the most suitable wells, intervals of thermal data analysis and different methodologies applied for such investigation are considered in this Chapter. Some nonlinear (and strongly nonlinear) aspects of thermal data examination may assist to re-estimate certain climate phenomena.

Identifying the mutual interrelations between different physical parameters of geological associations is highly important for correctly carrying out an integrated geophysical analysis, making the necessary corrections to observed physical values, the recognition of many geodynamic processes and solving various other problems. Since temperature is one of the fundamental physical properties, it is crucial to establish some correlations between temperature and other geophysical fields.

Integrated interpretation errors mainly depend on the type of applied algorithm. The ways for calculation of the reliability of single geophysical method and the reliability of information obtained by a set of geophysical methods is shown. The methodology of some information parameters calculation is shown in detail. Integrated interpretation can be deterministic, probabilistic and mixed (probabilistic–deterministic). A notion of Physical-Geological Model necessary for combined geophysical analysis is explained. Several examples illustrate effective integration of thermal and other geophysical methods.