Long-term matrix acidification experiments under reservoir conditions using the Thermo-Triaxial device

Matrix acidification is one of the most popular stimulation techniques to increase the porosity and permeability of reservoir systems. Usually, the thermal-hydro-mechanical properties of reservoir rocks affected by the matrix acidification process are studied using flow-through tests or autoclave experiments. In this study, a novel acidification approach was tested using a thermal triaxial device at the TU Darmstadt laboratory. Thereby, hydrochloric acid 0.0375% (HCl pH=2) was flushed continuously through a total of five Remlinger sandstone samples under reservoir conditions (90^oC temperature, s₁=25 MPa, and s₃=23MPa). Changes in matrix permeability and other petrophysical parameters due to the chemical reaction between the rock sample and HCl were recorded before, during, and after the reactive experiments. In addition, outflow fluid samples were collected and the pH was subsequently measured. After approximately 30 days of continuous flow for each sample, the permeability increased for all the samples, with a maximum increase of 300%. Likewise, porosity increased from 13.2% to 14.5%. In contrast, P- and -S-wave velocities decreased from 2608 to 2189 m‧s^-1 and from 1540 to 1380 m‧s^‑1, respectively. Test results provide important information for reservoir stimulation and can be used to benchmark THMC models.