A Study of Permeability Changes Due to Cold Fluid Circulation in Fractured Geothermal Reservoirs

Abstract

Reservoir behavior due to injection and circulation of cold fluid is studied with a shear displacement model based on the distributed dislocation technique, in a poro-thermoelastic environment. The approach is applied to a selected volume of Soultz geothermal reservoir at a depth range of 3600 to 3700 m. Permeability enhancement and geothermal potential of Soultz geothermal reservoir are assessed over a stimulation period of 3 months and a fluid circulation period of 14 years. This study-by shedding light onto another source of uncertainty-points toward a special role for the fracture surface asperities in predicting the shear dilation of fractures. It was also observed that thermal stress has a significant impact on changing the reservoir stress field. The effect of thermal stresses on reservoir behavior is more evident over longer circulation term as the rock matrix temperature is significantly lowered. Change in the fracture permeability due to the thermal stresses can also lead to the short circuiting between the injection and production wells which in turn decreases the produced fluid temperature significantly. The effect of thermal stress persists during the whole circulation period as it has significant impact on the continuous increase in the flow rate due to improved permeability over the circulation period. In the current study, taking into account the thermal stress resulted in a decrease of about 7 °C in predicted produced fluid temperature after 14 years of cold fluid circulation; a difference which notably influences the potential prediction of an enhanced geothermal system.

Evaluation of recoverable energy potential from enhanced geothermal systems: a sensitivity analysis in a poro‐thermo‐elastic framework

“…), Q H is the fluid sink/source, q T is the heat flux, c p is the specific heat capacity, b is the fracture apertures, Q T is the heat sink/source term, λ is the thermal conductivity, f‐ superscript denotes the corresponding fracture property, T ‐, s‐ and l ‐subscripts refer to transpose, solid and liquid, respectively, and d‐ subscript refers the fracture plane (Watanabe ; Gholizadeh Doonechaly et al . ).…”

“…Generated subsurface fracture map of S oultz geothermal reservoir (after, Gholizadeh Doonechaly et al . , ).…”

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Geothermal Energy: Delivering on the Global Potential

“…In doing so, a delicate compromise must be negotiated between injecting so close to the production zones that the temperature of the produced fluids is reduced and injecting so far away that the desired pressure maintenance effects are not achieved. This balancing act is made no simpler by the tendency for thermal contraction fracturing to increase the permeability where cooler reinjectates enter the high-temperature reservoir [44]; a zone that seemed suitably remote from the production zones can become more intimately hydraulically connected with it over time, as these thermal contraction fractures propagate. A further consideration is the minimization of undesirably large seismic events.…”

“…However, significant technical challenges remain, including the challenge of ensuring borehole stability when the wall rocks exceed melting point [55], and the difficulties of obtaining good records of the geology encountered when there is no core or even chippings to examine. Incremental advances in more conventional reservoir stimulation approaches must also be pursued, such as "soft" thermal [44] and chemical stimulation of permeability.…”