Hydrothermal Convection in Geothermal Reservoirs with Sloping Basement Aquitards

Abstract

The flow of hot water in a geothermal reservoir with an incl ined bottom has not been studied as extensivelY as the flat-bottom case. In many real fields. the rock formatiuns comprising the reservoir basement are far from horizontal. Hydrothermal convectinn is generally confined to a permeable aquifer region lying below the waterta~le and above the basement. owing to the impermeable character of the laUer. Thus. the geometry of the permeable zone may depart significantly from a rectangular vedical section. Most Japanese geothermal fields are located in volcanic areas; deep magma bodies supply heat conductivelY upward through ilTegular ti Ite(1 hasement structures to drive bunyant hydrothermal cirCUlation systems in the overlYing permeable strata. Under these circumstances. the effects of basement orientation may be of considerable importance. This paper presents preliminary results of a theoretical study( involving numerical simulation techniQues) of natural convection Ilaiterns in reservoirs with sloping lower houndaries. (Calculations involving non-h()rizontal basement layers are not unusual in simUlation studies of particular r.eothermal fields. hut the influence of bottom slope itself has nut yet heen systematicallY investigated. in r.eneral fashion.) First. simplA base cases are treated: these hase cases are then perturhed in various ways to examine the influence of pertinent key parameters. The significance of the parameters must be estahlished before appl icatians to practical field situations should be undertaken.