Positive-Displacement Motors (PDM) make use of a power generation section which is made up of a rotor/stator combination. In order to move a rotor part, a PDM requires hydraulic power from drilling fluid flowing through the power generation part. With a PDM, the stator and rotor work in tandem in the same way that gears do. The stator acts as the outer gear, and is made from a moulded elastomer featuring at least two lobes. The OD of the elastomer is protected by a secure metal casing. The rotor is positioned within the stator, and acts as an internal gear. This rotor is made of metal, and will have one less gear or lobe than the stator. Because of this difference, a cavity is created which is filled with drilling fluid when the PDM is downhole. This cavity acts as a wedge when it is put under pressure, and because the drilling fluid itself can’t be compressed, the force applied to the top of the wedge causes the rotor to move.
Figure 1 – Mud Motor
Not only are reamers important for directional drilling, but they can also be useful in straight hole applications. Reaming assemblies can straighten out and smooth over crooked holes, restore undergauge holes to gauge, and get rid of any irregularities or keyseats. They also help to prevent excessive hole curvature in short intervals, which may be experienced when entering and exiting a section of hole which forms a sharp curve. Finally, reamers can reduce the rotational torque in a wellbore, and may therefore be used as a substitute for a conventional string or near-bit stabilizer.
Reamers are made by almost all major downhole tool manufacturers, and have the same core features: sealed or open (mud lubricated) bearings, cutter types – either “nobbly” or “smooth”, and either one (so called “3-point”) or two (“6-point”) sets of cutters in a tool.
Reamer (Courtesy of NOV, 2017)
This article demonstrates how to design the well trajectory in J-shape from the surface location to the required target depth (TD).
- The surface location coordinate of Well-A is 6,543,065.00N 416,695.00E and the target is located at 6,542,213.00N 415,456.00E and the UTM zone is 31N.
- Kick off depth = 4,200’MD/4,200’TVD
- Planned build up rate = 2 degree/100 ft
- Well profile = J-profile (build and hold)
The surface location coordinate of Well-A is 6,543,065.00N 416,695.00E and the target is located at 6,542,213.00N 415,456.00E and the UTM zone is 31N. Therefore, the surface and the target for Well-A can be illustrated is Figure 1. Continue reading
Nowadays, many wells required complex well trajectory plans in order to reach reservoir sections and some of complicated well paths (Figure 1) cannot be drilled with either rotary drilling assemblies or mud motors. In order to achieve the drilling goal, rotary steerable tools are usually selected.
Figure 1 – Complex Well Paths
While the precise mechanics might vary, each rotary steerable tool uses much the same approach. Running the rotary steerable immediately above the bit serves as a sort of replacement for a near bit stab (NB stab). Most tools use three blades close to the drill bit, which act as stablizers and move in and out. While the tool turns, the blade which is turning in the opposite direction pushes against the side of the hole, giving the necessary side force to create a curved hole while drilling.
When using a steerable motor, the adjustment of the well path a series of slide drilling and rotary drilling doesn’t give a clean smooth edge, but rather creates a hole with multiple sharp edges, and straight sections between them. A rotary steerable tool, on the other hand, does give a smooth curved hole. This makes the wellbore more stable, and less resistant when tripping in and out of the hole. With higher inclinations, a smooth curve makes for an easier job of running casing or logging tools. Continue reading
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
- 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°.
- 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.
- 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.