Page 11 - Offshore Well Construction

CHAPTER 7
Hydraulics and Hole Cleaning
Petroleum Extension-The University of Texas at Austin

INTRODUCTION
Hydraulics planning is part of the overall drilling optimization process.

It involves a calculated balance of the various components of the circulating system to maximize ROP and keep the bit
and hole clean while remaining within any constraints of the wellbore, surface, and downhole equipment.

CONSIDERATIONS FOR HYDRAULICS PLANNING
Maximizing ROP

Cuttings removal from the bottom of the hole is related to the fluid energy dissipated at the bit (bit hydraulic power).
It has been shown that bit hydraulic horsepower is optimized when the pressure differential (pressure drop) across
the bit is equal to two-thirds of the total system pressure (pump pressure). Maximizing hydraulic horsepower can be
used to increase penetration rate in medium-to-hard formations.

Hole Cleaning
In soft formations or deviated holes, hole cleaning is often the dominant factor. There is little point in maximizing the
ROP by selecting nozzles that optimize bit hydraulic horsepower or impact force, if the resulting flow rate is insufficient
to lift the cuttings out of the hole. In these instances, it is preferable to determine a suitable flow rate first and then
optimize the hydraulics.

Annulus Friction Pressure
In slim-hole or deep wells, the annulus friction pressure needs to be considered. Too high an annulus friction pressure
increases the equivalent circulating density (ECD) and can lead to lost circulation, differential sticking, or hole instability.

Erosion
Soft, unconsolidated formations are prone to erosion if the annulus velocity and, therefore, flow rate are too high or
the annulus clearance is small leading to the possibility of turbulent flow. In these instances, a reduction in flow rate
will be required to minimize erosion.

Lost Circulation
If heavy lost circulation is anticipated and large quantities of LCM could be pumped, it may be necessary to install
larger bit nozzles to minimize the risk of bit plugging.

FACTORS THAT AFFECT HYDRAULICS
The rig equipment, drill string and downhole tools, wellbore geometry, mud type, and properties are all factors that
can affect hydraulics.

Rig Equipment

The single biggest factor of the rig equipment is the pump pressure limitation and volume output of the mud pumps
in use. Increasing pump liner sizes increases the volume output but decreases the maximum allowable pump pressure.
Most high-pressure pipe work from the mud pumps to the kelly or top drive is rated at a pressure higher than the
pump rating.

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