HENRY WOODS & ARTHUR LUBINSKI'S THEORY
Henry Woods and Arthur Lubinski stated in 1954 that the size of drill collar just above the bit is the limiting factor for the lateral movement of bit causing hole deviation.
They gave a formula to calculate the Minimum Effective Hole Diameter (MEHD) for lowering the casing. The formula is:
MEHD = (Bit Size + Drill Collar OD) / 2
ROBERT S. HOTCH'S THEORY
Later, Robert S. Hotch came up with a theory that while drilling with an unstabilized bit an abrupt change of hole angle can occur if hard ledges are encountered.
With reference to his theory, he suggested that the minimum permissible drill collar OD should always be greater than twice the casing coupling diameter to be run minus the bit size.
i.e. Minimum Drill Collar OD = 2 x Casing Coupling OD – Bit size
H.E. TRIECHER'S THEORY
At the Same time, H.E. Triecher pointed out that the lateral movement of an unstabilized bit in non dipping formation tends to cut a spiral hole. The spiralling will be more severe in soft formations where penetration rate is high and this will produce a hole of reduced drift diameter.