T - The distinction you hint at is rigs drilling vs rigs AVAILABLE. Here's some thoughts along that line from link below:
"With little demand for older, mechanical rigs that cannot drill horizontal wells and don’t have walking systems, contractors have retired these rigs in large numbers over the past two years, Mr Spears said. He pointed to the 2014 National Oilwell Varco (NOV) rig census, which showed that there were 3,250 total available rigs in the US. The same census two years later counted only 2,100 available rigs, a drop of more than one-third.
“From this point forward, we’ll probably see the number of available rigs continue to decline, but I’d guess it would only fall about 5% a year going forward,” he said. Of the rigs that have been retired, 70% to 80% of them are mechanical rigs that were built for drilling vertical wells. “I think most of the retirements that we’ll see in the next few years will also be these older rigs that just don’t have much demand in the marketplace,” Mr Spears added."
http://www.drillingcontractor.org/us-on ... ttom-42560And you can track the situation yourself at NOV:
http://www.drillingcontractor.org/dc-ar ... ember-2017And for the nature of "mechanical rigs": the entire drill string rotates which causes the drill bit on the end of it to rotate and cut the rock. Horizontal wells are not drilled with the drill string rotating. On the bottom of the drill string is a "mud motor" which has the drill bit attached to it. As the drilling mud is pumped down the non-rotating drill pipe it rotates the mud motor which rotates the bit that cuts the rock. Rotating the drill pipe in a hz hole is difficult and dangerous. The rigs drilling with mud motors enquire more powerful pumps and other equipment.