南海北部白云凹陷沉积压实作用对浅水流超压演化影响数值模拟

doi:10.11867/j.issn.1001-8166.2014.09.1055

深水超压是制约深水油气开发的重要问题,其中浅水流是对深水钻井最具破坏力的一种地质灾害。结合南海北部陆坡最新采集的高分辨率二维和三维地震资料,采用基于压实模型的流体压力模拟方法,通过井震联合分析,建立地震属性与岩石物性的关系,对研究区现今压力场状态进行模拟,获取了有效应力和超压系数等参数,探讨了沉积压实作用下的浅水流演化过程。通过模拟发现浅水流发育区的水动力特征与研究区的沉积模式密切相关,低渗透率地层的沉积物加载导致现今压力场具有明显的低有效应力异常和高超压系数,而如果仅考虑由沉积压实作用,现有的沉积速率尚不足以维持持续地超压来造成高风险的浅水流灾害,但其泄压过程也是个漫长的阶段。

关键词: 超压预测, 压力演化, 白云凹陷, 数值模拟

Overpressure in the deepwater basin has become a challenge, which constrains the development of oil and natural gas industry and is a global problem. Drilling risks associated with shallow water flow have received the most attention. High resolution seismic data in the northern South China Sea were acquired by the China Geology Survey in 2006. Detailed studies of this region reveal the presence of overpressured sands, which may cause the shallow water flow hazard. By combining the seismic data and logging, we created a mathematical model based on compaction theory and established relationship between the velocity and porosity, density and effective stress. The P-velocity was transformed to effective stress and overpressure ration. The simulating results show that the overpressure ratio is 0 at the seafloor, which indicates that there is no overpressure, or the fluid pressure is hydrostatic. The value of the overpressure ratio in the interest zone, which responds to the deepwater channel depositional system in Baiyun Sag, Pearl River Mouth Basin, is higher and increases with depth in deeper sediments. After 10 ka, the value becomes lower, the overpressure seems to be released, which corresponds to the formers’ achievement in pressure research.

Key words: Overpressure prediction, Pressure evolution, Baiyun Sag, Numerical simulation.

中图分类号:

P634

图1 研究区位置图 (a)南海北部陆坡白云凹陷区域位置图;(b)研究区海底地形图(据文献[28]修改)

Fig.1 the location of the study area (a) the location of the Baiyun Sag at the continental slope of northern South China Sea; (b) the 3D bathymetric map in study area(modified from reference [28])

图2 浅水流潜在区地震剖面及简化模型 (a)地震剖面;(b)波阻抗反演剖面;(c)速度剖面;(d)地质模型

Fig.2 Seismic profile including shallow water flow sands and simplified 2D model (a) Seismic profile; (b) Acoustic impedance section; (c) Velocity section;(d) 2D model with parameters

表1 模型恒定参数值

Table1 Simulation constants

参数	符号	值(单位)
流体密度		1030(kg/m³)
颗粒密度	ρ_s	2680(kg/m³)
初始沉积物密度	ρ₀	1530(kg/m³)
重力加速度	g	9.81(m/s² )
初始孔隙度		0.613(m³/m³)
体压缩模量	β	2.0×10^-7(Pa^-1)
粘度	μ	1(cp)
初始沉积速率	m	0.00015(m/a)

表1 模型恒定参数值

Table1 Simulation constants

参数	符号	值(单位)
流体密度		1030(kg/m³)
颗粒密度	ρ_s	2680(kg/m³)
初始沉积物密度	ρ₀	1530(kg/m³)
重力加速度	g	9.81(m/s² )
初始孔隙度		0.613(m³/m³)
体压缩模量	β	2.0×10^-7(Pa^-1)
粘度	μ	1(cp)
初始沉积速率	m	0.00015(m/a)

图3 SH2井拟合正常压实情况下获取的参考孔隙度和初始压缩系数

Fig.3 Fitting results of the reference porosity and bulk compressibility from SH2 under normal compaction

图4 孔隙度剖面 (a)测线孔隙度剖面;(b)孔隙度异常剖面

Fig.4 Porosity profile (a) Porosity profile of line; (b) Abnormal porosity differential

图5 SH2井的地层密度值

Fig.5 Density logging fitting result from SH2

图6 有效应力剖面 (a)现今有效应力;(b)有效应力异常剖面;(c)10ka后有效应力;(d)10ka有效应力异常剖面

Fig.6 Effective stress profile ( a)Effective stress and (b)abnormal effective stress differential profile from the 2-D model nowdays; (c)Effective stress and (d)the differential profile from the 2-D model after 10 ka of simulation

图7 超压系数剖面 (a)现今超压系数;(b)超压系数异常剖面;(c)10ka后超压系数;(d)10ka超压系数异常剖面

Fig.7 overpressure ratio profile (a) Overpressure ratio and(b) overpressure ratio differential profile from the 2-D model nowdays;(c) Overpressure ratio and (d) the differential profile from the 2-D model after 10 ka of simulation

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摘要

Numerical Simulation of Overpressure of Shallow Water Flow in Baiyun Sag of the Northern South China Sea