Resuspension, Flocculation, and Transport of Drilling Muds and Fine-grained Sediments 

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FINAL TECHNICAL SUMMARY 

STUDY TITLE: Resuspension, Flocculation, and Transport of Drilling Muds and Fine-grained 

Sediments 

REPORT TITLE: Resuspension, Flocculation, and Transport of Drilling Muds and Finegrained Sediments 

CONTRACT NUMBER: 14-35-0001-30471 

SPONSORING OCS REGION: Pacific

APPLICABLE PLANNING AREA(S): Southern California 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 89, FY 90, FY 91 

COMPLETION DATE OF REPORT: March 1993 

COST(S): FY 89: $70,000; FY 90: $76,532; FY 91: $76,532 

CUMULATIVE PROJECT COST: $223,064 

PROJECT MANAGER: Wilbert J. Lick 

AFFILIATION: University of California, Santa Barbara 

ADDRESS: Department of Mechanical and Environmental Engineering, University of 

California, Santa Barbara, CA 93106 

KEY WORDS: drilling muds; sediments; offshore oil; pollution; transport; resuspension; 

flocculation. 

BACKGROUND: The disposal of drilling muds from offshore oil platforms is of concern 

because of potential effects of pollution from these muds. A related problem is the concentration 

of pollutants from produced waters and other sources in bottom (especially fine-grained) 

sediments. Fine-grained sediments (silts and clays) are especially important because of their 

great capacity for adsorbing contaminants which are then transported with the sediments. Both 

drilling muds and fine-grained bottom sediments are similar in their general physical and 

chemical characteristics and can be studied by similar methods. 

In an accurate description of the transport of fine-grained sediments, it is essential to realize that 

(1) in a real mud sample, the particle sizes vary by more than three orders of magnitude, from 

less than one micrometer to several hundred micrometers, and (2) the effective particle size is a 

dynamic quantity which changes with time depending on the rates of aggregation and 

disaggregation (flocculation). Neither the parameters that govern these factors nor their influence 

on transport are well understood, but they were investigated as part of this project. 
Final Technical Summary – Lick and Huang 

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OBJECTIVES: The purpose of this project was to quantitatively understand and be able to 

predict the transport of drilling muds and bottom sediments. This included a quantitative study of 

their resuspension, deposition, and flocculation. This project was similar to previous work that 

we have done on fine-grained sediments in both lake and oceanic waters. It emphasized 

laboratory experiments but also incorporated the experimental results into mathematical models 

of resuspension, deposition, and flocculation of drilling muds and sediments. 

DESCRIPTION: Specific studies are (a) laboratory experiments on flocculation, particle sizes, 

and settling speeds of the flocculated particles; (b) resuspension experiments using our annular 

flumes in the laboratory and using the shaker to study resuspension of undisturbed sediments in 

the field; (c) numerical modeling of the resuspension, flocculation, and hence transport of muds 

and other finegrained sediments. 

STUDY RESULTS: 

Laboratory experiments on flocculation, particle sizes, and settling speeds: It should be 

emphasized that the formation of flocs is a dynamic process with the state of aggregation at any 

particular location and time depending on the rates of aggregation and disaggregation. The fact 

that flocculation is a dynamic process also indicates that aggregate properties, such as the settling 

speed, also change in a dynamic manner. For example, as a particle settles in the water column, it 

may increase or decrease in size depending on the rates of aggregation and disaggregation. 

Corresponding to this increase or decrease in size, the settling speeds will also increase or 

decrease. 

Figure 1 shows the result of some of our work (Xu 1989) on the flocculation of bentonite, a 

major component of most drilling muds. In these experiments, bentonite was first disaggregated 

in a blender and then placed in a Couette flocculator. Initially, the median particle size was about 

4 microns but then increased with time and eventually reached steady state. This steady-state 

particle size depends on the shear stress and also on the particle concentration. 

Figure 1. Time variations of median floc diameters for bentonite at a shear stress of 2 dynes/cm2. 
Resuspension, Flocculation, and Transport of Drilling Muds and Fine-grained Sediments 

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A numerical model which describes these flocculation processes has been developed and shows 

good agreement with the experiments. In the original model, thousands of states were used in the 

description of the flocs, i.e., each state corresponded to a different number of particles making up 

the floc. Although this model was quite accurate, it also required a great deal of computer time. 

In order for this model to be included in transport calculations, the model must be made more 

efficient. This has now been done, and the computation times have been reduced by several 

orders of magnitude. 

The above experiments and analyses investigated the effects of an applied fluid shear on 

flocculation. As the applied fluid shear decreases, the flocs increase in size, and the settling 

speeds of these flocs also increase. As this happens, the induced fluid shear due to flow around 

the flocs as they settle through the water becomes more significant than the applied shear. To 

investigate this situation, flocculation experiments were done in a unique rotating fluid disk 

device, essentially an infinitely long settling tube. Flocculation at concentrations from 100 mg/l 

to 1 mg/l was investigated. 

Of course, in predicting transport, the most important particle property is its settling speed. This 

is dependent upon the size, or diameter, of the particle but not simply through Stokes’ law, for 

example. Results of direct measurements of settling speeds of flocculated bentonite are shown in 

Figure 2. These flocs were produced in the flocculator at a sediment concentration of 100 mg/l 

and a fluid shears of 100, 200, and 400 s-1. It can be seen that all of the settling speeds are 

considerably less than those predicted by Stokes’ law for a solid particle, some by almost a factor 

of 1000. 

Figure 2. Settling speeds of bentonite flocs formed at a concentration of 100 mg/l and fluid shears of 1, 2, and 4 

dynes/cm2

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Similar experiments to those described above have been done for barite, another major 

component of most drilling muds, and also for used drilling muds. 
Final Technical Summary – Lick and Huang 

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Laboratory resuspension experiments: Drilling muds and effluents are generally disposed of 

from platforms by either discharging at or near the surface (for maximum dispersion) or by 

shunting to depths (to minimize dispersion). In either case, drilling muds are eventually 

transported to the bottom by a combination of density-driven flows, currents, turbulent diffusion, 

and settling of the particles relative to the surrounding waters. 

Here they may be buried and compacted by sediments and other particles being deposited over 

them, and they may also be processed and transformed by chemical reactions and benthic 

organisms. At some later time, depending on environmental conditions, they may be resuspended 

and again transported and dispersed by currents and wave action. This series of events will 

continue until the particles are buried deep enough so that they become a permanent part of the 

bottom sediments or until they are transported out of the region of interest. 

Considerable work has been done in general on the resuspension properties of fine-grained 

sediments, but nothing specifically on drilling muds previous to the present work. What is known 

in general about the resuspension of fine-grained sediments is as follows (Lick 1991). A 

sediment bed composed mostly of cohesive, fine-grained materials will be compacted with the 

compaction generally increasing with depth and time. Because of this, the sediments near the 

surface are relatively easy to resuspend while deeper sediments are more difficult to resuspend. 

Particle size variations also affect the resuspension since, at any particular shear stress, only the 

finer particles on the surface of the sediment bed can be resuspended while the larger particles 

are left behind and armor the bed. For both of these reasons, at any particular stress, only a finite 

and relatively small amount of sediment can be resuspended. This is in contrast to uniform-size, 

non-cohesive sediments where the resuspension rate is approximately constant with time. 

Numerical modeling of the transport of drilling muds: As part of our previous modeling efforts, 

both two-dimensional vertically integrated time-dependent and three-dimensional, time--

dependent hydrodynamic and transport models have been developed (Lick 1976; Sheng et al 

1978; Lick and Ziegler 1988). These models have been coupled with a three-dimensional, timedependent model of the sediment bed and its properties. The sediment bed model is layered in 

the vertical direction with an arbitrary number of layers and arbitrary initial thickness. The 

properties of each layer depend on time after deposition and composition (relative fraction of 

medium and coarse sediments). Settling speeds and sediment resuspension parameters needed in 

the model have been determined by laboratory and field tests. 

SIGNIFICANT CONCLUSIONS: 

Laboratory experiments on flocculation, particle sizes, and settling speeds: From this and related 

experiments, it can be shown that the median particle size decreases as the shear stress increases 

and decreases as the particle concentration increases. The rates of aggregation and 

disaggregation can also be deduced from these experiments. 

It is also clear that the settling speed of a floc depends on the conditions under which it was 

produced, i.e., flocs produced at low shears have lower settling speeds than do flocs produced at 

higher shears. It can also be shown that flocs produced at lower concentrations have lower 

settling speeds that do flocs produced at higher concentration (Burban et al 1990). 
Resuspension, Flocculation, and Transport of Drilling Muds and Fine-grained Sediments 

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Laboratory resuspension experiments: The experimental work on resuspension of bentonite, 

barite, and used drilling muds has determined the dependence of the resuspension rate and the 

total amount of sediment that can be resuspended at a particular stress as a function of (a) the 

turbulent stress at the sediment-water interface, and (b) the water content of the deposited 

sediments (or the time after deposition). 

STUDY PRODUCT: 

Huang, Hening. 1992. Transport Properties of Drilling Muds and Detroit River Sediments. 

Ph.D. Dissertation, University of California, Santa Barbara. 191 pp.