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

Lijuan Gu,Hailong Lu,Shilong Shang

doi:10.3390/en14227513

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.

Highlights

Natural gas hydrates, commonly occurring in offshore sediments and terrestrial permafrost with relatively low temperature and high pressure, are non-stoichiometric solid compounds in which small guest gas molecules are captured in the water cages [1,2]
A better return from a longer fracture is not presented at the early production stage, which is different from traditional oil and gas production in that the gas production rate increases with fracture length [46]
The effect of fracture length on production is closely related to the time frame of production, and the benefits from larger fracture length are not presented at the early production stage

Summary

Introduction

Commonly occurring in offshore sediments and terrestrial permafrost with relatively low temperature and high pressure, are non-stoichiometric solid compounds in which small guest gas molecules (mainly methane) are captured in the water cages [1,2]. The research of natural gas hydrate has entered the stage of development in exploitation technology after several production trials in terrestrial and marine hydrate reservoirs, including Mallik in Canada [6], Alaskan North Slope in USA [7], Eastern Nankai Trough in Japan [8,9], and the South China Sea [10,11]. In the pilot production conducted in Eastern Nankai Trough of Japan in 2017, two production tests were carried out by depressurization method; due to the failure of sand management, the first production lasted for 12 days, with 41,000 m3 of methane gas produced, which is way lower than the requirement for commercial exploitation of natural gas hydrate [9]. Increasing the permeability of the hydrate-bearing sediments by reservoir stimulation techniques might benefit promoting the gas production rate, as evinced by its effect in shale gas production

Methods

Results

Conclusion