Dielectric and Thermal Characterization of Reservoir Rocks for In-Situ Hydrogen Production Via Microwave Heating

This study investigates how dielectric properties, mineral composition, and fluid saturation influence microwave energy absorption, temperature rise, and hydrogen yield in reservoir rocks from the Permian Basin. We specifically examine the role of calcite content in enabling or inhibiting hydrogen production, even when the hydrocarbon source is identical. The aim is to establish dielectric testing as a predictive tool and to identify mineralogical parameters—particularly calcite—that directly impact hydrogen generation under microwave irradiation. Three core samples were characterized using X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), and dielectric property measurements with a Vector Network Analyzer (VNA) at 2.45 GHz. Direct microwave heating experiments were performed on both air-saturated rocks and fluid-saturated systems containing identical crude oil and brine compositions. Six total experiments were conducted: three on dry rocks (E1-0, E2-0, E3-0) and three on rock–oil–brine blends (E1-1, E2-1, E3-1). An additional test (E2-2) was performed by manually adding 10 wt% calcite to the intermediate-calcite rock to examine its effect on hydrogen production. Gas yields and temperature profiles were continuously monitored. VNA results revealed strong positive correlations between total organic content and both dielectric constant and loss index, confirming that organic matter significantly enhances microwave absorption. Direct heating experiments showed that hydrogen generation only occurred in fluid-saturated systems reaching sufficiently high temperatures. Rock 1 (negligible calcite) and Rock 3 (∼24 wt% calcite) produced hydrogen despite different maximum temperatures—2000 °C for Rock 1 and ∼950 °C for Rock 3. Surprisingly, Rock 2, which had intermediate calcite content (∼9 wt%), produced no hydrogen in two separate trials, despite using the same crude oil and brine. To investigate this inhibition effect, we blended Rock 2 with 10 wt% additional calcite (E2-2). This mixture reached ∼650 °, higher than the original Rock 2 blend (∼350 °C), and successfully produced hydrogen. The results suggest that calcite decomposition to CaO can promote hydrocarbon cracking reactions, particularly in the presence of other mineral phases, enabling hydrogen generation even at lower peak temperatures. These findings indicate that dielectric measurements can serve as a rapid screening method for predicting heating performance, while calcite content directly influences catalytic pathways for hydrogen production.