Summary: To solve the issue of seasonal gas peak shaving, building underground gas storage is an effective solution. Geophysical monitoring technology is necessary for dynamic gas storage management. However, as these technologies are not highly accurate in the lateral direction, new technologies need to be developed. In recent years, borehole-to-surface electrical imaging (BSEI) technology has been widely used in reservoir and fracturing monitoring. In the dynamic monitoring of gas storage, high-power direct current is applied to the borehole through the steel casing, and the potential is measured on the surface. Then, resistivity inversion is performed to analyze the transverse distribution of the injected gas. To prove the feasibility of monitoring injected gas, a 3D forward case is simulated using the finite difference method (FDM). The model-structure-constrained inversion approach is verified. Based on the forward simulation results and the formation pressure period, the gas storage observation scheme is designed. The case study shows that the gas storage inversion results constrained by the upper formation have a high resolution and clearly show the transverse boundary of the target reservoir, which is consistent with the fault boundary determined by seismic prospecting. Therefore, 3D resistivity imaging can clearly show the distribution of gas reservoirs during injection and production. This study shows that BSEI is an effective technique for dynamic monitoring of gas storage and has good application prospects.

The traditional source uses a square wave with a fixed fundamental frequency to excite transient electromagnetic (TEM) fields, with harmonic energy primarily concentrated in the low-frequency range, limiting the detection resolution of the TEM. The differential pulse, composed of two square waves with identical pulse widths but opposite polarities, concentrates harmonic energy more effectively. By adjusting the pulse width of the differential pulse, the concentration frequency band of harmonic energy can be changed, enabling multi-resolution detection of geological structures at different depths. In this study, TEM fields are excited using differential pulses of varying pulse widths during power supply. A preconditioned precise integration time-sweeping wavefield reverse transformation method is applied to interpret the virtual wavefield from the diffusion field, effectively improving the numerical accuracy and noise resistance of the virtual wavefield. Then, the finite-difference migration imaging method is used to obtain imaging profiles for differential pulses of different pulse widths, and stacking techniques are applied to acquire high-resolution characteristics of electrical interfaces at various depths. Finally, the feasibility of the method is verified through a complex geological model. By comparing the relative anomalies of square waves and differential pulses with different pulse widths, the results show that the electromagnetic anomalies for differential pulses are increased by 53.7%. Therefore, using differential pulses as the excitation source leads to higher-resolution electromagnetic responses, which in turn result in high-resolution imaging.

The borehole-to-surface electromagnetic (BSEM) method is widely employed in oil and gas exploration and downhole monitoring. However, the strength of the ground observation signals of the BSEM method is affected by the metal steel casing in the well. To investigate the response characteristics of the BSEM method under metal casing conditions, this study performed three-dimensional BSEM forward modeling based on a cylindrical grid. The finite volume method was adopted to discretize and solve the governing equations of the electromagnetic field, and the cylindrical grid was partitioned in accordance with the axisymmetric geometric features of the wellbore-casing system, thereby achieving high-precision adaptation to the well structure. To explore the impact of metal casing in an alternating electromagnetic field, four typical models were established: a linear source, a long metal wire, a metal casing, and a casing with a cement sheath. The characteristics of ground signals under low-frequency alternating emission conditions were systematically studied. By comparing the simulation results with the 1D analytical solution, this method was verified to have high numerical accuracy, which can accurately reflect the responses of a metal casing and multiple media interfaces to the alternating electromagnetic field. Based on comparative analysis, the differences in underground electromagnetic field distributions among different source models and their applicable ranges were clarified, and the applicable scenarios and effective detection depths of different models in actual monitoring were explored. This research provides numerical simulation cases to investigate the role of metal casings in BSEM observations, and also lays a theoretical foundation for the interpretation of downhole electromagnetic data, which is of positive significance for improving the effect of applying BSEM technology in oil and gas exploration.

Hydraulic fracturing is a crucial technology for developing unconventional oil and gas resources. However, conventional geophysical methods struggle to efficiently and accurately image proppant-connected channels created by hydraulic fracturing. The borehole-to-surface electromagnetic imaging method (BSEM) overcomes this limitation by utilizing a controlled cased well source. Placing the source close to the target reservoir and deploying multi-component receivers on the surface enable high-precision lateral monitoring, providing an effective approach for dynamic monitoring of hydraulic fracturing operations. This study focuses on key aspects of forward modeling for BSEM. A three-dimensional finite-volume method based on the Yee grid was used to simulate the borehole-to-surface electromagnetic system incorporating metal casings, validating the method of simulating metal casing using multiple line sources. The simulation of the observation system and the frequency-domain electromagnetic monitoring simulation based on actual well data confirm BSEM’s high sensitivity for monitoring deep subsurface formations. Critically, well casing exerts a substantial influence on surface electromagnetic responses, while the electromagnetic contribution from line sources emulating perforation zones necessitates explicit incorporation within data processing workflows.

Hydraulic fracturing is a core technology for unconventional oil and gas development. However, existing geophysical monitoring methods struggle to efficiently and accurately image the effective stimulated reservoir volume (SRV) of borehole fracturing. To address this, the borehole-to-surface controlled-source electromagnetic (BS-CSEM) monitoring technology is proposed: it transmits electromagnetic signals to the target fracturing interval through the well casing with the source positioned close to the target zone, while surface multicomponent receivers perform time-lapse monitoring for SRV imaging. This study innovatively adopts the 3-D finite volume method (FVM) with local grid refinement for BS-CSEM 3-D forward simulation. From logging data of a target well, a 3-D geoelectric model that incorporates refined grids for the target fracturing interval and a full-wellbore metal casing is built for simulation, generating surface multicomponent electromagnetic data. Subsurface fracturing dynamics’ impact on electromagnetic data variations is analyzed. An inversion method integrating Limited-memory BFGS (L-BFGS) with 3-D geoelectric model constraints is validated, with numerical results confirming its effectiveness. Ultimately, SRV inversion imaging using actual surface data from the target well verifies the BS-CSEM technique’s efficiency, accuracy, and broad application prospects for fracturing monitoring.