The presently preferred power generator in oilfield exploration or extraction applications is the mud alternator because it is powerless on the surface, and therefore safe to handle, and only generates power once down hole and spinning in the mud flow. However, one of the major problems faced by both the provider and end-user of drilling equipment has been the control of impeller speed of down hole mud alternators.
All mud alternators effectively run open loop and the only way to govern their speed is either to modify the flow rate of mud pumped past the impeller or to change the impeller. These methods are not always successful in controlling alternator speed and, hence, power output. An alternator that rotates too quickly generates too much voltage for the down hole apparatus and consequently causes the apparatus to overheat. If an alternator causes the apparatus to overheat for too long, the apparatus is likely to fail resulting in a costly interruption of the drilling programme. A loss of down hole power results in a loss of communication with the drilling apparatus and the entire drill string must be pulled from the well to effect repairs. For an extended reach hole, the extraction, repair and replacement can take in excess of two days and a drilling engineer therefore avoids excessive rotation speeds for the alternator if at all possible.
To ensure the alternator is always operated within safe limits, the alternator RPM or output voltage is transmitted to the surface at frequent intervals in order that the drilling engineer may monitor the performance of the alternator and effect alternator speed corrections by adjusting the mud pumping-rate.
The preferred method for avoiding excessive rotation speed for the alternator is to select a suitable impeller for the alternator during the planning stage of a drilling contract. Mud flow rates can vary widely, for example from 150 to 800 United States gallons per minute. One of the planning tasks is to establish the minimum and maximum mud flow rates for the task in order that an optimum impeller can be selected. The difficulty with this method is that an incorrect impeller is sometimes fitted to the alternator. The drilling engineer is then faced with the decision of not using the drilling apparatus until the correct impeller has been fitted, or risking damage to the drilling apparatus due to excessive alternator power output.