Typically, drilling hydraulics optimizations (maximum jet impact and maximum hydraulic horsepower) are based on an assumption of a fixed system. In real operation, you cannot fix flow rate to drill all the way to TD because the system pressure loss is changed because of hole depth, drilling mud properties, wellbore conditions, etc.
There are two ways of optimizing drilling hydraulics which are optimization at the beginning of the run and at the end of the run.
Drilling Hydraulics Optimized at the Beginning of the Run
If the system is designed to meet the maximum hydraulic horsepower at the beginning of the well and the pump pressure is maintained constant at the designed value, when the well is getting deeper, the flow rate must be reduced to maintain the pressure. With this situation, it will cause the jet velocity and hydraulic horsepower dropping (see the curve below) at the end.
Drilling Hydraulics Optimized at the End of the Run
For this case, the end of the run (the deepest depth) is the condition where we design to maximize hydraulic horse power. We know the pressure which we will maintain to get the best hole cleaning at the end. At the beginning of the run, the hydraulic horsepower will not be at the maximum point (see the curve below).
Decide the Optimum Point of the Bit Run for drilling hydraulics optimization
First of all, you need to keep in mind that the calculation for drilling hydraulics optimization is valid only for a fixed depth. Therefore, you need to work within a certain range. Practically, operating within 15 – 20 % deviation from the theoretical value is acceptable because either hydraulic horsepower or impact force will not be changed significantly. Moreover, the best point to optimize hydraulics is at the end of the bit run because it gives the best hole cleaning at the deepest depth and makes the best performance of the bit.
Reference: Drilling Hydraulic Books