A multi-zone characterization-based framework for effective storage capacity evaluation in water-bearing reservoirs

Abstract

The construction and operation of gas reservoir-type underground gas storage (UGS) facilities play a pivotal role in ensuring the safety and stability of natural gas supply. For gas reservoirs with edge or bottom water, the subsurface gas-water two-phase flow dynamics and high-speed injection/withdrawal (I/W) processes result in complex distributions of gas and water within the reservoir layers. Additionally, the boundaries of multiphase flow zones are often poorly defined, and the pore volume utilization efficiency (PVUE), which directly impacts effective storage capacity, remains difficult to quantify. These challenges hinder the accurate evaluation of gas storage capacity and complicate the design of optimal construction and operational parameters for UGS facilities. To address these issues, this study proposes an integrated approach combining multi-cycle I/W experiments, numerical reservoir simulations, and the mass balance method to accurately assess UGS storage capacity. The methodology was applied to an active UGS facility constructed in a water-bearing gas reservoir in northwestern China. The gas-bearing reservoir was categorized into four distinct flow zones: the gas zone, the gas-displacing-water zone, the transition zone, and the water zone. Key factors influencing immobile gas-bearing pore volume — such as water invasion and stress sensitivity — were identified for each zone. A mathematical model was developed to predict immobile gas-bearing pore volume, and a quantitative model was established to estimate effective gas storage space (underground) by incorporating PVUE variations across different flow zones. These models provided theoretical foundations for designing UGS construction and operational strategies. The results demonstrated: (1) After six I/W cycles, the measured PVUE in the gas zone was 99.3% and 94.9% for blocks B1 and B2, respectively. In the gas-displacing-water zone, the PVUE was 80.9% and 73.8%, while in the transition zone, it was 47.9% and 40.3%. (2) The total gas-bearing pore volume of the UGS was 9.65 million rm³ (subsurface conditions), with an effective gas storage space of 5.39 million rm³ after accounting for PVUE variations across flow zones. (3) Numerical simulations confirmed that the proposed UGS operational design would achieve a total inventory of 8.24 × 10⁸ sm³ (surface conditions) and an effective storage capacity of 6.67 × 10⁸ sm³. This study provided a robust framework for evaluating and optimizing UGS storage capacity in water-bearing gas reservoirs, offering valuable insights for the design and operation of such facilities.

Keywords

Underground gas storage / Water-bearing gas reservoirs / Storage capacity evaluation / Pore volume utilization efficiency

CRediT authorship contribution statement

Junyu You: Writing-review & editing, Writing-original draft, Methodology, Funding acquisition, Conceptualization. Xingxin Jiang: Writing-review & editing, Writing-original draft, Methodology, Formal analysis. Xiaoliang Huang: Writing-review & editing, Supervision, Project administration, Funding acquisition. Qiqi Wanyan: Writing-review & editing, Resources, Data curation. Ziang He: Visualization, Validation. Songze Li: Writing-review & editing. Hongcheng Xu: Resources, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Science and Technology Major Project of China (2025ZD1406806), the National Natural Science Foundation of China (52304023 and 52274034), the Science and Technology Research Program of Chongqing Municipal Education Commission (KJQN202501521 and KJZD-M202401501), Natural Science Foundation of Chongqing (CSTB2022NSCQMSX0403), Chongqing Municipal Support Program for Overseas Students Returning for Entrepreneurship and Innovation (2205012980950154). Thanks for the relevant data provided by the Underground Storage Research Center of PetroChina Research Institute of Petroleum Exploration & Development.

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