Reinjection of cooled geothermal brines into sandstone reservoirs

Injection of heat-depleted brines into clastic sedimentaryreservoirs with alternating clay, sand and sandstone sequences has long been considered a delicate subject among petroleum and geothermal operators. Not onlyis injection an environmental pre-requisite in waste disposal, but it is also a means of achieving sound reservoir management as regards optimum heat sweep and recoveryand pressure maintenance. However, without thorough and careful planning, injection can turn to disaster, for example where the formation and reinjected waters prove incompatible, or there is particle entrainment, capture and release, or unsuccessful well completion, which often leads to irreparable damage to the well and formation. This paper reviews the physics and chemistry governing these critical areas, and the problems likelyto occur while pumping cooled brines into sandstone reservoirs: (i) water incompatibilities and subsequent supersaturation/precipitation of solid particles and clayswelling; (ii) fine particle migration/settling processes and relative pore throat bridging, pore retention/ bridging and formation plugging; (iii) flow velocities and near well or well completion erosion impairment; and (iv) temperature-related electrokinetic and water–rock interaction effects. The foregoing are illustrated bylaboratory experiments, model runs and field trials. Application of these studies to water injection in the Upper Pannonian clastic reservoir in the Great Hungarian Plain is also discussed. # 2003 CNR. Published byElsevier Ltd. All rights reserved