High-gas-tightness ultrafine cement-based grouts for CAESA: Multi-scale synergistic mechanism and RSM-based geology-adaptive optimization

Compressed air energy storage in aquifers (CAESA) is promising because of its widespread site availability and low reservoir construction cost. However, air leakage through high-permeability formations remains a major challenge to energy-storage efficiency. Grouting offers an effective approach to constructing artificial low-permeability barriers, but dedicated high-gas-tightness cementitious grouts for this demanding engineering scenario remain scarce. In this study, a geology-adaptive ultrafine cement-based grout was developed via a systematic “Orthogonal–Single Factor–RSM” optimization strategy. First, a ternary binder system incorporating ultrafine fly ash (UFA) and silica fume (SF) was optimized to establish a dense particle-packing skeleton. Nano-silica sol (NSS) and cellulose nanofibers (CNF) were then introduced for multi-scale modification. Performance and microstructural analyses showed that NSS refined the pore structure through filling and pozzolanic reactions, while CNF was associated with improved splitting tensile strength and resistance to gas-driven microcracking. Meanwhile, 1.8 wt.% PCE maintained grout flowability by mitigating nanomaterial-induced rheological deterioration via steric hindrance. Response surface methodology (RSM) was employed to quantify component interactions and derive tailored formulations for porous, fractured, and mixed reservoir types under differentiated rheological constraints. Validation tests confirmed that the optimized grouts achieved high compressive strength (>30 MPa) and ultra-low gas permeability of 0.21–0.56 mD. These results demonstrate the engineering potential of the proposed material-design framework for underground gas containment in deep energy-storage applications.