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Geology/Well tops Vs drilling behavior

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Question
Dear sir ,
      How Well tops sometimes are also marked on the basis of the change of the drilling behavior, i.e, from fast drilling (ROP) in feet/meter  per minutes or hour  or to slow or slow to fast) ?



waqar
Baluchistan House
Islamabad

Answer
Dear Waqar,

I think I have answered in one of earlier asked question; this means that there are lithological characteristics which have effect on drilling, for example, a thick sequence of dense limestone will be expected to drill slower than a porous limestone (slower rate of penetration, ft per hr or minutes per ft) or vice versa. You need to review your well prognosis or well's geological drilling order of the formations and also their characteristics to understand the rock behavior.

Less dense rocks usually drill faster but claystone while  drilling  with freshwater mud tends to swallow and makes drilling slow due to   RPM slows down due to clay behavior. In Khairpur and Badin area of Sindh Pakistan, Khadro or Deccan traps (volcanic rocks) are slowest to drill whereas Rani Kot Formation in Badin Block the fastest to drill due to the unconsolidated nature of Rani Kot sandstone.

A sequence of a formation consisting of shale and sandstone, drilling expected to be faster in sandstone than shales.

However, this scenario has only been applicable if the drilling parameters are maintained consistently  (weight on bit, RPM and pump pressure and nature of bit and its age in hole too)if the drilling is inconsistent with so much fluctuation of the drilling parameters  Rate of Penetration help little to pick formation top.

Geology

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Syed Tariq Hasany

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