The Effects of the Length and Conductivity of Artificial Fracture on Gas Production from a Class 3 Hydrate Reservoir

Abstract

Natural gas hydrate is considered as a potential energy resource. To develop technologies for the exploitation of natural gas hydrate, several field gas production tests have been carried out in permafrost and continental slope sediments. However, the gas production rates in these tests were still limited, and the low permeability of the hydrate-bearing sediments is identified as one of the crucial factors. Artificial fracturing is proposed to promote gas production rate by improving reservoir permeability. In this research, numerical studies about the effect of fracture length and fluid conductivity on production performance were carried out on an artificially fractured Class 3 hydrate reservoir (where the single hydrate zone is surrounded by an overlaying and underlying hydrate-free zone), in which the equivalent conductivity method was applied to depict the artificial fracture. The results show that artificial fracture can enhance gas production by offering an extra fluid flow channel for the migration of gas released from hydrate dissociation. The effect of fracture length on production is closely related to the time frame of production, and gas production improvement by enlarging the fracture length is observed after a certain production duration. Through the production process, secondary hydrate formation is absent in the fracture, and the high conductivity in the fracture is maintained. The results indicate that the increase in fracture conductivity has a limited effect on enhancing gas production.

Review and Outlook on Fracturing Technology and Mechanism of Hydrate-Bearing Sediments

“…Hydraulic fracturing has become a prevalent technique in the oil and gas industry, 103−105 primarily leveraging technological interventions to induce artificial fractures within subterranean rock strata and natural gas reservoirs. 106,107 This process serves to enhance hydrocarbon recovery rates by facilitating increased fluid flow. 42,44,88,108 When examining the mechanical properties of HBS, the subsurface stress is a complex combination of radial, alonglayer, and vertical positions, and the triaxial instrument has the same peripheral pressure, which is unable to provide the displacement and deformation information related to other dominant stress directions, and cannot accurately simulate the real stress environment of the subsurface reservoirs.…”

Reservoir Stimulation Technologies for Natural Gas Hydrate: Research Progress, Challenges, and Perspectives

“…In addition, secondary hydrates formed in the fracturing areas of the low-temperature reservoirs. Shang et al 99 studied the fracture length and conductivity coefficient (the product of the fracture permeability and width). They found that the influence of the fracture length on the production performance depended on the length of the mining time, whereas increasing the fracture length did not show a good gas production effect in the early stages of mining.…”

Embedded Discrete Fracture Model for Investigating the Effect of Fractures on Gas Hydrate Production

“…10 Numerical studies about the effect of artificial fracturing on production performance are carried out in a Class 3 hydrate reservoir at the Mount Elbert Prospect on the Alaska North Slope. 31 The effects of hydraulic fracturing on the enhanced gas recovery were thoroughly investigated at the second test site in the South China Sea. 29…”

“…However, the current Tough+Hydrate simulator lacks a separate module that can explicitly represent fractures and calculate the mass and heat exchange between sediments and fractures. As shown in Table , a commonly used way to explicitly express fractures in the simulator is the scheme of local grid refinement (LGR) to depict fracture width. ,− Although this method is easy to implement, two issues arise: (1) its inability to construct fracture networks with complex distributions; (2) a large number of grids after local gird refinement around fractures, thereby slowing down the speed of computation. The former is a result of a simultaneous consideration of both sediments and fractures, while the latter is caused by the significant contrast in the scales of fracture width (a few millimeters to centimeters) and hydrate reservoirs (hundreds or even thousands of meters) …”