![]() Turbulent Flow : In turbulent flow there is an irregular random movement of fluid in a transverse direction to the main flow.Laminar flow systems are generally represented graphically by streamlines. There is no microscopic or macroscopic intermixing of the layers. ![]() Laminar Flow (Streamline or Viscous flow) : In this type of flow, layers of fluid move in streamlines or laminae.Transport or lift velocity is defined as the difference between the annular velocity of mud and the slip velocity of particle ie Vt = Va - Vs where Vt = transport velocity Va = annular velocity Dh = hole diameter ODp = outside diameter of pipe.What is the annular velocity (ft/min and ft/sec) of the mud? Assume a flow rate of 920 gpm in a 17.5-inch hole with 4.5-inch drill pipe.This minimum annular velocity is dependent on the properties of the mud and cuttings for any particular well, and is usually between 100 - 200 ft/min.Only then, should any further increase in fluid flowrate be used to improve the pressure loss across the nozzles of the bit and therefore the hydraulic power at the bit face. ![]() Considerations with respect to optimization should therefore only be addressed once the minimum annular velocity for which the cuttings will be removed is achieved. If these cuttings are not removed the drillstring will become stuck and theoretical optimization will be fruitless. In order to do this the velocity of the fluid in the annulus will have to be high enough to ensure that the drilled cuttings are efficiently removed. The primary function of the drilling fluid is to carry the drilled cuttings to the surface. Horsepower Used in Drillstring and Across Nozzles of Bit This expression for hydraulic horsepower is a general expression and can also be used to express the power which is expended in sacrificial losses and the power that is used to pump the fluid through the nozzles of the bit. ![]() Since the flowrate through all parts of the system is equal, attention is generally focused on the pressure losses in each part of the system.where, Pt = Total pressure (psi) Q = flow rate (gpm) The product of the circulating pressure losses and the flowrate through the system is equal to the hydraulic power that the mud pumps will have to generate.pressure losses through the nozzles The ejection of the fluid through the nozzles in the bit also results in significant pressure loss but does perform a useful function(it helps to clean the drilled cuttings from the face of the bit.Sacrificial pressure losses: The pressure required to circulate the fluid through the drillstring and annulus are often called sacrificial pressure losses, they do not contribute anything to the drilling process The resistance is expressed in terms of the amount of pressure required to circulate the fluid around the system and is therefore called the circulating pressure of the system.The magnitude of the resistance to flow is dependant on a number of variables.A significant amount of power is required to overcome the (frictional) resistance to flow of the fluid in the drillstring, annulus and through the nozzles in the bit.One of the primary functions of drilling fluid is to carry drilled cuttings from the bit face, up the annulus, between the drillstring and wellbore, to surface.
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