sentences:  The well is therefore drilled in sections, with each section of the well being sealed off by lining the inside of the borehole with steel pipe, known as casing, and filling the annular space (or at least the lower portion) between this casing string and the borehole wall with cement.Then drilling commences on the subsequent hole section, necessarily with a smaller bit diameter that will pass through the newly installed casing.A "liner" is a casing string that does not extend to the surface, being hung instead from a liner hanger set inside of the previous casing string but usually within about of its bottom.