Now that relatively fewer elephant reservoirs are being discovered, and technology improves, directional drilling becomes more critical each year. If you imagine a well that has broken through the blowout preventer and is gushing, how can you cap it? This depends on the amount of underground pressure. In some instances, a second control well is drilled so that it intercepts the same point where the original wellbore meets the reservoir.
Once the new directional well is completed, it can be pumped with kill fluid. Mud and water are pumped in from a different angle, to get the first well under control and back to proper working order. A directional driller has a guide that has been created by the engineers and geologists. Directional drilling software receives input from multiple measurements while drilling MWD sensors in the drill bit, and at any branches or junctions. The drill bit has vibration sensors that can detect the type of formation being drilled at any point.
Collars can be added along the length of the well, sending back information to the surface regarding torque, weight and bending. From the surface, electromagnetic sensors can also track the progress of the drill bit. When all of the data from the drill bit, collars, motors and the surface equipment enter the control panel, a complete representation occurs. As well as being able to know what is going on, even a mile along the drill bore, drilling engineers can make adjustments in real-time that ensure that everything is going to plan.
This is especially relevant when unexpected things occur concerning geology or severe equipment stress. If you were to imagine the mechanics of directional drilling without seeing the technology, you might wonder how the drill could suddenly change direction.
Since the motor that turns the drill is at the surface, how can the drill string continue to rotate at degrees while going around a corner? We now have downhole drilling motors, that can drive the drill bit in a completely different direction to the usual degree downhole starting point. The rotational speed of the drill and the weight and stiffness of the drillstring can also be used to influence direction.
One of the original methods was jetting, a high-pressure nozzle shot water or drilling fluid from one edge to the drill bit, creating a weaker side in the formation. Another traditional method was to use a whipstock. A whipstock is a type of wedge that can redirect the drill. At the desired depth the drill is withdrawn to the surface, a whipstock gets put in place, then the drill goes back down and gets redirected by the whipstock.
Next, the drill is brought to the surface again, the whipstock pulled out and then drilling resumes and the bore changes path.
Drill bit sensors can tell the driller about external weight, and rotary speed that can also be used to influence the trajectory. Mud motors can also be used to change direction. The drillstring stops turning, and then there is plenty of time to use chosen directional techniques to reposition the bit to the desired trajectory. More about steerable mud motors in the next section. Specialised drillbits are used to improve performance and reduce the chance of failure.
Schlumberger supply directional PDC drill bits for both push- and point-the-bit rotary steerable systems. Horizontal Technology, Inc. Mud Motors. Downhole steerable mud motors get positioned near the drill bit, which has a bend in it. What happens is that at the correct depth the drillstring stops rotating, then drilling fluid is pumped through the mud motor so that the drill bit starts to turn just due to the force of the liquid.
This mud pressure pushes the drill bit into a different angle, and also begins to bite into the formation at a different angle to the central well trajectory. Once the sensors verify that the drill bit is pointing in the right direction, the drillstring starts to turn again. Directional drilling by using the mud motor means that often the drill pipe needs to be slid forward while the drill is motionless. A rotary steerable system can drill and steer at the same time.
This means that previously inaccessible formations can be accessed. Bottom Hole Assembly BHA configurations are often bent in shape so that they can make turns by using physical manipulations. The video further up the page clearly shows the bend in the drill pipe.
Multi-Shot cameras are fitted inside the drillstring. Then these images are sent to the surface control. These are a significant advance on the old fashioned ones previously mentioned. More time can be spent drilling, and less time removing the drill bit and conventional whipstock. Networked or wired pipe. The Intelliserve system from National Oilwell Varco is a broadband networked drilling string system. It can transmit data from the sensors back to the surface. These are most of the specialist directional equipment used.
Well integrity is perhaps the most crucial aspect of directional drilling. Drilling at deeper, or extended distances, and especially changing direction causes a number of additional engineering challenges and stresses on the equipment. For example, a downhole drilling motor will always be far smaller and less powerful than one connected to a robust drilling rig above ground.
The drillstring itself will be less stressed when going in a straight line, every degree of turn add extra friction and unbalanced pressure. It could mean that a brand new set of equipment is needed, and a new well might need to be drilled again in a slightly different direction. Maintaining hydraulic pressure, and wellbore cleaning is much more challenging with these types of wells. Computer simulation programmes are used to simulate the well plan.
This software is created based on previous knowledge, current seismic and magnetic data, supplemented with real-time data from the MWD instruments. Field developments, particularly offshore and in the Arctic, involve drilling an optimum number of wells from a single platform or artificial island.
Directional drilling has helped by greatly reducing the costs and environmental impact of this application. A well is directionally drilled to reach a producing zone that is otherwise inaccessible with normal vertical-drilling practices. The location of a producing formation dictates the remote rig location and directional-well profile. A very cost-effective way of delivering high production rates involves intersecting multiple targets with a single wellbore. There are certain cases in which the attitudes bed dips of the producing formations are such that the most economical approach is a directional well for a multiple completion.
This is also applicable to multiple production zones adjacent to a fault plane or beneath a salt dome. This technique may be employed either to drill around obstructions or to reposition the bottom of the wellbore for geological reasons. Drilling around obstructions, such as a lost string of pipe, is usually accomplished with a blind sidetrack. Oriented sidetrack is required if a certain direction is critical in locating an anticipated producing formation.
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