Overcoming challenges in the classification of deep geothermal potential

The geothermal community lacks a universal definition of deep geothermal systems. A minimum depth of 400 m is often assumed, with a further sub-classification into middle-deep geothermal systems for reservoirs found between 400 and 1000 m. Yet, the simplistic use of a depth cut-off is insufficient to uniquely determine the type of resource and its associated potential. Different definitions and criteria have been proposed in the past to frame deep geothermal systems. However, although they have valid assumptions, these frameworks lack systematic integration of correlated factors. To further complicate matters, new definitions such as hot dry rock (HDR), enhanced or engineered geothermal systems (EGSs) or deep heat mining have been introduced over the years. A clear and transparent approach is needed to estimate the potential of deep geothermal systems and be capable of distinguishing between resources of a different nature. In order to overcome the ambiguity associated with some past definitions such as EGS, this paper proposes the return to a more rigorous petrothermal versus hydrothermal classification. This would be superimposed with numerical criteria for the following: depth and temperature; predominance of conduction, convection or advection; formation type; rock properties; heat source type; requirement for formation stimulation and corresponding efficiency; requirement to provide the carrier fluid; well productivity (or injectivity); production (or circulation) flow rate; and heat recharge mode. Using the results from data mining of past and present deep geothermal projects worldwide, a classification of the same, according to the aforementioned criteria is proposed

LONDON PENETRATION DEPTH IN THE ENSEMBLE OF SPIN-POLARON OF QUASIPARTICLES CUPRATE SUPERCONDUCTORS

In the work on the example of calculation London penetration depth λL is shown that the spin-polaron approach can successfully describe the electrodynamic properties of high temperature cuprate superconductors. To study concentration and temperature dependences of λL was developed a method of calculating the response of an ensemble of spinpolaron of quasiparticles in a uniform field the vector potential Aq=0. Discusses the nature of the calculated concentration and temperature dependences of λL.Исследование выполнено при финансовой поддержке Программы Президиума РАН №12 "Фундаментальные проблемы высокотемпературной сверхпроводимости", РФФИ (грант #18-02-00837). Работа К.К.К. поддержана Советом по грантам Президента РФ (проект MK-1398.2017.2)

London penetration depth in the ensemble of spin polarons of cuprate superconductors

Within the spin-polaron concept for hole-doped cuprates superconductors the temperature and doping dependence of the London penetration depth λ is studied. To calculate λ we developed a novel approach which (i) does not suppose the analytical expression for the quasiparticle spectrum to be known in advance, and (ii) allows to take into account the strong coupling between a spin localized on the copper ion and a hole residing on the four nearest oxygen ions rigorously. Within this approach the expression for supercurrent density j⃗ is obtained in the long-wavelength limit for external magnetic field vector potential. It is shown that j⃗ is mainly due to the spin-polaron quasiparticles rather then bare oxygen holes. Temperature dependence of λ−2 at various doping is calculated and compared with available experimental data. It is argued that the inflection point revealed experimentally in the temperature behavior of λ−2 in La1.83Sr0.17CuO4 may be considered as a manifestation of the spin-polaron nature of quasiparticles in cuprates