Deviating the Wellbore by Positive Displacement Motor (Directional Drilling)

A positive displacement motor (PDM) is one of the most popular tool for drilling a directional well. It works by boring downwards and pumping mud through the motor itself. As shown in figure 1, the bottom section of the motor has an adjustable bend housing.

Figure 1 – Positive Displacement Motor (Courtesy of Schlumberger)

Before the motor is run into the hole, a set-up process needs to be carried out

  1. The bend will be adjusted according to the directional performance that the motor needs to achieve. This bend is only very slight, usually being under 2°.
  1. The motor is hooked up to navigational tools, which are then calibrated, in order for the driller to see where the bend is pointing when drilling. These tools are known as measurement while drilling, or MWD, and are described in detail later in this document.
  1. The other parts of the system will be adjusted to account for the required directional performance- the severity of this will depend on the drill design.

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Water Phase Salinity of Oil Based Mud

Water phase salinity is a factor showing the activity level of salt in oil based mud. In order to control the water phase salinity, salt is added into the drilling fluid. The salt added into the system will be dissolved by water in the mud; therefore, the chloride content will increase.

By increasing the chloride concentration (adding salt), the activity level in the mud will decrease. Salt is added in order to create an activity level which is equal to or less than formation water. Therefore, the water phase in the mud will not move into formation and cause a clay swelling issue. Practically, calcium chloride (CaCl2) or sodium chloride (NaCl) is the chemical to be used.

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Solid Content in Drill Mud

Solid content is a fraction of the total solid in drilling mud, and it always increases while drilling ahead because of drilling solid (cuttings), mud chemical additives and weighting material. Solid content refers to soluble and insoluble solid content in the drilling fluid system.


There are three types of solid contents as listed below;

Soluble material such as salt
Insoluble high gravity solid (HGS) such as weighting agents (barite, calcium carbonate, hematite, etc.)
Insoluble low gravity solid (LGS) or drilled solid such as solids particles from cuttings

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Mud Filter Cake

Mud filter cake is a layer formed by solid particles in drilling mud against porous zones due to differential pressure between hydrostatic pressure and formation pressure.  For the drilling operation, it is preferred to have a filter cake that is impermeable and thin. Practically, the filter cake should be less than or equal to 1/16 inch. If drilling fluid is not in a good shape, which results in a thick filter cake in the wellbore, it will lead to a stuck pipe situation and high torque/drag.

Mud filter cake

Mud filter cake

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Determine Quantity of Material Used in Mixing a Simple Drilling Mud

This is a simple example to demonstrate how to determine quantity of material required in a simple mixing system. Let’s learn from the example below.

The objective is to build mud which has a weight of 11.0 ppg with 1,000 bbl.

The mud recipe is shown below;

  • 30 ppb bentonite
  • 5 ppb CMC polymer
  • 5 ppb caustic soda (NaOH)
  • 25 ppb Na2CO3
  • Weighting Material is barite.

The following are important data of water and chemical used for this calculation.

Base fluid = fresh water

Specific gravity of water = 1

Specific gravity of barite = 4.2

Specific gravity of bentonite = 2.4

Specific gravity of CMC polymer = 2.4

Fresh water weight = 8.34 ppg

NaOH and Na3CO3 have negligible volume because it is dissolvable.

What is quantity of material required to meet the mud specification?

What is the final volume? Continue reading

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