Methane
hydrates are a promising source of natural gas. Several
field tests have validated the feasibility of gas production from
marine methane hydrate-bearing sediments. However, sustained and commercial
production has not been obtained due to limited gas production efficiency
and production-induced damage to the mechanical properties in the
sediments. This study investigates the gas production efficiency and
the geomechanical responses in hydrate-bearing sediments developed
by depressurization in hydraulic fractures. A numerical model for
the simulation of coupled thermal–hydraulic–mechanical–chemical
behaviors is described. The model considers three phases of water,
gas, and hydrate in the sediments. Heat transport and the kinetics
for hydrate dissociation are also taken into account. A Mohr–Coulomb
criterion for marine methane hydrate-bearing reservoirs is employed
to describe the shear failure and damaged rock strength caused by
depressurization in hydraulic fractures. Then, a base case for a 750-day
depressurization in a hydraulic fracture is presented. The spatial
and temporal evolutions of the pore pressure, temperature, hydrate
dissociation, principal stress, and shear failure are described. Parametric
studies for reservoir permeability, depressurization pressure, and
fracture number are also presented. The results indicate that early-stage
gas rates are high, while they drop significantly with time. At late
stages, shear failure is highly correlated with the reduced rock strength,
and stress concentrations are obtained near fracture tips. The evolution
of strength is correlated with hydrate dissociation as dissociated
hydrates weaken the mechanical properties in sediments, which is also
time dependent. Local stress concentrations are observed around fracture
tips as well. Higher reservoir permeabilities lead to greater early-stage
production and lower late-stage production, indicating that the use
of hydraulic fractures is preferable in low-permeability reservoirs.
Also, increasing the hydraulic fracture number can improve the overall
gas production efficiency, while the average gas production efficiency
from individual fractures is decreased.
Sand production has been identified as a key reason limiting sustained and commercial gas production in methane-hydrate-bearing sediments. Production tests in Canada and Japan were terminated partially because of excessive sand production in pilot wells. It is meaningful to carry out numerical investigations and sensitivity analyses to improve the understanding of sand production mechanisms during the exploitation of methane hydrates. This study introduces a numerical model to describe the coupled thermal–hydraulic–mechanical–chemical responses and sand production patterns during horizontal well depressurization in methane-hydrate-bearing sediments. The model is benchmarked with a variety of methane hydrate reservoir simulators. Results show that the spatial and temporal evolution patterns of multi-physical fields are different and the hydromechanical evolutions are the fastest. Gas production and sand production rates are oscillatory in the early stages and long-term rates become stable. Gas production is sensitive to rock physical and operational parameters and insensitive to rock mechanical properties such as cohesion. In contrast, sand production is sensitive to cohesion and insensitive to rock physical and operational parameters. Although cohesion does not directly affect gas productivity, gas productivity can be impaired if excessive sand production impedes production operations. This study provides insights into the sand production mechanism and quantifies how relevant parameters affect sand production during the depressurization in methane-hydrate-bearing sediments.
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