DRILLING HYDRAULICS COURSE

Drilling Hydraulics Course

Introduction

Why It Is Important To Understand Drilling Hydraulics?
The drilling hydraulics system serves many purposes in the well. Since it is centered around the drilling mud system, the purposes of mud and hydraulics are often common to each other.
The hydraulics system has many effects on the well. Therefore, the reasons for giving attention to hydraulics are abundant. The more common reasons are as follows:

• Control subsurface pressures
• Provide a buoyant effect to the drillstring and casing
• Minimize hole erosion due to the mud’s washing action during movement
• Remove cuttings from the well, clean the bit, and remove cuttings from below the bit
• Increase penetration rate
• Size surface equipment such as pumps
• Control surge pressures created by lowering pipe into the well
• Minimize wellbore pressure reductions from swabbing when pulling pipe from the well
• Evaluate pressure increases in the wellbore when circulating the mud.
• Maintain control of the well during kicks

Quite often, these effects are interrelated, which increases the difficulty in optimization.

Hydrostatic Pressure

The hydrostatic pressure of the drilling fluid is an essential feature in maintaining control of a well and preventing blowouts. It is defined, in a practical sense, as the static pressure of a column of fluid.

The hydrostatic pressure of a mud column is a function of the mud weight and the true vertical depth of the well. It is imperative that attention be given to the well depth so that the measured depth, or total depth, is not used inadvertently.

Since mud weights and well depths are often measured with different units, the equation constants will vary. Common forms of the hydrostatic pressure equation are as follows:

Where:
PH = hydrostatic pressure, psi
0.052 = constant, psi/lb/gal

Equivalent Mud Weight

Drilling operations often involve several fluid densities, pressures resulting from fluid circulation, and perhaps applied surface pressure during kick control operations.

The approach most widely used is to convert all pressures to an “equivalent mud weight” that would provide the same pressures in a static system with no surface pressure.

Another term commonly used to describe the equivalent mud weight concept is ECD, or equivalent circulating density. The ECD in drilling usually considers the hydrostatic pressures and the friction pressure resulting from fluid movement.

EMW = equivalent mud weight, Ib/gal.
19.23 = reciprocal of the 0.052 constant Ib/gal/psi.

Buoyancy

The drilling fluid provides a beneficial effect relative to drill string weight or hook load. When pipe is lowered into the well, the mud system will support, or buoy, some of the pipe weight. This effect is termed buoyancy, or buoyant forces. The buoyed weight of the drill string will be less than the in-air weight of the pipe.

Buoyant forces are a function of the volume and weight of the displaced fluid. Heavier muds have greater buoyant forces than low-density muds. The buoyed pipe weight can be calculated from below Eq.

Where:
BW = Buoyed weight
BF = Buoyancy factor

Pumping a drilling fluid requires overcoming frictional drag forces from fluid layers and solids particles. The pump pressure (Pp) can be described as the summation of the frictional forces in the circulation system: