生産還元による地熱貯留層の垂直変位について

抄録

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.