天然气水合物藏多层联合开采数值模拟研究

doi:10.13809/j.cnki.cn32-1825/te.2023.03.015

摘要/Abstract

摘要：

海域天然气水合物藏往往伴随着大量游离气的存在，而储层的低渗透性限制了不同层系的气水流动，利用储层中的水合物分解气和游离气是提升水合物藏产气效率的关键。基于中国南海神狐海域的实际地质数据，使用TOUGH+HYDRATE模拟器，建立3种不同水合物储层垂直井联合开采的数值模型，分析了产气、产水、温度、压力场和水合物饱和度的空间变化，进而提出天然气水合物的优化降压开采策略。结果表明：降压开采过程中，气和水不断向开采井汇集，井周围的温度压力迅速下降；持续降压开采10 a，随着产气速率的减小，产水速率不断增大，三层联合开采方式的累计产气量最高可达4.59×10⁶ m³，其累计产水量为8.31×10⁵ m³；水合物的分解由降压的压力梯度控制，井周围的水合物优先分解，下伏层水的流动会加快储层水合物的分解。

关键词: 天然气水合物, 产气效率, 联合开采, 数值模型, 神狐海域

Abstract:

Offshore natural gas hydrate reservoirs are often accompanied by a large amount of free gas. However, the low permeability of the reservoir limits the flow of gas and water in different layers. Therefore, the fully exploitation of the hydrate-dissociation gas and free gas in the reservoir is the key to improve gas production efficiency. Based on the actual geological data in the Shenhu area of the South China Sea, the TOUGH+HYDRATE code is used to establish the numerical model of the combined exploitation of three different hydrate reservoirs with vertical wells. The spatial changes of gas production, water production, temperature, pressure field and hydrate saturation are analyzed, and then the optimized depressurization production strategy of natural gas hydrate is put forward. The results show that during the depressurization production, gas and water are continuously collected to the production well, and the temperature and pressure around the well drop rapidly. After continuous production for ten years, the water production rate continues to increase with the decrease of gas production rate. The cumulative gas production of three-layer combined mining method is up to 4.59×10⁶ m³, and the cumulative water production is 8.31×10⁵ m³. Hydrate dissociation is controlled by the depressurized gradient, hydrates around the well are preferentially dissociated, and the flow of underlying water will accelerate the dissociation of reservoir hydrates.

Key words: natural gas hydrate, gas production efficiency, combined exploitation, numerical model, Shenhu area

中图分类号:

TE53

杨作亚,武晓敏. 天然气水合物藏多层联合开采数值模拟研究[J]. 油气藏评价与开发, 2023, 13(3): 393-402.

Zuoya YANG,Xiaomin WU. Numerical simulation study on multi-layer combined exploitation of natural gas hydrate reservoirs[J]. Petroleum Reservoir Evaluation and Development, 2023, 13(3): 393-402.

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参数	数值
非稳定水合物层底部初始压力/MPa	15.50
非稳定水合物层底部初始温度/°C	16.59
初始孔隙度及各相饱和度/%	水饱和度S_A=100.0,φ=35（上盖层）
	S_H=34.0,S_A=66.0,φ=35（水合物层Ⅰ）
	S_H=31.0,S_A=61.2,S_G=7.8,φ=33（水合物层Ⅱ）
	S_A=92.2,S_G=7.8,φ=32（游离气层）
	S_A=100.0,φ=32（下盖层）
CH₄/%	100
地温梯度/（°C/m）	0.044 3
孔隙水盐度/%	3.05
本征渗透率k/10^-3 μm²	2.9（上盖层&水合物层Ⅰ）
	1.5（水合物层Ⅱ）
	7.4（游离气层&下盖层）
开采井内渗透率/10^-3 μm²	1
湿热导率k_ΘRW/[W/（m·K）]	1.7
干热导率k_ΘRD/[W/（m·K）]	1.0
毛细压力模型^[20]	p_cap=-p₀[（S）^-1/λ-1]^1-λ
毛细压力模型^[20]	S=（S_A-S_irA）/（S_mxA-S_irA）
残余水饱和度S_irA/%	30
毛细管系数λ	0.45
静水压力p₀/Pa	10⁵
相对渗透率模型^[20]	k_rA=（S_A）ⁿ,k_rG=（S_G）ⁿ^G
	S_A=（S_A-S_irA）/（1-S_irA）
	S_G=（S_G-S_irG）/（1-S_irA）
水相相对渗透率衰减因子n	3.572
气相相对渗透率衰减因子nG	3.572
残余气饱和度S_irG/%	3

案例	井位置	深度范围/m	长度/m
方案1	稳定水合物层+ 非稳定水合物层	1 495~1 545	50
方案2	非稳定水合物层+游离气层	1 530~1 572	42
方案3	稳定水合物层+ 非稳定水合物层+游离气层	1 495~1 572	77

案例	累计产气量/10⁴ m³	单位井长产气量/10⁴ m³	累计产水量/10⁴ m³	单位井长产水量/m³
方案1	220	44	22	4 300
方案2	397	94	61	14 500
方案3	459	60	83	10 800

案例	本征渗透率/ 10^-3 μm²	累计产气量/ 10⁴ m³	日均产气量/ m³
方案3	1.5	459	1 258
方案4	15.0	716	1 962
方案5	100.0	1 585	4 343