Abstract
The Shenhu area of the South China Sea (SCS) is one of the most promising fields for natural gas hydrate (NGH) exploitation. However, previous studies conclude that using only depressurization is inefficient for this challenging hydrate deposits surrounded by permeable water zones, which requires assistance by thermal stimulation to promote hydrate decomposition and methane recovery. However, traditional thermal stimulation methods with hot water or steam injection induce massive heat loss along the wellbore. In addition, in situ electrical heating only results in a limited high temperature region due to low thermal conductivity of hydrate deposits. Therefore, we numerically investigate the performance of combined brine flooding with electrical heating–assisted depressurization in horizontal wells for exploitation of natural gas hydrate in the SCS, which simultaneously possesses the merits of low heat loss and enhanced heat transfer by convection. Our simulation results show that thermal stimulation by combined brine flooding with electrical heating can significantly enhance hydrate dissociation and methane recovery. After 20 years of production, the cumulative methane production of combined brine flooding with electrical heating–assisted depressurization is 1.41 times of that conducted by the only depressurization method. Moreover, the energy efficiency can be improved by reducing electrical heating time, and terminating electrical heating with 70% hydrate dissociation achieves the highest net energy gain. In addition, methane recovery and net energy gain increases with electrical heating power and brine injection pressure but with a decreasing rate. Therefore, the selection of electrical heating power and brine injection pressure should be performed carefully and comprehensively considering both the efficiency of gas production and risks of geological hazard. It is hoped that our research results will provide reference and guidance for the development of a similar NGH reservoir in order to promote the industrial development process of NGH.
Highlights
Natural gas hydrates are crystalline substances comprising water molecules and gas molecules, in which a solid water lattice accommodates gas molecules in a cage-like structure (Sloan, 2003; Wang et al, 2017; Wu et al, 2020)
The two successful production tests proved the technical feasibility of gas production from the clayey silt natural gas hydrate (NGH) reservoir, which accounts for 90% of the total hydrate reservoirs but tend to be the most difficult to exploit owing to low permeability and high content of clay (Boswell and Collett, 2011)
We focus on electrical heating, and many laboratory experiments have been conducted to clarify the effects of electrical heating–assisted depressurization on hydrate dissociation and methane recovery (Falser et al, 2012; Li et al, 2018a; Liang et al, 2018; Minagawa et al, 2018; Wan et al, 2020a; Wan et al, 2020b; He et al, 2021)
Summary
Natural gas hydrates are crystalline substances comprising water molecules and gas molecules, in which a solid water lattice accommodates gas molecules in a cage-like structure (Sloan, 2003; Wang et al, 2017; Wu et al, 2020). In 2007, 2015, and 2016, three gas hydrate drilling expeditions (GMGS1, 3, and 4) were conducted in this area by the Guangzhou Marine Geological Survey (Wu and Wang, 2018). Based on aforementioned numerical studies and many other laboratory experiments on the NGH in the SCS, the China Geological Survey successfully conducted the first and second production tests in the Shenhu area of the SCS in 2017 and 2020, respectively, using a vertical well and a horizontal well. The two successful production tests proved the technical feasibility of gas production from the clayey silt NGH reservoir, which accounts for 90% of the total hydrate reservoirs but tend to be the most difficult to exploit owing to low permeability and high content of clay (Boswell and Collett, 2011)
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