Deformation of subsurface formations leading to subsidence or upheaval due to fluid withdrawal or injection is a volume reduction or expansion, which is most severe in unconsolidated sediment when reservoir fluid pressure is allowed to decline (production) or increase (injection). Compaction theory has been applied for groundwater and petroleum engineering.However unlike groundwater and petroleum reservoir as isothermal condition, geothermal reservoirs may undergo significant thermal changes due to cold water injection or production-boiling. In these cases, it is not adequate for modeling formation as the isothermal condition.
In order to study the thermal effect on the deformation of geothermal reservoirs, the simplified non-linear uniaxial compaction theory forthermoelastic fluid-saturated porous media are applied for numerical simulation. The relations among the deformation and reservoir parameters such as bulk modulus, permeability, porosity and production/injection conditions were studied. In any cases of injection or production or injection-production simultaneous operation, both of the production and the injection area show subsidence. Surface deformation rate is in proportion to increase of production (injection) rate, and in inverse proportion to permeability porosity and bulk modulus. However, in case of cold fluid injection (fluid temperature: -15°C), thermal cooling due to injection causes thermal contraction of reservoirs, though injection causes reservoir expansion (increase of pore pressure). If average reservoir rock properties are applied, subsidence due to thermal contraction overwhelms the upheaval due to injection. The effect of cold fluid injection strongly influences the reservoir deformation. In order to keep less subsidence, it should be considered to maintain adequate pressure balance due to production and injection and to maintain relatively high temperature for injection fluid.

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We estimate heat-discharge rates for a geothermal area formed during the 2000 eruption of Usu volcano, Japan. Field observations carried out in September 2006 reveal that heat-discharge rates from fumaroles, areas of steaming grounds and crater lakes are 8, 9.2, and 2 MW, respectively. The total heat-discharge rate measured in September 2006 represents just 1% of the rate immediately following the eruption.
Integration of the heat-discharge rate from April 2000 to September 2006 yields an accumulated discharge of approximately 2.3×10¹⁶J, equating to the cooling of 8×10⁶m³ of magma. This amount of magma corresponds to several percent of the total intruded magma volume estimated from analyses of ground deformation associated with the 2000 eruption.
Compared with the 1977 eruption, the 2000 eruption involved the discharge of large amounts of heat from fumaroles. Fumaroles that developed during the 2000 eruption showed a decline in activity in short time. Areas of steaming ground associated with the 2000 eruption showed more rapid growth compared with those of the 1977 eruption but discharged less heat. We suspect that differences in the hydrological environments of the two eruptions (e. g., permeability around the intruded magmas) led to contrasting patterns of propagation of the hydrothermal systems around the intruded magmas.

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A simulation model of GeoHP air conditioning systems using the pile foundation was constructed using a analytical modeling approach.The simulation model combined a heat storage tank of 140m³, a 60HP heat pump, 75 vertical heat exchangers with double U-tubes of 50m deep, and horizontal piping systems. Using the simulation model, the available heat exchange rates from the pile foundation in the GeoHP systems in Sannou Junior High School were studied through case studies. As a result, it is concluded that the heat exchanges from the pile foundation (143, 658kWh) can be increased by 1.6times from the present value (90, 067kWh) without significant reduction of COP.

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