What is the tool choice for drilling a hole in the rock?

Some details needed to be answered before we can progress on this subject:

How tough is the rock?
Do we know what kind of rock it is?
What rig are you thinking about to use?
What does it cost?
What Mud Pump Volume (GPM) does your pump produce?
For how long is the bore?

These are the opening matters that if answers are known, usually produce additional concerns. If the rock has been evaluated to be 15,000 psi, we can shift to the next question, however, normally all that is available is blow counts from a core sample run in the location. If that holds true, I need to make some educated presumptions and proceed. Taking a look at the type of rock gives me an indication of the potential damage to the motor– for instance, shale is much more flexible than sandstone. Some client informs me he intends on using an 80,000 lbs drill with a 250 GPM mud pump and the bore is 800 ft long. I now have enough information to make a suggestion for a mud motor for this project. Rock solidity is 15,000 psi in strength. The formation has been identified to be limestone and shale. We have an 80,000-lb drill with 250 GPM pump. Based on this info I can suggest a 4-3/4-in. Mud Motor.

This positive displacement motors, as all made engines do, has a particular flow variety it runs in. For this size, it is 150 – 250 GPM. We aim to optimize the flow rate to the motor to utilize its optimal torque output and rotation at the bit box. This must produce the best penetration rates and steerability in the rock– two things we always want to take full advantage of (within the limitations of hole cleaning capability). Other factors to consider are the drill pipe external size (OD). We wish to have the pipe similar in OD as the motor, which should suggest similar mechanical restrictions.

Let’s take a look at the bit size. As with the proposed circulation range, there is recommended bit size (OD) range for every use. For a 4-3/4 in mud motor, it is 6 to 7-7/8 in. Sometimes we want to see a bit in the 6- 1/2- to 6- 3/4-in. Size used. This will give the motor enough annulus space to effectively stay in and also enable it to have the capability to steer properly. Oversized bits, for instance, an 8- 1/2-in a bit on a 4- 3/4-in Mud motor, will not enable the motor to guide. The motor cannot call the inner diameter of the bore hole, which removes the fulcrum result of the bend in the mud motor.

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