The impact of threshold pressure gradient on the production dynamics of gas hydrate reservoirs in the northern South China Sea

doi:10.13745/j.esf.sf.2024.00.00

Abstract

Abstract:

The South China Sea holds abundant gas hydrate resources with significant potential for industrial development, as demonstrated by successful natural gas hydrate pilot tests in the Shenhu area. Most gas hydrate-bearing sediments in the South China Sea are classified as Class Ⅰ gas hydrate deposits, underlain by a two-phase zone containing mobile gas. These reservoirs are characterized by unconsolidated argillaceous siltstones and a threshold pressure gradient (TPG), which significantly impacts gas production. A mathematical model incorporating TPG was developed to simulate multiphase flow in typical subsea sediments, and a TPG module was integrated into the TOUGH+HYDRATE simulator in this study. The Well SHSC4, located in the Shenhu test field, was selected to evaluate the effects of TPG on gas production. Simulation results reveal that TPG restricts pressure propagation, preventing hydrate dissociation near the reservoir boundaries. A phenomenon of low water output and a high gas-to-water ratio was observed during gas production. The water production from the horizontal well nearly ceased in the later stages. The TPG plays a critical role in preventing the formation of secondary gas hydrate shells in the free dissociation zone and eliminating gas traps. A review of 10 years of production data indicates that TPG can enhance gas production from Class Ⅰ gas hydrate deposits while TPG limits the recoverable reserves of gas hydrate. Notably, gas production increased by nearly 40%, attributed to TPG’s ability to improve free gas drainage in vertical wells. These findings highlight the dual role of TPG in both restricting and promoting gas production, offering valuable insights for optimizing gas hydrate exploitation in the South China Sea.

Key words: natural gas hydrate, threshold pressure gradient, numerical simulation, Class Ⅰ hydrate deposits, South China Sea

CLC Number:

YU Lu, LI Xian, CUI Guodong, XING Donghui, LU Hongfeng, WANG Yejia. The impact of threshold pressure gradient on the production dynamics of gas hydrate reservoirs in the northern South China Sea[J]. Earth Science Frontiers, 2025, 32(2): 178-194.

Figures/Tables 21

Fig.1 Geological units in the northern South China Sea and the location of the hydrate production test area. Adapted from [17,58].

Fig.2 Seismic section characteristics of the production test well SHSC4 and the geological model for methane hydrate production. Adapted from [17].

Table 1 Physical parameters of the geological model for gas hydrate reservoirs. Adapted from [17,59].

参数	数值	参数	数值
上覆层厚度/m	31	水合物层厚度/m	35
三相混合层厚度/m	15	游离气层厚度/m	27
下伏层厚度/m	32	上覆层水饱和度	1
上覆层孔隙度	0.3	上覆层渗透率/mD	0.1
水合物层水合物饱和度	0.34	水合物层水饱和度	0.66
水合物层孔隙度	0.35	水合物层渗透率/mD	2.9
三相混合层水合物饱和度	0.31	三相混合层气饱和度	0.15
三相混合层水饱和度	0.54	三相混合层孔隙度	0.33
三相混合层渗透率/mD	1.5	游离气层气饱和度	0.078
游离气层水饱和度	0.912	游离气层孔隙度	0.32
游离气层渗透率/mD	7.4	下伏层水饱和度	1
下伏层孔隙度	0.3	下伏层渗透率/mD	0.1
孔隙水盐度	3%	储层压缩系数/Pa^-1	1×10^-8
骨架密度/(kg·m^-3)	2 650	饱和水储层热传导率/(W·m^-1·℃^-1)	3.1
干样储层热传导率/(W·m^-1·℃^-1)	1.0	骨架比热/(J·kg^-1·℃^-1)	1 000

Fig.3 Schematic model of the horizontal well

Fig.4 Effect of TPG on gas production rate and hydrate dissociation gas flow rate during vertical well production

Fig.5 Effect of TPG on cumulative gas production in vertical wells

Fig.6 Effect of TPG on water production rate and gas-water ratio in vertical wells

Fig.7 Evolution of hydrate saturation distribution: (a) without TPG, (b) with TPG.

Fig.8 Evolution of reservoir pressure distribution: (a) without TPG, (b) with TPG.

Fig.9 Detailed reservoir pressure distribution in the GHBL and TPL without TPG

Fig.10 Radial distribution of pressure gradient after 10 years of production at a depth of -1481 m

Fig.11 Evolution of reservoir temperature distribution: (a) without TPG, (b) with TPG.

Fig.12 Evolution of gas saturation distribution: (a) without TPG, (b) with TPG.

Fig.13 Schematic diagram of gas flow channels and gas accumulation

Fig.14 Effect of TPG on gas production dynamics in horizontal wells

Fig.15 Effect of TPG on water production rate and gas-water ratio in horizontal wells

Fig.16 Evolution of hydrate saturation distribution in horizontal wells: (a) without TPG, (b) with TPG.

Fig.17 Evolution of reservoir pressure distribution in horizontal wells: (a) without TPG, (b) with TPG.

Fig.18 Evolution of reservoir temperature distribution in horizontal wells: (a) without TPG, (b) with TPG.

Fig.19 Evolution of gas saturation distribution in horizontal wells: (a) without TPG, (b) with TPG.

Fig.20 Schematic diagram of gas flow channels and gas accumulation in horizontal wells

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