I got some questions from Mike R Hogolan regarding mud motor concerns. The questions are very interesting and I would like to share some answers to you all as well.
Why is it more difficult to steer a motor the deeper section of wellbore?
It is harder to steer the motor when well is deeper because the friction exerted from formation to drillstring in open hole section increases. Motor cannot be effectively used to drilled deeper along all well path because high friction force exerted on BHA, higher temperature as well deeper can cause rotor, made of synthetic rubber, failure.
Why is the most effective steering by using the pump pressure gauge rather than the weight indicator?
Driller will use the pump pressure gauge as opposed to the weight indicator because WOB is not accurate while steering. High friction force between drillstring and formation is created when steering. If there are consistent circulating mud properties, flow rate and formation characteristics should be within the normal motor operating range, an increase or decrease in weight on bit will result in a directly proportional increase or decrease in pump pressure.
What is meant by stalling a motor?
Stalling motor means that steerable motor stalls at bottom hole (can not rotate) because of higher WOB, harder formation, not enough torque to turn the bit, etc. When motor stalling, stand pipe pressure increases significantly and ROP significantly drops.
What are indicators a motor is wearing out?
Indicators demonstrate a motor worn out as follows:
• Lower ROP without any changes of parameter on surface
• Difficult to control well direction as per designed well trajectory
• Increase in pump pressure
• Easily motor stall
Referencer book: 
Drilling Engineer Books
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