Underestimated risks for application of hydraulic fracturing into hydrate exploitation: In the perspective of formation deformation and sand production

Abstract

Hydraulic fracturing in hydrate-bearing formation enhances reservoir permeability, thereby promoting gas production, but simultaneously compromises the mechanical strength integrity of the solid skeleton, amplifying risks of reservoir deformation, seafloor subsidence, and sand production. Despite these challenges, the interplay between hydraulic fracturing and such risks remains inadequately quantified, resulting in underestimation of exploitation risks. In this study, we developed a fully coupled thermal-hydraulic-mechanical-chemical model that incorporates sand production dynamics, validated against experimental data and numerical benchmarks. Using the Shenhu hydrate reservoir as a case study, we evaluated the effects of hydraulic fracturing (characterized by a fracture length of 5 m, a fracture permeability of 1 D, and a damage zone permeability of 50 mD) on long-term gas production, formation deformation, and sand production behaviors. Our analysis reveals that hydraulic fracturing increases cumulative gas production by 57 %, reaching 2.34 × 10⁶ m³ after 1000 days. However, it also triggers significant mechanical degradation: the volumetric strain in the damaged zone exceeds 2.5 %, which exacerbates formation collapse, inducing an additional 7 cm of seafloor subsidence, for a total of 21 cm and intensifying sand production to 903 m³, 2.4 times higher than production without fracturing. Further increases in fracture length beyond 5 m or fracture permeability above 0.1 D yield diminishing returns in gas production but exacerbates sand production. Enhancing the damaged zone's permeability from 25 mD to 75 mD increases gas production by 20 %, but also raises sand production by 49 % and seafloor subsidence by 4 cm.