Human intervention in geothermal systems, such as fluid injection, can reactivate faults in the subsurface. This study investigates the Lower Carboniferous system and specifically the Dinantian formation. This specific formation is particularly interesting for deep geothermal energy (>100ºC) in the Netherlands, Belgium and Germany but can serve as a proxy for fractured carbonate deep geothermal reservoirs worldwide. This study experimentally investigated the stress conditions under which Dinantian carbonate faults, filled with gouge material, reactivate, heal and relax. Moreover, we tested different grain size distributions, which serve as a proxy of grain dissolution during fluid injection in geothermal systems. Finally, we investigated the effect of shearing history on the reactivation envelopes, healing and relaxation of carbonated faults.
Experiments were performed in a rotary shear apparatus to shear a simulated gouge obtained from quarry material of the Dinantian carbonates. It was sieved to be in grain size clusters of 45 μm, 50% 45 μm - 50 125 μm, 15 μm, 5-125 μm and 125-212 μm. Experiments were performed at room temperature, dry or wet, using de-ionised water or, in 2 cases, bine as pore fluid, under drained conditions. The brine was obtained from an existing geothermal well and contained mainly different Na and Cl salts. In every single experiment and sample, we stepped the normal stress up from 1 to 10 MPa, and down from 10 to 1 MPa, using stress steps of 1 to 2 MPa. During each stress interval, we performed a slide-hold-slide procedure, where the hold times were 10-100-1000 sec, and the sliding interval lasted 10 seconds, with a velocity of 20 μm/sec. Using the peak shear stress after each hold period, we calculated the Mohr-Coulomb reactivation envelopes of each experiment, as well as the quantification of the fault healing and relaxation as a function of hold time. Furthermore, we quantified the fault healing rate and relaxation rate as a function of grain size and shearing history during stepping up and down the applied normal stress.
Our preliminary results showed that carbonate laboratory faults exhibited a friction coefficient of up to 0.74 for both dry and wet experiments. However, the friction coefficient of wet experiments showed a greater range of values as a function of hold time compared to dry. Dry and wet experiments displayed a clustered behaviour in healing and relaxation with the wet experiments preserving the highest values of both healing and relaxation throughout the shearing history of the samples. Wet samples display higher values of healing/relaxation rate compared to the dry ones. Grain size differentiation appears to have an effect on the healing/relaxation rate for wet samples (DI water and brine) and no effect on dry samples, but further investigation is required.
Future work includes the investigation statistical significance of the current regressed reactivation envelopes, microstructural analysis of sheared samples and further analysis of the impact of shearing history on the fault strength.
