Abstract
Natural gas hydrate (NGH) has been widely focused on having great potential for alternative energy. Numerous studies on gas production from hydrate-bearing sediments have been conducted in both laboratory and field. Since the strength of hydrate-bearing sediments depends on the saturation of NGH, the decomposition of NGH may cause the failure of sediments, then leading to reservoir deformation and other geological hazards. Plenty of research has shown that the reservoir deformation caused by hydrate decomposition is considerable. In order to investigate this, the influence of sediment deformation on the production of NGH, a fully coupled thermo-hydro-chemo-mechanical (THMC) model is established in this study. The interaction effects between reservoir deformation and hydrate dissociation are discussed by comparing the simulation results of the mechanical coupling and uncoupled models on the laboratory scale. Results show that obvious differences in behaviors between gas and water production are observed among these two models. Compared to the mechanical uncoupled model, the mechanical coupling model shows a significant compaction process when given a load equal to the initial pore pressure, which leads to a remarkable decrease of effective porosity and reservoir permeability, then delays the pore pressure drop rate and reduces the maximum gas production rate. It takes a longer time for gas production in the mechanical coupling model. Since the reservoir temperature is impacted by the comprehensive effects of the heat transfer from the boundary and the heat consumption of hydrate decomposition, the reduced maximum gas production rate and extended gas production process for the mechanical coupling model lead to the minimum reservoir temperature in the mechanical coupling model larger than that of the mechanical uncoupled model. The reduction of the effective porosity for the mechanical coupling model causes a larger cumulative water production. The results of this paper indicate that the reservoir deformation in the gas production process should be taken into account by laboratory and numerical methods to accurately predict the behaviors of gas production on the field scale.
Highlights
Natural gas hydrate (NGH) is an ice-like crystal in which gas molecules, most often methane, are trapped by water molecules
These pore pressure changes caused by NGH dissociation vary according to the initial effective permeability of hydrate-bearing sediments, and the overpressure is often generated in low permeability layers with rapid dissociation of NGH, thereby decreasing the effective pressure and shear strength of unconsolidated sediments [26,27]
Since the equilibrium pressure of NGH is temperature-dependent, as mentioned former, with the decrement of model temperature caused by sensible heat consumption, the equilibrium pressure of NGH drops, which leads to the slowdown of NGH decomposition rate for both mechanical coupling and uncoupled models
Summary
Natural gas hydrate (NGH) is an ice-like crystal in which gas molecules, most often methane, are trapped by water molecules. A vast number of triaxial test results based on synthetic and nature hydrate samples have indicated that the mechanic properties of hydrate-bearing sediment are closely related to NGH saturation and distribution morphology [11,12,13,14,15,16,17,18,19]. It has been widely used to simulate the gas production behavior in hydrate-bearing sediment by different production methods from laboratory to reservoir scale [35,36,37,38] It can predict geomechanical responses during hydrate production by linking FLAC3D through a coupling code [25]. The present work is based on a laboratory-scale model, the methodology behind this work could apply for field-scale modeling with detailed stratum parameters
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