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Rheological investigation in the mining and oil exploration

While mining is one of the oldest industries, with metal and stone being mined in prehistoric times, it continues to grow and expand today. Mining targets range from basic energy materials, such as coal and gas, to building materials such as iron, and precious metals such as nickel, titanium, lithium, gold, and others. 

The main processes involved in mining include drilling, processing, and transport of the excavated materials, and removal and storage of waste products. Since most mines deal with extremely large areas and quantities, small differences in the efficiency of these processes can have a big impact on the productivity and economic viability of the mine.

Rheological behavior of materials used in mining and oil exploration

Rheological measurements provide important information about the efficiency of the processes described above, which largely revolve around the flow of slurries and liquids:

Drilling

Drilling fluids play a number of important roles in the drilling process, such as lubricating the bit and drill string, clearing cuttings from the bit face, and keeping formation fluids at bay. Rheological measurements are needed to monitor and predict the performance of drilling fluids in diverse environments, from deep-sea mines as cold as 5 ˚C, to geothermal wells up to 180 ˚C. 

Processing and transport

In some cases, excavated materials need to be transported over long distances before they can be processed and refined. Rheological data is needed to define the optimum pumping conditions, including pipe size, fluid composition, and pump pressure.

Waste treatment

The mining industry is the world’s largest producer of waste. Removing much of the water before the waste is transported can result in savings of water, transport energy, and storage costs; however, concentrating the waste can also dramatically change its flow behavior, so rheological measurements are needed to design the best waste-disposal strategy.

Typically measured materials from the mining industry

Drilling fluids

The most critical function that a drilling fluid performs is to minimize the concentration of cuttings around the drill bit and throughout the well bore. The drilling fluid must also stabilize the wellbore and lubricate the bit and drill string to ensure high drilling rates. The fluid must therefore be viscous enough to lift the cuttings to the surface, but at the same time the viscosity must not be so high that it minimizes friction-pressure loss. The best way to control the viscosity balance is to manage the drilling-fluid rheology.

Rheological tests on drilling fluids

Drilling fluids have been through a major technological evolution: previously they were a simple mixture of water and clays; today they are complex synthetic mixtures of various organic and inorganic products known as “drilling muds”. By controlling the components, the rheological properties can be improved and adapted. For example, if water is added to the mixture, the viscosity decreases over a wide shear-rate range. If bentonite is added, the viscosity increases. Polymers help to increase the viscosity at high shear rates, whereas white lime is used to increase the viscosity at lower shear rates. 

The figure below shows how increasing the amount of thickener in a drilling fluid increases its viscosity over a wide shear-rate range.

This test requires a rheometer equipped with a cylinder measuring system.

Slurries

The processing and transport characteristics of slurries are heavily dependent on their rheological properties. The yield point, for example, tells us the minimum force required to initiate the flow of the slurry, while the viscosity is a measure of the resistance of the slurry to flow. A disproportionate amount of energy is required to overcome the yield point, and until the yield point is reached the resistance and pressure in the pipe increase proportionally while the slurry is at a standstill. Thus, if the yield point is too high, it is impossible to start the pumping process. It is vitally important to monitor and control these rheological properties because a small change in pumping efficiency can have significant effects on energy and costs.

Rheological tests on slurries

The yield point and viscosity can be influenced by varying the slurry composition, e.g. the amount of water, additives (solids, polymers, liquid), particle size, and pumping temperature. Often a shear-thinning slurry is required, whereby increasing the shear stress reduces the viscosity of the slurry, making it easier to pump.

The relationship between shear rate and viscosity in a shear-thinning sample can be seen in the figure below. At low shear rates and low volumetric flow, the shear stress and pressure difference in the pipe are relatively high. However, increasing the volumetric flow of slurry in the pipe requires only a slight further increase in pressure. This is due to the shear-thinning behavior of the slurry, i.e. the viscosity decreases with increasing shear rate.

This test requires a rheometer equipped with a sandblasted cylinder measuring system.