Objective Against the coordinated advancement of new energy system construction and national energy security strategies, the engineering value of deep underground space has gradually extended beyond traditional resource exploitation to energy security guarantee, power system regulation and low-carbon geological storage. Projects including underground gas storage, geological hydrogen storage, Compressed Air Energy Storage (CAES) and CO₂ geological storage all rely on the storage capacity, sealing performance and long-term safe operation capability of underground space. Nevertheless, existing research cannot clearly distinguish geological resource volume, theoretically utilizable space and engineering-accessible space. In addition, further optimization is needed for the matching relationships between geological carriers and application scenarios, and there is a lack of systematic analysis of key technical bottlenecks.

Methods Based on research achievements in underground gas storage, geological hydrogen storage, CAES and CO₂ geological storage projects, we conduct an inductive analysis of geological carrier properties, storage-sealing mechanisms, operating conditions, and risk control requirements. Supported by typical engineering cases, we systematically compare the differences among various deep underground spaces in terms of boundary identification, formation modification, injection-production operations, and long-term monitoring. Furthermore, driven by scenario demands, we analyze the matching relationships between different geological carriers and specific storage applications, and identify the major constraints that limit the transition from geological resource assessment to engineering accessibility evaluation.

Results From the perspective of engineering utilization, deep underground spaces are categorized into three types: cavern type, porous type and fractured/fractured-vuggy type. We clarify the distinctions of different geological carriers in storage boundary delineation, space identification, sealing modes, and operational risks, and establish a matching framework between geological carriers and typical application scenarios. It is proposed that the resource assessment of deep underground space should shift from simple statistical measurement of resource scale to engineering-accessible space evaluation, with geological resource volume, theoretically utilizable space and engineering-accessible space set as progressive evaluation tiers.

Conclusion The core of deep underground space development and utilization lies in judging the matching degree among geological carriers, engineering scenarios and long-term operating conditions. Future research should improve engineering-accessibility evaluation methodologies, prioritizing the detailed characterization of geological carriers, wellbore and caprock integrity assessment, sealing capacity degradation under cyclic thermal-pressure loads, multi-source monitoring and early warning systems, and digital-twin-based operational management. The research findings can provide critical technical support for site selection, construction, operation, and safety management in deep underground space development.