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Self-Sensing Graphene Wellbore Cements for Geothermal Applications
Ali ETTEHADI, Mileva RADONJIC
[Oklahoma State University, USA]
Ensuring the long-term integrity of wellbore construction materials is a fundamental challenge in geothermal and other deep subsurface environments where direct inspection is infeasible. The development of self-sensing cement systems that maintain reliability under high-temperature, water-saturated conditions offers a transformative opportunity for continuous integrity monitoring. Electrical conductivity characterization provides a promising foundation for embedded sensing, though the dominant effect of conductive pore fluids can obscure signals associated with microcracking and matrix degradation. This study establishes an AC-conductivity framework for Class H geothermal cement to investigate the effects of graphene nanoplatelets modification, saturation state, thermal conditioning, and mechanical loading on electrical conductivity. The objective is to define frequency-dependent conductivity metrics capable of providing robust indicators of material integrity under realistic geothermal conditions. The experimental workflow consisted of three integrated steps. In the first step, the electrical response of fresh Class H slurry was continuously monitored using a conductivity probe integrated with an atmospheric consistometer operating between 27 °C and 82 °C, enabling real-time tracking of slurry conductivity during hydration and comparison with thickening-time behavior. In the second step, hardened cylindrical specimens (2 × 4 in.) comprising unmodified and graphene nanoplatelet–modified cements were prepared and cured at 90 °C and 95% relative humidity in a controlled environmental chamber. Conductivity was measured at 23–25 °C under fully saturated conditions using a broadband AC impedance analyzer equipped with guarded parallel electrodes over the 1 Hz–10 kHz frequency range, focusing on the stable 1–10 kHz plateau representative of bulk transport behavior. To probe the effect of moisture, the same specimens were conditioned at 60 °C and 50% relative humidity for one week and re-tested. Thermal durability was further assessed through a two-stage cycling protocol (5–60 °C and 0–90 °C) designed to thermally induce microstructural fracturing, with electrical characterization performed before and after each phase. In the third step, mechanical loading tests including unconfined compression and Brazilian tensile configurations, were conducted with in-situ conductivity monitoring to capture correlations between microcrack development and electrical response. Additional measurements on fractured and oven-dried specimens were used to isolate moisture-related effects. Overall, results confirm that AC conductivity provides consistent, frequency-resolved indicators of microstructural and moisture-related changes in geothermal well cements. Graphene nanoplatelet modification stabilized the conductive network, enhanced signal sensitivity, and improved measurement reproducibility. These outcomes demonstrate the potential of frequency-based conductivity diagnostics to underpin self-sensing cement systems for real-time geothermal well integrity monitoring.
Topic: Emerging Technology