Wednesday, 5 February 2020


Drilling Hydraulics Course 


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:

PH = 0.052 x (mud weight, Ib/gal) x (depth, ft)

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

PH = 0.00695 x (mud weight, Ib/cu ft) x (depth, ft)

PH = 9.81 x (mud weight, g/cm3) x (depth, m)

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 usually considers the hydrostatic pressures and the friction pressure resulting from fluid movement.
EMW= total pressures X 19.23 ÷ true vertical depth
EMW = equivalent mud weight, Ib/gal.
19.23 = reciprocal of the 0.052 constant Ib/gal/psi.


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.

BW = (BF) x (in-air weight)

BW = Buoyed weight
BF = Buoyancy factor
BF = 1 - (mud weight / 65.5)
The constant of 65.5 is the density of a gallon of steel 


Flow Regimes are the behavior of the fluid while flowing in a well. The most common regimes are laminar, turbulent, and transitional. Unfortunately, it is impossible to clearly define each type in the well. As an example, mud flow may be predominantly laminar, although the flow near the pipe waIls during pipe rotation may be turbulent.

  1. Identify different types of drilling fluids flow regimes
  2. Understand flow regimes turbulence criteria and its relation with Reynolds Number
  3. How to identify flow regimes usinf z factor method
  4. Learn why we need mathematical models
  5. Understand the differences between Newtonian model, Bingham model & Power law model assumptions and equations
  6. How to calculate PV, YP for drilling mud


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:

PP = PDS + PB + PA

  1. Calculating the pressure drop in surface equipment.
  2. Calculating the pressure drop in Annulus.
  3. Calculating the pressure drop in drill string.
For many years, engineers have known that hydraulics play an important role in cleaning the face of the formation so that a bit can drill faster. This first became evident when larger pumps increased the drill rate because more mud was being pumped through the large throat of the regular circulation bit. Jet bits were developed to improve on the jetting action of the high mud velocities at the bit. In addition, features such as extended nozzles and varying the number of nozzles were shown to affect drill rate.

Chapter.4: Surging & Swabbing Pressure Calculations

Surge pressures describe pressure changes in the annulus resulting from pipe movement. As the drill pipe is pulled from the well, mud flows down the annulus to fill the void left by the pipe. As the drill pipe is lowered into the well, mud is forced out of the flow line. Pressure changes caused by lowering the pipe into the well are called surge pressures and are generally considered to be added to the hydrostatic pressure.


  1. Drilling Surge /Swab Pressure Definition.
  2. What Is The Risks / Effects For Increasing Surge / Swab Pressures While drilling?
  3. Is It Difficult To Calculate Swab / Surge Pressure ?
  4. How Could Burkhardt build his model to calculate Swab / Surge Pressure while drilling?
  5. Example on how to run Surge / Swab Pressure Calculations.
  6. Notes About Surging & Swabbing Pressures Calculations.
  7. What Is The Maximum Allowable surging Pressure & Maximum Allowable Swabbing Pressure.
  8. Example On How To Calculate Maximum running or pulling velocity in drilling oilfield.

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