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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Description of Subsidence Phenomena Due to Gas Extraction in Deep Layers with Advanced Three-Phase Constitutive Model

Mathieu Nuth1; Lyesse Laloui1; Bernhard A. Schrefler2

(1) Soil Mechanics Laboratory, Swiss Federal Institute of Technology Lausanne EPFL, Lausanne, Switzerland.

(2) Department of Construction and Transportation Engineering, University of Padua, Padua, Italy.

In coastal regions, the land subsidence due to industrial pumping of underground fluids such as methane is documented on the basis of in situ surveys. Some laboratory characterization of the soils hosting those fluids have also been published to complement the knowledge on compaction due to changes of fluid pressures. The withdrawal of gas is simulated in the laboratory by injecting water under a constant uniaxial or hydrostatic load, which results in the plastic compaction of the samples. The paper proposes a new attempt to model the observed collapse of samples, as well as the changes in compressibility and preconsolidation pressure during the process of wetting. The conceptual framework essentially relies on unsaturated soil mechanics, as the subsidence phenomenon concerns a three-phase material with solid grains, liquid water and gas. The developed constitutive model provides a description of the water retention capability of the studied soils that is coupled with the mechanical behaviour. Consequently, the elasto-plastic volumetric changes within the porous medium incorporate the effects of saturation and suction, also called capillary effects. The formulation of the preconsolidation stress is such that the shape of the yield limit depends on suction so that the apparent added stiffness brought by low saturation is predicted. The modelling framework, based on the generalization of the effective stress principle to three-phase media, also provides an elasto-plastic comprehension of the well-known “wetting pore collapse” phenomenon. The ACMEG-s model shows consistent understanding of changes of compressibility with the quantity of retained water. The successive phases of isotropic compression and uniaxial mechanical compaction are used for the model calibration. Interestingly, the phases of plastic compression during injection are captured with accuracy, which evidence the applicability of this model to the boundary value problems that are the large scale cases of subsidence.