CFD simulation on the generation of turbidites in deepwater areas: a case study of turbidity current processes in Qiongdongnan Basin, northern South China Sea

Turbidity currents represent a major agent for sediment transport in lakes, seas and oceans. In particu-lar, they formulate the most significant clastic accumulations in the deep sea, which become many of the world＇s most important hydrocarbon reservoirs. Several boreholes in the Qiongdongnan Basin, the north-western South China Sea, have recently revealed turbidity current deposits as significant hydrocarbon res-ervoirs. However, there are some arguments for the potential provenances. To solve this problem, it is es-sential to delineate their sedimentary processes as well as to evaluate their qualities as reservoir. Numerical simulations have been developed rapidly over the last several years, offering insights into turbidity current behaviors, as geologically significant turbidity currents are difficult to directly investigate due to their large scale and often destructive nature. Combined with the interpretation of the turbidity system based on high-resolution 3D seismic data, the paleotophography is acquired via a back-stripping seismic profile integrated with a borehole, i.e., Well A, in the western Qiongdongnan Basin; then a numerical model is built on the basis of this back-stripped profile. After defining the various turbidity current initial boundary conditions, includ-ing grain size, velocity and sediment concentration, the structures and behaviors of turbidity currents are investigated via numerical simulation software ANSYS FLUENT. Finally, the simulated turbidity deposits are compared with the interpreted sedimentary bodies based on 3D seismic data and the potential provenances of the revealed turbidites by Well A are discussed in details. The simulation results indicate that a sedimen-tary body develops far away from its source with an average grain size of 0.1 mm, i.e., sand-size sediment. Taking into account the location and orientation of the simulated seismic line, the consistence between normal forward simulation results and the revealed cores in Well A indicates that the turbidites should have been transported from Vietnam instead of Hainan Island. This interpretation has also been verified by the planar maps of sedimentary systems based on integration of boreholes and seismic data. The identification of the turbidity provenance will benefit the evaluation of extensively distributed submarine fans for hydro-carbon exploration in the deepwater areas.

Features and Origin of Turbidity Current Sediment Waves in the Huatung Basin off the Eastern Taiwan Island

Based on numerous high-resolution seismic profiles,sediment waves and their distribution,morphological characteristics,internal structure,and potential origins were revealed in the eastern waters of Taiwan.The sediment waves are located at the junction between the Taitung Canyon and other canyons in the slope.The wave length and the wave height of a single waveform ranged from 0.8 to 7.2 km and from 18 to 75 m,respectively(NE-SW direction).Sediment waves,located inside the bend of the Taitung Canyon,were characterized by an upward migration and showed mass transport deposits(MTDs)at the bottom,while the inner curve of the bend was subdivided into lower and upper wavy transition units.The sediment waves on the outer curve of the bend were characterized by vertical accumulation,and there was no mass flow deposit at the bottom.According to the geometry of the sediment waves,the calculated flow thicknesses across the entire wave field ranged from 196 to 356 m,and the current velocity ranged from 15 to 21 cm/s.The morphological characteristics,the internal structure,and the distribution of sediment waves,as well as the numerical calculations,evidenced that these sediment waves had formed by turbidity currents.The development of the sediment wave field in eastern Taiwan was found to be similar to that in southwestern Taiwan.It was the sedimentary response of the tectonic movement between 3 and^1 Ma which created the sedimentary systems where gravity flow processes predominated.Turbidity current sediments settled in the place of less topographical constraints or overflowed in the bend section of the Taitung Canyon,which resulted in the formation of sediment wave fields.

Evaluation of flow regime of turbidity currents entering Dez Reservoir using extended shallow water model

In this study, the performance of the extended shallow water model （ESWM） in evaluation of the flow regime of turbidity currents entering the Dez Reservoir was investigated. The continuity equations for fluid and particles and the Navier-Stokes equations govern the entire flow of turbidity currents. The shallow water equations governing the flow of the depositing phase of turbidity currents are derived from these equations. A case study was conducted on the flow regime of turbidity currents entering the Dez Reservoir in Iran from January 2002 to July 2003. Facing a serious sedimentation problem, the dead storage of the Dez Reservoir will be full in the coming 10 years, and the inflowing water in the hydropower conduit system is now becoming turbid. Based on the values of the dimensionless friction number （ Nf ≤1 ） and dimensionless entrainment number （ NE≤ 1 ） of turbidity currents, and the coefficient of determination between the observed and predicted deposit depths （R2 = 0.86） for the flow regime of negligible friction and negligible entrainment （NFNE）, the flow regime of turbidity currents coming into the Dez Reservoir is considered to be NFNE. The results suggest that the ESWM is an appropriate approach for evaluation of the flow regime of turbidity currents in dam reservoirs where the characteristics of turbidity currents, such as the deposit depth, must be evaluated.

CFD Modeling of Turbidity Current Deposition

Simulation of the flow and deposition from a laboratory turbidity current, in which dense mixtures of sediment move down a narrow, sloping channel and flow into a large tank. SSIIM CFD software is used to model 3-D flow and deposition. SSIIM predicts the height of the accumulated mound to within 25% of experimental values, and the volume of the mound to 20%-50%, depending on the concentration of sediment and slope of the channel. The SSIIM predictions were consistently lower than experimental values. In simulations with initial sediment volumetric concentrations greater than 14%, SSIIM dumped some of the sediment load at the entry gate into the channel, which was not the case with the experimental runs. This is likely due to the fact that the fall velocity of sediment particles in SSIIM does not vary with sediment concentration. Further simulations of deposition from turbidity currents should be attempted when more complete experimental results are available, but it appears for now that SSIIM can be used to give approximate estimates of turbidity current deposition.

The failure propagation of weakly stable sediment:A reason for the formation of high-velocity turbidity currents in submarine canyons

The long-distance movement of turbidity currents in submarine canyons can transport large amounts of sediment to deep-sea plains.Previous studies show obvious differences in the turbidity current velocities derived from the multiple cables damage events ranging from 5.9 to 28.0 m/s and those of field observations between 0.15 and 7.2 m/s.Therefore,questions remain regarding whether a turbid fluid in an undersea environment can flow through a submarine canyon for a long distance at a high speed.A new model based on weakly stable sediment is proposed(proposed failure propagation model for weakly stable sediments,WS S-PFP model for short)to explain the high-speed and long-range motion of turbidity currents in submarine canyons through the combination of laboratory tests and numerical analogs.The model is based on two mechanisms:1)the original turbidity current triggers the destabilization of the weakly stable sediment bed and promotes the destabilization and transport of the soft sediment in the downstream direction and 2)the excitation wave that forms when the original turbidity current moves into the canyon leads to the destabilization and transport of the weakly stable sediment in the downstream direction.The proposed model will provide dynamic process interpretation for the study of deep-sea deposition,pollutant transport,and optical cable damage.

Research on Characteristics of Density Current Under the Action of Waves

In this paper, the characteristics of density current under the action of waves are described with the help of flume experiment and theoretical analysis. The study shows that turbid water under the action of the waves can present three types of motion, i. e. significant stratification, fragile stratification and strong mixing. The motion gf turbid;,ater presents significant stratification when (H/D)/root Delta rho/rho less than or equal to 4.5, generally this state is known as density current. The formulas of motion velocity, thickness, and discharge of density current moving on horizontal bottom are derived by use of basic equations such as momemtum equation, equation of energy conservation and continuity equation of fluid. The time-average velocity and the thickness of density current under the action of waves have a relationship with such parameters as relative density (Delta rho/rho), wave height (H), and water depth (D). When these parameters are determined, the time-average thickness and motion velocity of density current are also determined. The relative thickness of density current (D-t/D) decreases with the increase of Delta rho/rho and increases with the increase of H/D. On the other hand, the motion velocity of density current increases with the increase of Delta rho/rho and decreases with the increase of the relative thickness (D-t/D) of density current. It is shown that the calculated results are in agreement with those of the flume experiment.

Steady Lateral Growth of Three-Dimensional Particle Laden Density Currents

In this paper the steady lateral growth of three-dimensional turbulent inclined turbidity current is investigated. To simulate the current, an experimental setup is developed to analyze the turbidity current for different regimes in the particle laden density currents environment. The Buckingham’s π theorem together with a dimensional analysis is implemented to derive the appropriate non-dimensional variables. The experimental results were normalized and plotted in the form of non-dimensional graphs from which a theoretical model is developed and analyzed. Based on the results obtained for the steady lateral growth, three different regimes, namely, inertia-viscous one as the first regime, buoyancy-viscous and gravity-viscous as the second and third regimes are distinguished within the current.In these regimes, the force balance is between the driving and resisting forces. Namely, in the first regime, the force balance is between the inertia and viscous forces, in the second regime, the buoyancy and viscous forces, and in the third regime, gravity and viscous forces are balanced. The experimental results indicate that the lateral growth rate in the first regime is smaller than that in the second and third regimes due to the magnitude and type of the forces involved in those regimes. According to the graphical results, the three different lateral growth rates appear when the normalized current length is smaller than about 3, between about 3 and 10, and larger than about 10. In those regions,the slopes of the data are different with respect to one another.

海底峡谷浊流汇流后含沙量与速度变化研究

高速的浊流具有强大的破坏力,威胁着海底结构物的安全。海底峡谷是浊流向深海运动的重要通道,其中许多海底峡谷具有多条分支峡谷,而分支峡谷与主干峡谷浊流发生汇流后,含沙量、速度可能会增加,进而破坏力增强。本文通过室内水槽试验和数值模拟,研究了分支峡谷中的浊流汇流到主干峡谷中含沙量和速度的变化,并与仅有主干峡谷浊流的情景进行了对比。研究发现,发生汇流时,浊流的高度、含沙量和速度在头部均有增加,在汇流发生过后会有所减小,但含沙量和速度仍大于不发生汇流时的情况。本文试验结果可为有分支峡谷发生浊流汇流的现场监测位置及项目、速度推算提供指引。

鄂尔多斯盆地东南部三叠系长7油层组深水沉积特征及演化规律

以沉积学理论为指导,利用野外露头、钻井岩心、测井等资料,对鄂尔多斯盆地东南部三叠系长7油层组深水沉积特征包括相标志、沉积微相类型等进行了研究,并揭示了其沉积演化规律。研究结果表明:(1)鄂尔多斯盆地东南部三叠系长7油层组深水沉积的相标志包括:岩石中常见水平层理、鲍马序列、槽模沟模、滑动与滑塌构造、撕裂屑、泥包砾等沉积构造,含有深水双壳类和鱼类动物化石,粒度概率曲线中悬浮总体含量大且分选差,测井曲线上可见锯齿状、齿化箱形-钟形-指形、泥岩基线等特征。(2)研究区长7物源主要来自东北和南部2个方向,发育深湖—半深湖、湖底扇、浅湖沉积,湖底扇包括浊流和砂质碎屑流2种重力流类型,可进一步划分为重力流主水道、溢流沉积、重力流分支水道、分支水道间、朵叶体等微相。湖底扇主水道主要为砂质碎屑流沉积,分支水道和朵叶体主要为浊流沉积。(3)研究区长7沉积期,中南部主体发育深湖—半深湖、湖底扇沉积,东北部发育浅湖沉积;其中,东北方向发育4个湖底扇体,南部发育2个湖底扇体,半深湖/浅湖界线呈北西向延伸于延安—甘泉一带。长7^(3)亚段沉积期,深湖—半深湖范围最大,仅局部发育湖底扇;长7^(2)、长7^(1)亚段沉积期,湖底扇逐步增多,深湖—半深湖范围有所缩小,整体呈深湖—半深湖与湖底扇交互沉积态势。

CHARACTERISTIC ANALYSIS OF THE PLUNGING OF TURBIDITY CURRENTS

Turbidity currents are flows driven by suspended sediment of flood-induced turbid river water with excess density.Such currents are often the governing factor in reservoir sedimentation by transporting fine materials over long distances and delivering the majority of deposition,which thus reduces the storage capacity.Therefore,the design and operation of a reservoir requires an accurate prediction of its occurrence condition and plunging position,which is the objective of the present study.This article presents a verified algebraic slip mixture model including momentum,continuity and algebraic velocity expressions to simulate 2-D turbidity currents.Test experiments in a multiphase flume were carried out.Reynolds number,sediment concentration and densimetric Froude number were used as parameters in the occurrence condition analysis.The plunging of turbidity currents may produce reflux and backflow due to the diving flow at the surface of the clear water.The similar experimental results were also obtained by PIV measurements

On a Shallow Water Model for the Simulation of Turbidity Currents

We present a model for hyperpycnal plumes or turbidity currents that takes into account the interaction between the turbidity current and the bottom,considering deposition and erosion effects as well as solid transport of particles at the bed load due to the current.Water entrainment from the ambient water in which the turbidity current plunges is also considered.Motion of ambient water is neglected and the rigid lid assumption is considered.The model is obtained as a depth-average system of equations under the shallow water hypothesis describing the balance of fluid mass,sediment mass and mean flow.The character of the system is analyzed and numerical simulations are carried out using finite volume schemes and path-conservative Roe schemes.

页岩油水平井全油藏立体开发理念及庆城油田大平台实践

目前国内页岩油开发尚处于起步阶段,其中,鄂尔多斯盆地庆城油田探索形成了全油藏立体开发理念,率先实现了国内陆相页岩油工业规模开发的成功。为阐明全油藏立体开发理念的内涵、形成条件和关键作用,根据陆相湖盆沉积模式和微地震资料,解剖刻画了长7页岩油油层展布特征和水平井压裂缝网轮廓形态,并对典型大平台开发效果进行了评价。结果表明,全油藏立体开发需要具备有地质储层条件和工程造缝能力,长7页岩油储层平面上大面积连片展布、纵向上砂泥叠置互层,源储一体使得油层在立体空间充分发育,压裂缝能够穿层延伸,缝网正视角呈近端高缝穿层、远端收敛的菱形,整体轮廓呈“棍子面包”形态,“W”型交错部署的水平井立体开发井网可实现全油藏储量一次性动用,因此庆城油田大平台开发能够取得成功。现阶段加大全油藏立体开发理念的实践应用,对提升国内陆相页岩油产量、助推页岩油革命具有重要意义。

合水地区长6_(3)深水重力流沉积特征研究

合水地区是鄂尔多斯盆地近年来原油勘探的重点区块,在系统分析沉积构造、岩石学特征、结构粒度特征等沉积相识别标志的基础上,结合区域沉积背景,开展了合水地区长6_(3)重力流沉积类型及展布特征研究。结果表明,合水地区长6_(3)油层亚组为湖泊-重力流沉积体系,发育半深湖-深湖沉积亚相,可进一步划分为浊流沉积、砂质碎屑流、滑塌沉积三种重力流沉积类型;其中砂质碎屑流及浊流为主要控砂微相,主要分布在研究区的中部、南部区域,为区内有利储集相带。

Numerical simulation on the evolution of sediment waves caused by turbidity currents

Interest in the forming mechanism of sediment waves increases recently because of its significance on submarine engineering, sedimentary dynamics and hydrocarbon reservoir prediction in deep water. In this paper, the time-averaged continuity equations and Reynolds-averaged Navier-Stokes equations are applied in the numerical simulation of fluid dynamics. The modeling results are used to illuminate the effects of topography on turbidity current and explore the origin of submarine sediment waves. The research results show that (1) deposition occurs firstly at the lower ramp due to the deceleration of fluid, increase of density, loss of flow capacity and longer duration of flow passage; (2) density increase at the upslope due to the local jam results in velocity decrease and pressure increase; (3) sediment waves begin to be formed and migrated toward upstream in an area far away from the source with in- crease of the turbidity events; (4) deposition becomes more slowly with decrease of grain sizes, but the shape and sequences of these deposits are controlled by topography, not grain size.

南海北部陆坡X区MTDs特征及形态动力学控制因素

海底斜坡地形对浊流的走向及其沉积物的分布与几何形状具有控制作用。越来越多的研究表明,块体搬运沉积体系(MTDs)顶面地形起伏不平,也会影响后续浊流沉积分布。为了进一步探究海底复杂地形如何对后期浊流的沉积分布产生影响,基于南海北部陆坡X区三维地震数据,对研究区发育的MTDs及其顶界面地形进行刻画,利用Fluent软件中多相流双欧拉法和Realizable k-ε湍流模型对MTDs顶面地形进行数值模拟分析。结果表明,在研究区内识别出大规模的块体搬运沉积,地震剖面显示出空白杂乱反射的特征,同时还识别出逆冲断层、挤压脊等典型构造,这些构造导致MTDs顶界面的地形起伏不平。MTDs趾部发育挤压脊,浊流路过时动能逐渐消散,充填挤压脊之间的凹槽,趾部平缓区域沉积分布明显。入流初速度越大,MTDs顶界面受到的侵蚀程度就越严重,且随着模拟过程的结束,后期浊流对MTDs顶界面的侵蚀和沉积分布走向与入口方向平行。

Numerical and experimental study of continuous and discontinuous turbidity currents on a flat slope

When the sediment and the dissolved matter laden in the river meet a clear water in reservoirs, the turbid water will plunge and spread into the clear water, forming the turbidity current and influencing the water quality and the life of the reservoir. Due to the unsteady nature of the flood, the turbidity current is unsteady. In the present study, we use the MIKE 3 computational fluid dynamics code to simulate continuous and discontinuous turbidity currents on a flat slope. With the model used by us, the turbulence is divided into two parts: the horizontal turbulence and the vertical turbulence, which are separately modeled by the Smagorinsky model and our model to capture the anisotropic turbulence. In this model, the sediment settling and deposition are considered. The simulation results concerning the flume water surface level, the front velocity and sediment concentration profiles are found consistent with the experimental data, particularly, for the sediment concentration profiles with an absolute mean error of 0.026 kg/m3and the root mean square error of 0.046 kg/m3. This finding suggests that this model can be used to well predict the turbidity current on the flat slope.

高密度浊流和砂质碎屑流关系的探讨

高密度浊流和砂质碎屑流作为深水环境重要的地质营力,但如何区分两种流体及其沉积,一直存在着争议。通过探讨两种流体的关系,有助于澄清深水沉积过程的相关认识,有助于正确区分两种流体的沉积,也有利于开展相关砂岩储层的预测。在分别回顾高密度浊流和砂质碎屑流的概念、特征及具体实例的基础上,对比了两者的流体性质和沉积特征,结合触发机制、形成过程及影响因素,从理论运用和实际运用两方面探讨了两者的关系。高密度浊流沉积可划分为底部、中部和上部三部分,垂向上具逆——正粒序。砂质碎屑流沉积富砂质,具块状层理,垂向上可由块状砂岩叠置形成。碎屑流形成的触发机制多样,且相对于中等至弱粘性碎屑流,强粘性碎屑流不易向浊流转换。与砂质碎屑流相比,高密度浊流在术语使用上更合理。如果碎屑流发生了流体转换,形成高密度浊流沉积;如果未发生,且物源富砂质,可以形成砂质碎屑流沉积。块状砂岩与上覆泥质沉积物呈突变接触时,可能为砂质碎屑流成因;与上覆沉积物为渐变接触时,构成正粒序,为高密度浊流成因。

涌流型浊流形成及发展的实验模拟

对涌流型浊流及底流型浊流的动量方程进行了比较 ,结果表明涌流型浊流不仅从其前部卷吸水体 ,同时从其顶面卷吸水体。在 0°、5°、10°底坡上开展的涌流型浊流的模拟实验发现 :涌流型浊流的悬浮云是悬伸而向前凸出的 ,在横向上形成近乎周期性出现的船艄形的凸起和凹陷 ;涌流型浊流的主体比头部运动速度快 ,运动过程表现为波浪式前进、后波超前波的特征 ;涌流型浊流的流体厚度及速度与搬运距离和底坡成正比 ;流体密度在其底部较大 ,顶部较小 ,而底流型浊流不具上述特征。

重力流沉积:理论研究与野外识别

重力流沉积是(半)深海和深湖环境中一种重要的沉积现象,因此准确识别重力流沉积对恢复古代沉积环境具有重要意义。从沉积物重力流的基本理论出发,介绍四类重力流沉积的特点和野外鉴别特征。碎屑流沉积表现为颗粒大小混杂,底面平坦,板条状砾石平行层面排列;超高密度流沉积的砂岩呈厚层状或块状,砂岩内部经常出现较大砾石或泥岩碎片,泄水构造和液化构造常见,具逆粒序的颗粒流也属于超高密度流;高密度流沉积兼具超高密度流和浊流沉积的特点;鲍玛层序Ta-d段是一次浊流事件的产物,但只有Ta段为浊流沉积,Tb-c段为牵引流沉积,Td段为悬浮沉积。在重力流发展过程中可出现各种流动转换。底流和过路流对重力流沉积的改造也是一种普遍现象。

聚合氯化铝-有机高分子复合絮凝剂处理造纸废水

用流动电流技术研究了聚合氯化铝（PAC）分别与不同类型有机高分子制备复合絮凝剂（PACP）的电荷特性，并利用透射电镜技术探讨了PACP的结构形貌。同时利用自制的复合絮凝剂对造纸综合废水的浊度、色度和COD的脱除效果进行了研究。结果表明：与PAC相比，PACP的脱除效果有明显地提高；PAC-阳离子型有机高分子复合絮凝剂的效果又优于PAC-阴离子型有机高分子复合絮凝的效果。