Geomechanical response of permafrost-associated hydrate deposits to depressurization-induced gas production

Abstract

In this simulation study, we analyzed the geomechanical response during depressurization production from two known hydrate-bearing permafrost deposits: the Mallik (Northwest Territories, Canada) deposit and Mount Elbert (Alaska, USA) deposit. Gas was produced from these deposits at constant pressure using horizontal wells placed at the top of a hydrate layer (HL), located at a depth of about 900 m at the Mallik and 600 m at the Mount Elbert.The simulation results show that general thermodynamic and geomechanical responses are similar for the two sites, but with substantially higher production and more intensive geomechanical responses at the deeper Mallik deposit. The depressurization-induced dissociation begins at the well bore and then spreads laterally, mainly along the top of the HL.The depressurization results in an increased shear stress within the body of the receding hydrate and causes a vertical compaction of the reservoir. However, its effects are partially mitigated by the relatively stiff permafrost overburden, and compaction of the HL is limited to less than 0.4%. The increased shear stress may lead to shear failure in the hydrate-free zone bounded by the HL overburden and the downward-receding upper dissociation interface. This zone undergoes complete hydrate dissociation, and the cohesive strength of the sediment is low. We determined that the likelihood of shear failure depends on the initial stress state as well as on the geomechanical properties of the reservoir. The Poisson's ratio of the hydratebearing formation is a particularly important parameter that determines whether the evolution of the reservoir stresses will increase or decrease the likelihood of shear failure.

Numerical simulations for analyzing deformation characteristics of hydrate-bearing sediments during depressurization

“…Kimoto et al (2010) developed a chemo-thermo-mechanical numerical simulator to predict the deformation of hydrate reservoirs during hydrate dissociation induced by depressurization and thermal stimulation. A numerical simulator which combines TOUGH+HYDRATE and FLAC3D was commonly used to study the gas production behavior as well as the geomechanical response during hydrate recovery (Rutqvist and Moridis, 2008;Rutqvist et al, 2009).…”

Multiphase Flow Behavior of Layered Methane Hydrate Reservoir Induced by Gas Production

“…In the following days, both R and G begin oscillating around a coarsely invariable value, which clarify the struggle between the impacts of temperature and pressure on hydrate decomposition and gas production [3]. Decreasing pressure causes hydrate decomposition with the increase in R and G .…”

“…In nature, the dominant gas in NGH is methane, which forms at low temperature and high pressure with extensive distribution in the permafrost and in deep marine sediments [1,3,4]. The evaluation results show that the global quantity of hydrocarbon gas hydrates varies widely between 10 15 and 10 18 ST m 3 (ST represents the standard conditions) [2,5].…”

“…Both of their results indicated that the gas production rate is expected to increase with time and the simulated water recovery rate cannot match field measured data. In addition, the geomechanical behaviors (such as seafloor subsidence and sand production) of the marine HBS induced by gas production have been widely studied [3,7,[18][19][20][21]. The geomechanical analysis indicated that the spatial evolution of the temperature, pressure, hydrate saturation, and gas saturation is the most relevant to the geomechanical behavior of HBS.…”

“…Gas hydrate reservoirs are composed of alternating beds of sand and clay in sediments with various conditions of permeability, porosity, and hydrate saturation [22,25]. Consequently, the development of suitable geological model is critical to identify the dissociation layers and dissociation front induced by depressurization, which are key factors to ensure the success of the next long-term offshore production test [3,4,19,22]. Additionally, the sensitivity analyses of gas production potential in previous studies were concentrated on production pressure, reservoir permeability, porosity, hydrate saturation, thickness, and initial temperature and pressure of HBS [11,12,14,15,26], whereas comprehensive studies related to the effects of well production interval on gas recovery have been reported sparsely, which are important for the optimization of gas production and reducing the water production rate.…”