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92 Part I Liquid Drilling Systems

Illustrative Example 4.2
General data are given as follows for drilling hydraulics analysis:
Surface equipment: Type III
Casing size: 8.535 in, bottom at 8,200 ft
Hole size: 8.500 in, bottom at 11,500 ft
Drill pipe: 5 × 4.276 in
Drill collar: 6.25 × 2.813 in, 656 ft
Nozzle coefficient: 0.95
Nozzle size: 3 × 9/32 in
Bit size: 7.875 in
Flow rate: 285 gpm
Weak zone
Measured depth: 11,500 ft
Pore pressure gradient: 9.2 ppg
Fracture pressure gradient: 10.65 ppg
Mud weight: 10 ppg, Bingham plastic fluid
Plastic viscosity: 23 cp
Yield point: 14.79 lb/100 ft 2
Maximum pump pressure: 3,200 psi
Maximum pump horsepower: 1,184 hp
Minimum annular velocity: 82 ft/min
Cuttings properties
Rate of penetration: 33 ft/hr
Cuttings diameter: 0.197 in
Cuttings density: 21 ppg
The goal of the drilling hydraulics analysis is to make sure the following con-
cerns of drilling engineers are addressed:
1. The required pump pressure can be delivered by the selected pump.
2. The downhole equivalent circulating density (ECD) is between the pore
and fracture gradient so there will be no kick or loss of circulation.
3. The cuttings concentrate is controlled. It is normally required that the cut-
tings concentrate should be less than 5%.
4. Under the condition of meeting other requirements, the flow rate should be
chosen so either bit hydraulic horsepower or jet impact force is maximized.
Table 4.1 shows an output of HYDPRO, which is a drilling software model
developed by Pegasus Vertex. It indicates that the required pump pressure is
2,910 psi, which is less than the maximum pump pressure of 3,200 psi. The
pressure profile in the system is presented in Figure 4.1. The computed cuttings
concentrate profile is shown in Figure 4.2, which indicates that the maximum cut-
tings concentration, 0.706%, is lower than the normally permissible value of 5%.