In the early days of land drilling most wells were drilled vertically, straight down 
into the reservoir.An operating company usually hires a directional drilling service company to: 
provide expertise in planning the well; supply special tools; and to provide onsite 
assistance when operating the tools.In 
this chapter we will discuss: the applications of directional well drilling; the design 
of these wells; and the techniques used to drill a well with controlled deviation from 
the vertical. The next chapter will discuss the tools and techniques used to survey 
the position of the well (determine the three dimensional position of all points in the 
wellbore relative to the wellhead).Although these wells were considered to be vertical, they rarely 
were.