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It is known that the plastic Bingham model is inadequate to represent some types of drilling fluids, such as low solids and emulsions, especially at medium and low shear rates. Previous experiments on carrying capacity concluded that the average annular velocity and rheological properties affect the fluid transport more significantly than other parameters. Many of these experiments refer to the yield point of the Bingham model, calculated at high shear rates as the most important parameter.
This experiment studies how the initial gel strength and plastic yield point affect the carrying capacity of the drilling fluids employing the transport ratio concept. For this purpose the terminal settling velocity of the drilled solids through quiescent fluids was experimentally determined. The results showed a better correlation with the initial gel strength than with the yield point. Also, the analysis of the results demonstrated that a gel strength of about 1.92 Pa (4.0 lbf/100 ft2) is necessary and substantial to obtain a transport ratio higher than 0.50 when the annular velocity is in the range of 0.41 m/s (80 ft/min) to 0.66 m/s (130 ft/min), using low solids and polymer drilling fluids.
For any type of water based drilling fluids a gel strength in the range of 3.35 Pa (7 lbf/100 ft2) to 4.79 Pa (9.0 lbf/100 ft2) is sufficient to remove the cuttings at more than 0.50 of transport ratio, even when the annular velocity is about 0.25 m/s *50 ft/min).
The transport of drilled solids from the bottom hole to the surface, throughout the annulus, is one of the main objectives of the drilling fluids. The terms removal velocity and transport ratio are often used to estimate the ability of a drilling fluid to transport drilled solids in a vertical well. The removal velocity is a mean value of the relative cuttings velocity which is equal to the difference between the average annular fluid velocity and the ave rage slip velocity of the cuttings.
The average annulus velocity is defined as a function of the flow rate and the dimensions of the annulus, while the slip velocity is defined as the average velocity at which the particles tend to settle in a fluid. This velocity depends on the size of the particle, its geometry, its density, and properties of the fluid, mainly rheological and density.
The transport ratio is defined as follows:
The analysis of the equation (2) shows that the transport ratio increases when the slip velocity decreases or the annular velocity increases. If the slip velocity is equal to zero, then the transport ratio will be transported with the same annular velocity. Conversely, if the slip velocity is high the transport ratio will be low. In this case, note that the concentration of the solid in the annulus will increases.