Mud Motors 101

Need to buy a mud motor but not sure what size is required for your job? Or what about the common mistakes contractors make when using this HDD tool? Steve Cornwall with Sharewell LP takes on the subject of what contractors need to think about when selecting the proper mud motor for a job and how to get the maximum use out of it.

1) What is the tooling selection procedure for drilling a hole in rock?

There are several questions that need to be addressed before we can move forward on this topic: How hard is the rock? Do we know what type of rock it is? What rig are you considering to use? How much mud pump volume (gpm) does your pump produce? How long is the bore? These are the opening questions that if answers are known, typically produce additional questions. If the rock has been tested to be 15,000 psi, I can move to the next question, but often all that is available is blow counts from a core sample run in the area. If that’s the case, I need to make some educated assumptions and move on. Looking at the type of rock gives me an indication of the potential wear on the motor — for example, shale is far more forgiving than sandstone. My customer now tells me he plans on using an 80,000-lb drill with a 250-gpm mud pump and the bore is 800 ft long. I now have enough information to make a recommendation for a mud motor for this project. Rock hardness is 15,000 psi in strength. The formation has been determined to be shale and limestone. We have an 80,000-lb drill with 250-gpm pump. Based on this information I can recommend a 4-3/4-in. mud motor.

This mud motor, as all manufactured motors do, has a specific flow range it operates in. For this size, it is 150 to 250 gpm. We try to optimize the flow rate to the motor to utilize its optimum torque output and rotation at the bit box. This should produce the best penetration rates and steerability in the rock — two things we always want to maximize (within the limits of hole cleaning ability). Other considerations are drill pipe outer diameter (OD). We hope to have the pipe similar in OD as the motor, which should mean similar mechanical limitations.

Now we can look at a bit size. As with the recommended flow range, there is a recommended bit size (OD) range. For a 4-3/4-in. mud motor, it is 6 to 7-7/8 in. Most of the time we like to see a bit in the 6-½- to 6-¾-in. size used. This will give the motor enough annulus area to properly reside in and also allow it to have the ability to properly steer. Oversized bits, for example an 8-½-in. bit on a 4-¾-in. mud motor, will not allow the motor to steer. The motor can’t contact the inner diameter of the bore hole, which eliminates the fulcrum effect of the bend in the mud motor.

2) What is the life expectancy of a mud motor?

If run within the operations manual specifications, the mud motor should perform within the published parameters up to 100 to 125 hours of drilling. At that time, it is recommended to be returned to the supplier for service and inspection. Typically at that time, we see the bearings out of tolerance and needing replaced. Obviously this is what we like to see but in our industry we face situations daily that can affect operations on the jobsite. We see many motors returned with 150-plus hours on them and usually these additional hours show on the repair bill. The internal workings of the transmission section are usually affected by the added hours, which will increase repair cost.

The most expensive replacement on the mud motor is the power section. This is comprised of the stator and rotor. The rotor is typically chrome-plated steel and has one less lobe than the stator, which its inner diameter is made of a highly engineered rubber compound. The drilling mud is pumped through the rotor-stator assembly, creating torque and rotation at the drill bit. This means the drilling mud needs to be clean of solids and sand. If it is not clean, it will prematurely wear the stator and the rotor, increasing the fit between the two, which dramatically reduces the pressure and power this section is designed to produce.
We once had a customer who bought a 6-¾-in. mud motor and after a month, I called to check to see if it needed service. I was told it was doing great and was in the hole on its third bore. I reminded the customer that service of the motor was critical to the overall life of the tool, but apparently it fell on deaf hears. Six months later, the customer called and said he was sending it in for service. I asked if he knew how many hours they had on it — I was told in excess of 750! Needless to say, the tool was retired and the rotor was made into a front bumper for one of their work trucks.

3) How do you take care of a mud motor to extend its life?

Service the motor every 100 to 125 hours of use and the overall life of the motor will be extended. Minimize motor stalls — this is when the motor stops turning internally while the mud pump is still pumping mud through it. After a motor is used on a job and doesn’t require service, we recommend that fresh water be pumped through it to flush out the drilling mud which will dry out and can cause damage when the motor is reused.

4) Can mud motors be used on any rig?

As long as a rig has the required gpm and pressure rating — I can answer a qualified yes. Let’s assume a contractor has a rig with a pump rated at 150 gpm by its manufacturer. This rating is based on pumping water with a new pump. So realistically, we have a pump that will probably pump mud at 100 to 120 gpm. This is enough flow to power a 3-½-in. mud motor in its mid flow range requirements. At that flow rate, we can check the motor’s performance curve and find we have around 550 ft-lbs of torque at 150 rpm. This will work well enough in rock up to 15,000 psi. But if the project is in rock that is 25,000 psi, it will dramatically reduce the penetration rate, lowering the ROP to a point that it becomes economically not feasible. In this case, the contractor would need a larger rig with a bigger mud pump to power a larger mud motor to overcome the compressive strength of the rock.

Another issue to consider when using a mud motor is the contractor’s ability to recycle and clean his drilling mud. If you are pumping 100 gpm, you will drain a 1,000 gal mud mixing system in 10 minutes, an obvious reason to recycle drilling mud.

5) What mistakes do drillers make with mud motors?

Probably the most common mistake is what is done when a motor stalls out. A stall-out occurs when the bit stops turning while pumping. We have three mud pump pressure readings we monitor while using a mud motor. The first is the off bottom pump pressure reading; the second is the on bottom pump pressure reading; the third is the stall pressure reading.

Let’s say our contractor has an off bottom pressure reading of 500 psi and an on bottom reading of 750 psi. While drilling, he applies too much weight on the bit, causing it to stop turning. The pressure now reads 825 psi. At this point most drillers pull the motor off bottom and resume drilling. This method will send reactive torque up the drill string and could cause the motor or drill pipe to unscrew. The correct action to take when a motor stalls is to turn the pump off, and then pull the motor off bottom. Then, restart the pump and advance to bottom.

Another area of concern is the fact that you are doing your locating behind the mud motor. You have your drill bit, mud motor and crossover sub, then your steering system. It doesn’t matter if you are using a wireline steering tool or a walkover system — if the rig is using a 4-¾-in. mud motor, your steering system is 20 ft behind the drill bit. This usually causes the driller to overreact and drill an S-curved pilot hole. He sees he is heading right of centerline so he steers left, then he sees he is heading left so he steers right so on and so on. This situation will correct itself as the operator becomes used to drilling with a mud motor.

Sharon M. Bueno is managing editor of Trenchless Technology.
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