FlowCharacteristics

A comparison is made of the flowcharacteristics of mud drilling and air drilling in an example deep well. A schematic of this example well is shown in Figure 14. 7. The well is cased from the surface to 7000 ft with API 85/8 in diameter，28. 00 lbf / ft nominal casing. The well has been drilled out of the casing shoe with a 77/8 in diameter drill bit. The comparison is made for drilling at 10000 ft. The drill string in the example well is made up of ( bottom to top) 77/8 in diameter drill bit， about 500 ft of 6 3/4 in outside diameter by 21316in inside diameter drill collars，and about 9500 ft of API 41/2 in diameter，16. 60 lbf / ft nominal，EU-S135，NC 50，drill pipes.

The mud drilling hydraulics calculations are carried out assuming the drilling mud weight is10 lbf / gal( 75 lbf /ft³) ，the Bingham mud yield is 10 lbf /100 ft²，and the plastic viscosity is 30 cts. The drill bit is assumed to have three 13/32 in diameter nozzles and the drilling mud circulation flowrate is 300 gal/min. Figure 14. 8 shows the plots of the pressures in the incompressible drilling mud as a function of depth. The figure is a plot of the pressure inside the drill string. The pressure is approximately 1400 psig at injection and 6000 psig at the bottom of the inside of the drill string just above the bit nozzles. Also the figure is a plot of the pressure in the annulus. The pressure is approximately 5440 psig at the bottom of the annulus just belowthe bit nozzles and 0 psig at the top of the annulus at the surface.

Figure 14. 7 Comparison example well and drill string

Figure 14. 8 Mud drilling pressures versus depth

This figure reflects the hydrostatic weight of the column of drilling mud and the resistance to fluid flowfrom the inside surfaces of the drill string and the surfaces of the annulus. This resistance to flowresults in pressure losses due to friction. The total losses due to friction are the sum of pipe wall，open hole wall，and drill bit orifice resistance to flow. This mud drilling example shows a drilling string design which has a open orifice or large diameter nozzle openings in the drill bit. This is reflected by the approximate 700 psi loss through the drill bit. Smaller diameter nozzles would yield higher pressure losses across the drill bit and higher injection pressures at the surface.

The air drilling calculations are carried out assuming the drilling operation is at sea level. There are two compressors capable of 1200 ft3/ min each，so the total volumetric flowrate to the drill string is 2400 ft3/ min. The drill bit is assumed to have three open orifices ( about 0. 80 in diameter) . Figure 14. 9 shows the plots of the pressures in the compressible air as a function of depth. The figure is a plot of the pressure inside the drill string. The pressure is approximately 260 psia at injection and 270 psia at the bottom of the inside of the drill string just above the bit orifices. Also in the figure is a plot of the pressure in the annulus. The pressure is approximately 260 psia at the bottom of the annulus just belowthe bit orifices and 14. 7 psia at the end of the blooey line at the surface ( top of the annulus) .

Figure 14. 9 Air drilling pressures versus depth

As in the mud drilling example，the pressures in Figure 14. 9 reflects the hydrostatic weight of the column of compressed air and the resistance to air flowfrom the inside surfaces of the drill string and the surfaces of the annulus. This resistance to flowresults in pressure losses due to friction. In this example the fluid is compressible. Considering the flowinside the drill string，the hydrostatic weight of the column dominates the flow( relative to friction losses) and this results in the injection pressure at the surface being less than the pressure at the bottom of the drill string ( inside the drill string above the bit open orifices) .

Figure 14. 10 shows the plots of the temperature in the incompressible drilling mud as a function of depth. The geothermal gradient for this example is 0. 01 / ft. Subsurface earth is nearly an infinite heat source. The drilling mud in a mud drilling circulation system is significantly more dense than compressed air or other gases. Thus，as the drilling mud flows down the drill string and up through the annulus to the surface，heat is transferred from the rock formations through the surfaces of the borehole，through the drilling mud in the annulus， through the steel drill string to the drilling mud inside. It is assumed that the drilling mud is circulated into the top of the drill string at 60.

Figure 14. 10 Mud drilling temperature versus depth

As the drilling mud flows down the inside of the drill string the drilling mud heats up as heat flows from the higher temperature rock formations and drilling mud in the annulus. At the bottom of the well the drilling mud temperature reaches the bottom hole temperature of 160. The drilling mud flowing up the annulus ( usually laminar flowconditions) is heated by the geothermal heat in the rock formation.

The heated drilling mud flowing in the annulus heats the outside of the drilling string and this in turn heats the drilling mud flowing down the drill string. Because of its good heat storage capabilities，the drilling mud exits the annulus with a temperature greater than the injection temperature but less than the bottom hole temperature. In this example，the temperature of the drilling mud exiting the annulus is approximately 130.

Figure 14. 11 shows the plots of the temperature in the compressible air drilling fluid as a function of depth. The compressed air drilling fluid is significantly less dense than drilling mud. Thus，compressed air has poor heat storage qualities relative to drilling mud. Also，compressed air flowing in the drilling circulation system is flowing rapidly and therefore the flowis turbulent inside the drill string and in the annulus. Turbulent flowis very efficient in transferring heat from the surface of the borehole to the flowing air in the annulus and in the inside the drill string. Assuming the compressed air entering the top of the drill string is at 60 the heat rapidly transfers to heat ( or cool ) the air flowin the well. Under these conditions the compressed air exiting the annulus has approximately the same temperature as the air entering the top of the drill string.

Figure 14. 11 Air drilling temperature versus depth

Figure 14. 11 shows that the temperature of the compressed air at any position in the borehole is approximately the geothermal temperature at that depth. Thus，the temperature of the flowing air at the bottom of the hole is the bottom hole temperature of 160. There is some local cooling of the air as it exits the open orifices of the drill bit at the bottom of the hole. This cooling effect is more pronounced if nozzles are used in the drill bit ( when using a downhole motor) . This cooling effect is known as the Joule － Thomson effect and can be estimated. However，it is assumed that this effect is small and that the air flowreturns very quickly to the bottom hole geothermal temperature.

Figure 14. 12 shows the plot of the unit weight of drilling mud for this example calculation. The drilling mud is incompressible and，therefore，the unit weight is 75 lbf / ft3( or 10 lbf /gal) at any position in the circulation system. There is some slight expansion of the drilling mud due to the increase in temperature as the drilling mud flows to the bottom of the well. This effect is quite small and is neglected in these engineering calculations.

Figure 14. 12 Mud drilling unit weight versus depth

Figure 14. 13 shows the plot of the unit weight of the compressed air in this example. The compressed air is injected into the top of the drill string at a unit weight of 1. 3 lbf / ft³( at a pressure of 260 psia and temperature of 60) . As the air flows down the drill string the pressure remains approximately the same. At the bottom of the drill string the unit weight is1. 2 lbf / ft³( at a pressure of 270 psia and a temperature of 160) . The compressed air exits the drill bit orifices into the bottom of the annulus ( bottom of the well) with a unit weight of1. 1 lbf / ft³( at a pressure of 260 psia and a temperature of 160) .

Figure 14. 13 Air drilling unit weight versus depth

As the compressed air flows to the surface through the annulus it decompresses as it flows to the lowatmospheric pressure at the surface. At the surface the air exits the annulus ( via the blooey line) with a unit weight of 0. 0763 lbf / ft³. The surface atmosphere for this example is assumed to be API Mechanical Equipment Standards standard atmospheric conditions ( dry air: pressure of 14. 696 psia and a temperature of 60) . This figure shows a typical friction resistance dominated drill string flow( as opposed to hydrostatic column weight dominated) . This type of flowhas a drill string injection pressure at the top that is higher than the pressure above the drill bit at the bottom. Friction dominated flowresults when the drill bit is run with no nozzles.

The drilling mud flows in the annulus around the drill collars with an average velocity of about 7. 6 ft / s. The drilling mud slows to an average velocity of about 3. 0 ft / s in the annulus around the drill pipe.

For the air drilling case，the compressed air flows in the annulus with an average velocity of about 30 ft / s around drill collars. The velocity increases up the annulus to about 125 ft / s at the exit to the annulus.

It is instructive to compare the power ( per unit volume) of example flows at the positions in the annulus where the power is likely the lowest. For both of these examples the lowest power is just above the drill collars in the annulus around the bottom of the drill pipe. The kinetic energy per unit volume，KE，is

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where KE is the kinetic energy per unit volume ( ft·lbf / ft³) ; ρ is the density of the fluid( lbf·sec²/ ft⁴) ; v is the average velocity of the fluid ( ft/s) .

The density of the fluid，ρ，is

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where γ is the specific weight of the fluid ( lbf / ft³) ; g is the acceleration of gravity( 32. 2 ft/s²) .

For the mud drilling example the specific weight of the drilling mud in the annulus just above the drill collars is 75 lbf / ft³. Using these values in Equations ( 14. 1) — ( 14. 2) the density of the drilling mud is

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石油钻井部分词汇扩展

( 1) 石油钻井设备 ( Oil drilling equipment)

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( 2) 定向钻井 ( Directional drilling)

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( 3) 固井与完井 ( Cementing and well completion)

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( 4) 油气井压力控制 ( Pressure Control of oil and gas well)

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( 5) 测井 ( Well logging)

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( 6) 钻井泥浆 ( Drilling mud)

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