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
Rotary drilling assemblies can typically control a directional of a well by having proper stabilizer placement. With this kind of drilling assembly, only inclination can be controlled and a well cannot be directionally oriented to required direction. In this article, it briefly describe how stabilizer placement can affect the well direction.
Rotary Build Assembly (Fulcrum Assembly)
When a drill collar is supported at both ends but not held vertically, its own weight causes it to sag in the middle. This phenomenon is used in rotary drilling assemblies to create the necessary side force at the bit to alter the angle (Figure 1).
Figure 1 – A build rotary assembly
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.
To deviate a well from a vertical path, and get it to follow an intended well trajectory , it is necessary to put some side force onto the bit. The amount of this, as well as its direction, are vital in order to keep the bit to its intended path. Other factors will also have an influence, including the hardness of the rock which is being drilled, as well as bedding plane angles. There are numerous different ways of developing a controlled side force on the bit. Two of the earliest developed methods are whipstock and jetting which will be discussed in this article.
Jetting as the Directional Drilling Tool
A tricone drill bit possesses three drilling cones, with a nozzle in between each one. Should a large nozzle be set into a single nozzle pocket, and two smaller nozzles used alongside it, then the majority of the mud flow would pass through the larger nozzle. Drilling fluid will be ejected from the drill bit with a significant amount of force, and so long as the formation is not overly hard, will erode the rock in its path. As the large nozzle directs the majority of the flow to a single point, a pocket will be carved into the rock in this direction. The well may be deviated simply by aligning the bit in the necessary direction, and then circulating without rotation.
Once between 5-6 feet have been washed away, the bit is then rotated, and drilling continues as normal. This process can be repeated continuously until an angle of around 12° is produced, or until rock is reached which is too solid to jet through. Figure 1 to 3 illustrate a jetting operation by a rotary drilling assembly, which is used to allow the well to keep building an angle while drilling and rotating take place.
Figure 1 – Jet a well to desired direction
The well planning process starts from geologists and reservoir engineers who decide the best place for the wellbore. They may only need to determine a single target, which will often be a tolerance of about 330 ft (100 m) around a certain target point. In this case, the angle at which the well enters the target can have various degree of deviation from the plan since a plan requires to hit only one target. On the other hand, it might be necessary for the well to penetrate multiple targets, with the final target being increasingly complex. This requires what is known as “geosteering”, a process which will be discussed later in the directional drilling series. The drilling engineer therefore needs to examine potential surface locations (if more than one is available) and design a well path which meets all necessary target requirements at the lowest possible cost. Cost can be minimized most effectively when there is a certain degree of flexibility when it comes to the surface location.