Fines migration as a challenge for reservoir-wide chemical stimulation using retarded acid systems

Declining permeability of geothermal reservoirs due to fines migration is often considered with regard to existing fines like clay particles that are mobilized. However, fines are also generated due to dissolution and chemical stimulation techniques can induce the generation of fines. For near-wellbore regions, this is uncritical as fines can be extracted after stimulation using cleaning lifts. With the aim of replacing hydraulic stimulations for enhanced geothermal systems by a reservoir-wide chemical stimulation, the penetration depth of the acids needs to be substantially increased. However, cleaning lifts to remove fines are no longer possible in greater distance from the wellbore, so alternative strategies are required.

Flow-through experiments with citric acid on dolostone were used to investigate the dependence of fines generation on the reaction conditions. For all dissolution regimes over a range of different Damköhler numbers, fines were generated and could greatly reduce the permeability of the rock samples. Only when large pathways were created by the dissolution process, the generated fines were transported out of the major pathways as well as dissolved therein, thus not interfering with the increase of permeability due to dissolution. The results highlight the great challenges for reservoir-wide chemical stimulations techniques using retarded acid systems, but also indicate possible solutions.