Chongyuan Zhang

Conventional Diagnostic Fracture Injection Tests (DFITs) are widely used for formation pressure estimation, but in practice, they frequently require days, weeks, or even months of extended shut-in periods, a challenge particularly pronounced when large injection volumes are coupled with ultra-low formation permeability. While recent studies have proposed various modified DFIT approaches to reduce testing time, direct physical validation confirming the reliability of the derived formation pressure estimates remains scarce in the literature. This study applies a low-rate/volume injection mini-frac approach that integrates flow regime identification and Horner analysis. Two complementary field cases are presented: a standard DFIT in a shale reservoir to validate the baseline methodology, and a low-volume mini-frac in a tight granite formation to demonstrate rapid estimation. Results show that low-volume injections exhibit a flow regime evolution identical to standard DFITs, yet this approach is expected to accelerate the transition to the pseudo-radial flow regime. To verify the reliability of formation pressure estimates derived from such methods, the formation pressure estimated in the low-rate/volume injection mini-frac case was benchmarked against a decade of continuous downhole fluid pressure monitoring data from the same well, yielding a relative error of less than 5%. The findings suggest that employing a lower injection rate and volume can improve formation pressure testing efficiency, with potential applications in unconventional hydrocarbon development and deep geo-energy engineering.

Identifying the primary triggering factors that govern injection‐induced seismicity is essential for reliable seismic‐hazard assessment. Here, we present observations from 20 surface stations deployed at China's first enhanced geothermal system (EGS) project in the Gonghe Basin during hydraulic stimulation and circulation tests in 2021. Using a machine‐learning workflow, we compiled a high‐precision catalog of 7,346 events ( M _L −2.0–2.18) that reveals a donut‐like seismicity distribution comprising four major clusters with variable faulting styles. Principal‐stress modeling indicates profoundly weak faults with a low friction coefficient ( μ ∼ 0.23 with 95% confidence interval of 0.19–0.48), consistent with chlorite‐rich gouges, and a subcritical stress state in which low‐to‐moderate fluid overpressure (∼0.8–34 MPa) is required to initiate seismic slip. We find that the seismicity distribution is controlled by scale‐dependent factors: at the site‐scale (i.e., within ∼1 km of the wellheads), events localize on mapped natural faults, indicating shear reactivation of subcritically stressed, pre‐existing structures rather than the creation of new tensile fractures; at the borehole‐scale (i.e., within ∼1–100 m of the borehole trajectories), the main seismogenic zone coincides with pronounced SHmax rotation, reflecting lithologic contrasts and localized fault‐damage zones. Overall, our integrated analysis underscores the necessity of explicitly incorporating the low‐friction mineral phases, and scale‐dependent controls, that is, heterogeneities of stress and structures, into seismic hazard assessments for EGS projects.

Identifying the primary triggering factors that govern injection-induced seismicity is essential for reliable seismic-hazard assessment. Here we present observations from 20 near-surface stations deployed at China’s first enhanced geothermal system (EGS) demonstration project in the Gonghe Basin during hydraulic stimulation and circulation tests in 2021. Using a machine-learning workflow, we compiled a high-precision catalog of 7,346 events (ML -2.0 – 2.18) that reveals a donut-like seismicity distribution comprising four major clusters with variable faulting styles. Principal-stress modeling indicates profoundly weak faults with a low friction coefficient (μ~0.24), consistent with chlorite-rich gouges, and a subcritical stress state in which low-to-moderate fluid overpressure (~0.85–30.9 MPa) is required to initiate seismic slip. We find that the seismicity distribution is controlled by scale-dependent factors: at the ~1 km site-scale, events localize on mapped natural faults, indicating shear reactivation of subcritically stressed, pre-existing structures rather than the creation of new tensile fractures; at the ~1–100 m borehole-scale, the main seismogenic zone coincides with pronounced SHmax rotation, reflecting lithologic contrasts and localized fault-damage zones. Overall, our integrated analysis underscores the necessity of explicitly incorporating the low-friction mineral phases, and scale-dependent controls, i.e., heterogeneities of stress and structures, into seismic hazard assessments for EGS projects.

The Bohai Strait is a special tectonic region in North China, characterized by strong fault activity and frequent seismic events. In this study, we analyzed the stress state across the Bohai Strait in detail by integrating the stress data derived from the hydraulic fracturing measurements in four boreholes along the strait (i.e., Pingdu, Xiangli, Changdao, and Gaizhou from south to north) and evaluated its implications for seismicity. The results reveal that the gradient coefficients of the maximum (S_H) and minimum horizontal stresses (S_h) with depth in Xiangli and Changdao are over 1.59 and 1.87 times the corresponding stresses of the North China Block. However, the S_H and S_h in Pingdu and Gaizhou do not exceed 50.2% and 59.4% of those of the North China Block. The stress values increase as the distance approaches the interaction of the regional faults in the Bohai Strait. The S_H orientation in the Bohai Strait region is N68.67 ± 9.30° E, consistent with the prevailing NEE–E-W regional stress direction. According to the Coulomb friction failure criterion, the friction coefficients of the four boreholes range from 0.24 to 0.52, lower than the theoretically critical limit for inducing fault slip in the upper crust (i.e., Byerlee’s law). The faults in the strait region are considered to be contemporarily stable but need to be further evaluated, considering more influencing factors. This study provides a new, instructive understanding of the variations in the stress state in the Bohai Strait region.