Comprehensive Evaluation of In‐Situ Stress Field in Ultra‐Deep Bedrock Gas Reservoirs

ABSTRACT Ultra‐deep bedrock gas exploration remains at a preliminary stage, where comprehensive in‐situ stress evaluation can provide crucial guidance for sweet spot prediction. Taking the bedrock reservoirs in Block K2 of the northern Qaidam Basin as an example, this study systematically assesses in‐situ stress by integrating drilling data, array acoustic logging, conventional well logging, fracturing tests, seismic interpretation, and numerical simulation. A novel in‐situ stress prediction method for ultra‐deep bedrock reservoirs is proposed, which is based on rock mechanical parameter fields and overcomes the limitations of conventional approaches regarding solution multiplicity and computational complexity. The results demonstrate that the bedrock reservoir follows a σ v > σ H > σ h stress regime with significantly lower stress activity compared to shallower reservoirs, showing σ H − σ h differential stresses predominantly ranging 8–12 MPa. Borehole breakout and drilling‐induced fracture analysis reveals a NNW‐SSE orientation of the maximum horizontal stress. Building upon measured stress constraints, we first established a well‐log interpretation method. Subsequently, we developed a 3D stress field modeling approach through Poisson's ratio optimization, leveraging the positive correlation between wave impedance and Poisson's ratio. This was achieved by integrating Petrel‐based genetic inversion with spectral decomposition, CNN, and GA optimization techniques. The bedrock displays strong horizontal stress heterogeneity, featuring alternating high‐low stress zones. This stress distribution closely correlates with tectonic deformation and faulting patterns. Thrust faults typically create 200 m‐wide stress concentration zones, causing severe borehole instability. These zones also contain poorly‐connected mineralized fractures that reduce permeability and productivity. In contrast, peripheral stress‐relief zones show better reservoir quality, with effective open fractures and consequently higher gas productivity. These findings highlight in‐situ stress as a critical indicator for both well placement and productivity evaluation in ultra‐deep bedrock reservoirs.