The construction of underground gas storage mostly focuses on depleted gas reservoirs. However, the depleted gas reservoir used to build underground gas storage in China is located far from the main gas consumption economic zone. It is necessary to reconstruct underground gas storage using nearby reservoirs in order to meet the needs of economic development. The complex three-phase seepage characteristics encountered in the process of reconstruction of underground gas storage reservoirs seriously affect their storage and injection production capacities. Combined with the mechanism of multiphase seepage and the multicycle injection production mode during the process of gas storage construction, the feasibility of rebuilding gas storage in medium- and low-permeability reservoirs was evaluated through relative permeability experiments and core injection production experiments. The results showed that the mutual driving of two-phase oil–water systems will affect the storage space and seepage capacity, that the adverse effect will be weakened after multiple cycles, and that increasing the gas injection cycle can enhance the gas-phase seepage capacity and improve the crude oil recovery. Therefore, we found that it is feasible to reconstruct underground gas storage in medium- and low-permeability reservoirs, which lays a foundation for the development of underground gas storage in China.

The Jidong Oilfield No. 2 Underground Gas Storage (UGS), located in an active fault zone in Northern China, is a key facility for ensuring natural gas supply and peak regulation in the Beijing–Tianjin–Hebei region. To evaluate the effectiveness of a combined surface and shallow borehole monitoring system under deep reservoir conditions, a 90-day microseismic monitoring trial was conducted over a full injection cycle using 16 surface stations and 1 shallow borehole station. A total of 35 low-magnitude microseismic events were identified and located using beamforming techniques. Results show that event frequency correlates positively with wellhead pressure variations instead of the injection volume, suggesting that stress perturbations predominantly control microseismic triggering. Events were mainly concentrated near the bottom of injection wells, with an average location error of approximately 87.5 m and generally shallow focal depths, revealing limitations in vertical resolution. To enhance long-term monitoring performance, this study recommends deploying geophones closer to the reservoir, constructing a 3D velocity model, applying AI-based phase picking, expanding array coverage, and developing a microseismic-injection coupling early warning system. These findings provide technical guidance for the design and deployment of long-term monitoring systems for deep reservoir conversions into UGS facilities.

Fault sealing evaluation is an important part of the preliminary evaluation of gas storage reconstruction. This article takes the 1-29 fault block of the Nanpu No. 1 structural belt in the Bohai Bay Basin of China as an example, with the goal of constructing a reservoir-type gas storage facility in this fault block and conducts research on the evaluation of the fault sealing of this fault block. A method for evaluating the vertical and lateral sealing properties of faults has been established based on a large amount of well-logging data interpretation, well formation comparison, numerical simulation, and downhole tracer methods. Based on the data from the work area, qualitative evaluation indicators such as fault occurrence, fault properties, combination characteristics, and lithological configuration combinations of the upper and lower walls were selected for fault sealing evaluation. The study area developed two types of sealing: docking sealing and fault rock sealing. The mass fraction (SGR) of fault mudstone in the work area is generally high, reaching up to 78%. The normal pressure of the eight fault sections in the research area ranges from 14.98 MPa to 15.45 MPa, and the normal pressure of the fault plane is greater than 8 MPa. Combined with the SGR value of the fault mud ratio, which is generally greater than 50%, the corresponding sand-to-ground ratio is less than 50%, and the vertical sealing of the fault belongs to the moderate to good category. Based on the comprehensive evaluation above, it is believed that the fault sealing in the research area is good and suitable for the reconstruction of gas storage facilities.