Thermal-hydraulic-chemical coupling mechanism of porous-fractured karst geothermal reservoirs
Water-rock reactions in karst thermal reservoirs during exploitation may lead to deformation of flow pathways.Neglecting this reaction will yield a large deviation in predicted productivity of geothermal reservoirs.A(pore-fracture)two-medium thermal-hydraulic-chemical coupling model was developed to reveal the evolution of the pore-fracture morphology in karst geothermal reservoirs under water-rock reaction,and it was verified.With this model,the distribution characteristics of multi-physics(thermal-hydraulic-chemical)field and pore-fracture morphology are analyzed,and the influences of water-rock reaction on the pore/fracture deformation and system thermal performance are identified.The following results are obtained.First,dissolution reaction occurs at the injection well under the condition of under-saturated injection.The fracture aperture increases by 0.32％and the porosity increases by 75.76％after 30 years.Second,after 30 years of production,the average fracture aperture in the case of considering water-rock reactions in both pores and fractures decreases by 0.05％,and that in the case of only considering water-rock reactions in fractures increases by 17.12％,showing the opposite trend.Third,after 30 years of production,the production temperature in the case of considering water-rock reactions in both pores and fractures is similar to that in the case of only considering water-rock reactions in fractures,but the injection-production pressure difference increases by 1.57 times.In conclusion,for karst geothermal reservoirs,the water-rock reaction in pores has great influences on pore/fracture deformation and system thermal performance,and it changes the mechanism at which the water-rock reaction acts on the fracture aperture and affects the system's injection-production pressure difference.Thus,the water-rock reaction in pores should not be ignored,but should be fully considered in accurate productivity prediction and economical evaluation of karst geothermal reservoirs.This understanding provides a theoretical and technical support for the development planning and productivity prediction of karst geothermal reservoirs.
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