This paper examines the application of surface drilling parameters and drilling optimization techniques, such as mechanical specific energy (MSE) and equivalent strength (EST), in detecting hydrate gas-bearing sediments during drilling operations. Gas hydrates are usually detected from 3D seismic imaging and later confirmed with Measurement-While-Drilling (MWD)/Logging-While-Drilling (LWD) data and collected core samples. Here, we describe an analysis of the time-based surface drilling parameters recorded from two wells drilled during the International Ocean Discovery Program (IODP) Expedition 372A offshore of New Zealand and the Indian National Gas Hydrate Program Expedition 02 (NGHP-02) offshore of India. The investigation revealed that drilling parameters, as well as MSE/EST methods, can and should be used to monitor and optimize the drilling process and to detect lithological/tectonic (fractures, fault zones, rock hardness, etc.) changes in the drilled substrata and signs of the dynamic changes in the downhole environment (tool vibration, washouts, pack-offs, etc.). However, surface drilling parameters with MSE models cannot explicitly determine the hydrate gas-bearing sediments. This qualitative analysis of whether the gas-bearing sediments consist of hydrates can only be accomplished with the use of the MWD/LWD suite, preferably located as close as possible to the bit. Full article

We investigate the feasibility of electromagnetic (EM) geophysics methods to detect the dissociation of gas hydrate specifically from a gas hydrate deposit located in the Ulleung Basin, East Sea, Korea via an integrated flow-geomechanics-EM geophysics simulation. To this end, coupled flow and geomechanics simulation is first performed with the multiple porosity model employed, where a mixed formulation with the finite volume (FV) and finite element (FE) methods are taken for the flow and geomechanics, respectively. From the saturation and porosity fields obtained from the coupled flow and geomechanics, the electrical conductivity model is established for the EM simulation. Solving the partial differential equation of electrical diffusion which is linearized using the 3D finite element method (FEM), the EM fields are then computed. For numerical experiments, particularly two approaches in the configuration for the EM methods are compared in this contribution: the surface-to-surface and the surface-to-borehole methods. When the surface-to-surface EM method is employed, the EM is found to be less sensitive, implying low detectability. Especially for the short term of production, the low detectability is attributed to the similarity of electrical resistivity between the dissociated gas (CH${}_4$) and hydrate as well as the specific dissociation pattern within the intercalated composites of the field. On the other hand, when the surface-to-borehole EM method is employed, its sensitivity to capture the produced gas flow is improved, confirming its detectability in monitoring gas flow. Hence, the EM geophysics simulation integrated with coupled flow and geomechanics can be a potential tool for monitoring gas hydrate deposits. Full article

Although various simulation studies on gas hydrate production have been conducted, a single vertical well in the cylindrical system has been adopted in most research. However, this system has a limited ability to predict commercial production in gas hydrate reservoirs. In order to facilitate commercial production, a field-scale reservoir model with a multi-well system must be constructed using geological data, such as seismic data, well logging data, core data, etc. The depressurization method is regarded as a practical production strategy because it has high levels of production efficiency and economical effectiveness. However, this method can lead to subsidence due to the increased effective stress. In this work, we studied a production simulation strategy for commercial gas hydrate production. A three-dimensional geological model with a realistic field scale is constructed using seismic and well logging data from the Ulleung Basin of the Korean East Sea. All of the grids are refined in the I and J direction, and the grids near the production well are very small to consider realistic hydrate dissociation. The cyclic depressurization method is adopted for the increase in the geomechanical stability, rather than the non-cyclic depressurization method. Various case studies are conducted with alternating bottomhole pressures for the primary and secondary depressurization stages over 100 days. Geomechanical stability is significantly enhanced, while cumulative gas production is relatively less reduced or nearly maintained. In particular, all cases of the cumulative gas production at 6 MPa during the secondary depressurization stage are similar to the non-cyclic case, while the geomechanical stabilities of those cases are restored. This study is thought to have contributed to the development of technology for commercial gas hydrate production with a geomechanical stability study using a reservoir-scale model with a multi-well system. Full article

Compared with the deeply buried marine gas hydrate deposits, gas hydrates in the shallow subsurface, close to and at the seafloor, have attracted more attention owing to their concentrated distribution, high saturation, and easy access. They accumulate at relatively shallow depths <100–120 m and occur as gas hydrate-bearing mounds (also known as hydrate outcrops, pingoes) at the seafloor derived from the growth of hydrates in the shallow subsurface or as pure hydrate chunks formed by gas leakage. This paper reviews and summarizes such gas hydrate systems globally from the perspective of gas sources, migration pathways, and accumulation processes. Here, we divided them into four categories: fault-chimney-controlled, diapir-fault-controlled, fault-controlled, and submarine mud volcano-controlled deposits. Gas chimneys originate immediately above the restricted regions, mostly affected by faults where high gas concentrations trigger elevated pore fluid pressures. Diapirism derives a dendritic network of growth faults facilitating focused gas discharge and hydrate formation near the seafloor. Furthermore, pre-existing faults or fractures created by overpressured gas from greater depths in accretionary tectonics at convergent margins act as preferential pathways channeling free gas upwards to the seafloor. Gas flux rates decrease from the submarine mud volcano center to its margins, creating a concentric pattern of distributing temperature, gas concentrations, and hydrate contents in shallow sediments around the mud volcano. Hydrate-bound hydrocarbons are commonly of thermogenic origin and correspond to high-background geothermal conditions, whereas microbial gas is dominant in a few cases. The presence of heavier hydrocarbons mitigates the inhibition of hydrate formation by salt or heat. Fluid migration and pathways could be compared to the “blood” and “bones” in an organic system, respectively. The root of a pathway serves as the “heart” that gathers and provides considerable free gas concentrations in a restricted area, thereby triggering pore fluid pressures as one important drive force for focused fluid flow in impermeable sediments (the organic system). Besides the suitable temperature and pressure conditions, a prerequisite for the formation and stability of hydrate deposits in the shallow subsurface and at the seafloor is the sufficient supply of gas-rich fluids through the hydrate stability zone. Thus, the proportion of gas migrating from deep sources is significantly larger than that trapped in hydrates. As such, such marine hydrate deposits seem more like temporary carbon storage rather than the main culprit for climate warming at least in a short period. Full article