Unit 10 - Hydraulics Flashcards
How do you call the necessary amount of pressure in a system that is required to overcome the resistance to flow of a fluid in a system?
circulating pressure
Hydraulic power needed to circulate the fluid is a direct function of ?
pressure losses and flowrate through the system?
does the flowrate change during different parts of the system?
no
what are sacrifical pressure losses?
pressure required to circulate the fluid through the drillstring and annulus → because they do not contribute anything to the drilling process
why is it desirable to optimise the pressure losses through the nozzles?
- because it helps to clean the drilled cuttings from the face of the bit
What are the principial factors, which influence the magnitude of the pressure losses in the system?
- The geometry of circulating system (e.g. I.D. of drillpipe, length of drillpipe)
- The flowrate through the system
- The flow regime in which the fluid is flowing (laminar/turbulent)
- The rheological properties of the circulating fluid
What are the two types of flow pattern?
- laminar flow
- turbulent flow
If you circulate a newtonian fluid through a pipe, the onset of turbulence is dependent on which parameters?
- Pipe diameter, d,
- Density of fluid, r
- Viscosity of fluid, µ
- Average flow velocity, v
and if the combination of these values exceeds a value of 2100
What are the different rheological models used by drilling engineers to describe drilling fluids?
- newtonian model
- non-newtonian model
- bingham plastic model
- power-law model
What are examples of newtonian fluids?
- water
- gases
- high gravity oil
How will a model according to the bingham plastic model behave?
- will not flow until applied shear stress, exceeds a certain minimum shear stress value known as the yield point
- but after yield point has been exceeded, changes in shear stress are directly proportional to changes in shear rate, with the constant of proportionality being called the plastic viscosity, µp.
What does the gel strength represent?
the shear stress to shear rate behaviour of the fluid at near zero shear rate conditions
- the power law can be used to represent what kind of fluids?
- pseudoplastic fluids ( n<1)
- newtonian fluids (n=1)
- dilatant fluids (n>1)
the deviation of the non-newtonian exponent from 1 describes what?
- the degree to which the fluid behaviour is non-newtonian
What is the apparent viscosity?
the shear stress to shear rate relationship measured at a given shear rate in a non - newtonian fluid
When do you call a non - newtonian fluid pseudoplastic?
if the apparent viscosity decreases with an increasing shear rate
When do you call a non-newtonian fluid dilatant?
- when the apparent viscosity increases with increasing shear rate
which fluids are generally pseudo-plastic?
- drilling fluids
- cement slurries
When will you get a turbulent flow?
- when fluid is pumped at a high rate the fluid laminae become unstable and break into a chaotic, diffused flow pattern
The frictional pressure loss associated with the turbulent flow of a Bingham plastic fluid is affected primarily by ?
density and plastic viscosity.
What are the two major aims for an optimum hydraulics programme?
- clean hole effectively
- make best use of power available to drill the hole
what are the two different approaches to optimum hydraulics design?
- maximise hydraulic horsepower at bit → assumes that best method of cleaning the hole is to concentrate as much fluid energy as possible at the bit.
- maximise hydraulic impact at bit → assumes that most effective method is to maximise the force with which the fluid hits the bottom of the hole.
What is the real purpose of the jet nozzles?
- improve cleaning action of drilling fluid at the bottom of the hole
What is the pressure of the returning mud at the flowline?
- atmospheric pressure
Where do you have the biggest pressure drop in the system?
What kind of flow do you generally have in the annulus?
- laminar flow
What kind of flow do you generally have in the drillstring?
- turbulent flow
Why do you usually try to avoid turbulent flow in the annulus?
- it may cause washouts in the formation by erosion