Drilling Fluid: HYDRAULIC HORSEPOWER HYDRAULIC HORSEPOWER The drilling fluid is the medium which transmits available hydraulic horsepower to the system.  This horsepower is needed to move the fluid through the surface system, down the drill string, through the bit, up the annulus (the space between the hole wall and the drill pipe), through the pits and back to the suction pump.  Fluid flowing from the bit nozzles exerts a jetting action that keeps the face of the hole and the teeth edge of the bit clear of the cuttings.  The horsepower required to move the mud through the remaining system should be minimized in order to maximize horsepower at the bit.  The heavier a fluid becomes, the greater the horsepower that is required to move it through the system. This results in less horsepower at the bit and slower penetration rates. Hydraulic energy can be used to maximize the rate of penetration by improving cuttings removal at the bit. It also provides power for mud motors to rotate the bit. Hydraulic energy is measured in terms of hydraulic horsepower.  Hydraulic horsepower is determined by multiplying pump pressure by the flow rate and dividing it by the constant 1714.  example  Given a standpipe pressure of 3000 psi A pump rate of  750 gallons a minute. The constant being 1714  what is the hydraulic horsepower. hp. = (3000 psi * 750 gallons) / 1714 = 1312.71 hp. When pumping through a pipe pressure is lost due to friction. Drill string pressure losses are higher in fluids with higher densities, plastic viscosities and solids.  The use of small diameter drill rods and mud motors all reduce the amount of pressure available for use at the bit. Low-solids, shear thinning drilling fluids or those that have drag reducing characteristics, such as polymer fluids, are more efficient at transmitting hydraulic energy to drilling tools and the bit. In shallow bores, sufficient hydraulic horsepower usually is available to clean the bit efficiently.  Because drill string pressure losses increase with length a point will be reached where there is insufficient pressure for optimum bit cleaning. This length can be extended by carefully controlling the fluid properties. As stated the hydraulic horsepower starts at the stand pipe and finishes at the flow line out let at "0" psi having been lost due to the circulating system