Gas entrapment and migration in deltaic soils containing biogenic methane: challenges for site investigation and volume estimation
Gas entrapment and migration in deltaic soils containing biogenic methane: challenges for site investigation and volume estimation
The presence of biogenic methane in soft deltaic sediments poses a major challenge for geotechnical characterisation, particularly in marine environments where access is limited and sampling inevitably disturbs the natural stress state. This issue is especially relevant in the Llobregat prodelta, where multiple field campaigns have documented persistent gas releases during drilling and penetration tests. These emissions, sometimes lasting more than 48 hours, indicate localised gas accumulations within specific sedimentary layers. However, despite extensive field evidence, no current insitu technique can quantify gas content or pressure conditions without inducing significant perturbations. As a result, the behaviour of these partially saturated soils remains poorly understood, and the mechanisms governing gas entrapment, migration, and release require a more rigorous analytical framework.
Previous investigations in the area (e.g., [1, 2]) identified surface gas emissions and geotechnical anomalies consistent with subsurface gas pockets. These anomalies include reductions in undrained shear strength ratios, attenuated porepressure responses, and irregular dissipation curves during CPTu and SDMT testing. Although these indicators suggest the presence of gas, they do not permit quantification, nor do they distinguish between discontinuous gas bubbles and connected gas pathways. The heterogeneity of the sedimentary environment further complicates interpretation: gas occurrences do not consistently correlate with organicmatter content, nor do they follow predictable stratigraphic patterns. Instead, gas tends to accumulate within siltysand or sandysilt layers that act as temporary reservoirs, confined beneath lowpermeability clayey silts.
Gassy sediments are a challenging topic in geotechnical engineering because they involve multiphase, multiscale, and multiphysical phenomena. Understanding the effects of biogenic methane on the geotechnical properties of gassy soils can help improve the design, construction, and management of offshore and coastal infrastructures and mitigate potential risks associated with gassy soils (e.g., [3, 4]).
This study develops a methodological framework to evaluate the unsaturated behaviour of these gasbearing sediments and to estimate the initial gas volume released during drilling. The approach integrates capillary pressure-saturation curves, effective gas permeability assessment, and a generalised Darcys law for compressible fluid.
The objective is twofold: (i) to determine the gas pressure required to initiate desaturation and enable gas breakthrough in the sediment matrix, and (ii) to estimate the volume of gas released during documented escape events, using field observations and soilspecific hydraulic properties.
Dani Tarrago; Enrique Romero; Antonio Gens
International Conference on Advances and Innovations in Soft Soil Engineering (ICAISSE2026)
Multiphysics Behaviour
https://doi.org/10.53243/ICAISSE2026-173