Mobil Oil Company first used this term in the early 1980s for drilling directional wells in which the drilled horizontal reach (HR) attained at total depth (TD) exceeded the true vertical depth (TVD) by a factor greater than or equal to two. Extended-reach drilling (ERD) is particularly challenging for directional drilling and requires specialized planning to execute well construction.
Since the term was coined, the scope of extended-reach drilling has broadened and the definition, which is now more flexible, includes deep wells with horizontal distance-to-depth, or H:V, ratios less than two. The drilling industry’s ERD database classifies wells, with increasing degree of well construction complexity, into low-, medium-, extended- and very extended-reach wells. Construction complexity depends on many factors, including water depth (for offshore wells), rig capability, geologic constraints and overall TVD. For example, a vertical well with TVD greater than 7,620 m [25,000 ft] is considered an extended-reach well. Also, depending on the conditions, drilling a well in deep water or through salt may be classified as ERD even if the well’s horizontal extent is not more than twice its TVD.