Impacts of Clay Type and Concentration on the Structure of Turbidity Current Flows
Melissa J. Crai, 2014
Honours Degree of Bachelor of Science (Petroleum Geology and Geophysics)
Australian School of Petroleum
The University of Adelaide
Experimental turbidity currents have been the subject of research for over 50 years and are the primary source of our present understanding of turbidity current fluid dynamics and deposits. Despite evidence of mud-rich turbidity currents, the use of clay in experimental flows has been largely avoided due to the network of flocculated particles that form in clay suspensions. This network has been observed to significantly alter flow behaviour, but the present understanding of its impact on flow structure and depositional processes is limited.
This thesis presents a series of clay-rich lock-exchange experimental turbidity currents conducted to obtain direct measurements of the velocity and turbulence structure of this flow type. Two clay types, bentonite and kaolinite, were used at a range of volumetric clay concentrations. Ultrasonic Doppler Velocity Profiling was used to measure stream-wise velocities, from which velocity and turbulence intensity profiles would be generated. Initial processing revealed surface waves that were affecting data quality and a Fourier frequency domain based filter was developed to remove them.
The results revealed changes in the flow structure of kaolinite suspensions as the volumetric clay concentration increased. At concentrations of less than 1%, the flow remained turbulent, but further addition of kaolinite induced changes in the velocity and turbulence profiles of the flows. Turbulence intensity increased over the entire profile and was markedly enhanced directly above the velocity maximum. This structure was observed in all flows up to a volumetric concentration of 3.5% at which the flow demonstrated another structure change. Turbulence in the upper flow region was strongly enhanced and the velocity profile featured a more pronounced velocity maximum as velocities were reduced elsewhere in the profile. The Reynolds numbers of the kaolinite flows showed a decreasing trend with increasing concentration, indicative of increasing viscous forces as the clay network developed.
In contrast, the bentonite flows demonstrated no apparent change in flow structure across the range of concentrations used. This was determined to be a result of the concentration of sodium chloride in the fluid that the bentonite clay, sodium montmorillonite, was mixed with to generate the suspension. These ionic conditions cause the clay particles to associate in a particular way that reduces the yield stress
iii and fluid viscosity of the suspension, permitting turbulent flow even at high clay concentrations of 3.5%.
These results have indicated that the type of clay structure in the clay suspension has a significant impact on the flow rheology. Clay particles will associate differently under different chemical conditions, and these associations impact flow turbulence and velocity to different degrees. This study has highlighted that suspension chemistry, in addition to clay type, has an impact on clay-rich turbidity current flow dynamics.