Fines migration poses challenge for reservoir-wide chemical stimulation of geothermal carbonate reservoirs

Fines material, which can compromise the permeability of geothermal reservoirs, is often only considered with regard to existing fines like clay particles that are mobilized. However, fines are also generated due to dissolution. This is a significant risk coming from chemical stimulation techniques in geothermal reservoirs, that try to increase the range of stimulation with retarded acid systems. Deeper in the reservoir, fines cannot be extracted as is done in the near wellbore region after stimulation and the generation of fines has to be prevented. This work investigates the dependence of fines generation on the reaction conditions. Flow-through experiments with citric acid on dolostone were conducted, creating dissolution regimes with a range of different Damköhler numbers. During the stimulation experiments, the creation of wormholes and the widening of existing flow paths could be differentiated by the shape of the differential pressure curves measured across the samples. Fines were always generated and could greatly reduce the permeability of the rock samples. But for very high Damköhler numbers, where the dissolution created large pathways, the fines were transported out of the major pathways as well as dissolved therein, thus not interfering with the increase of permeability due to dissolution.