Used for drilling vertical holes. A vertical well is always the preferred, easiest and cheapest option. No petroleum engineer or geologist is going to recommend horizonal wells when and where vertical wells will efficiently drain the reservoir. even with greater efficiency, the additional cost of a horizontal well may not–even probably will not, warrant the extra cost. In addition, there is more engineering risk in horizontal wells. All horizontal (directional wells) start out as vertical wells, theoretically one coud back out of the reservoir and kick off in a new direction.