Hydrochemical Stimulation in Fractured Carbonate Rocks - Monitoring and Simulation

Hydrochemical stimulation by acidification of geothermal wells is a standard procedure to remove drilling mud and to improve hydraulic contact between borehole and reservoir. Several successive stimulations using hydrochloric acid were monitored by both online measurement and conventional analysis. The results show recovery curves with distinct two-step exponential temporal decrease of the chloride concentration. The initial decrease is fast, representing water and acid flow along pathways which are very well connected to the borehole. After the fluid from these flow paths has been recovered, the concentration decreases at a lower rate. This can be attributed to water flowing in less well connected flow paths. With additional stimulations the chloride concentration curve approaches a mono-exponential decrease. This indicates that the flow paths within the reach of the stimulation get more homogeneous.

A numerical model of flow and solute transport in the borehole and the surrounding geothermal reservoir was developed in order to simulate the observed chloride-recovery behaviour in the course of a number of successive hydrochemical stimulations. The finite-element model was adapted to match the observed hydraulic and hydrochemical data range.

Simulation hereby allows to separate time-dependent single contributions from the different flow paths to the total recovery concentration. Based on this information, indications of structural change due to the successive acidification steps can be derived from the chloride-recovery curves of each step. Furthermore, for typical settings the minimum time and volume of solution can be estimated which is required to achieve a significant structural signal

The derived structural information can be useful to predict the long-term behaviour of a geothermal injection well which during operation is exposed to a mild but constant chemical stimulation by the injected cold and, with respect to chloride in the rock matrix, undersaturated water.