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Geology/well site question


   1:how we pick or set or judge the casing point on the biases on pressure's confused in these tell me please.??
2: what is drilling exponent tell me in easy wording ?
3:what is difference between leak off test & formation integrity test in easy word?
internee geologist

Dear Waqar Ahmed,

Thanks for sending me your question.

Casing is very important for any drilling well as it provides the safe pathway for bit to drill new formation. Therefore, it's essential that seat of the casing should be anchored in a formation which is strong enough to hold the pressure of casing and circulation pressure while cementing. For example foundation of any house are set on the cohesive base which can hold the weight of the building without failure, ideally on top of sandstone or limestone. Same principal can be applied for the well casing. However, if ideal rock surface is not available or operational urgency demands an early casing then casing shoe can be set in any relatively stronger rocks.

D exponent is the extrapolation of drilling parameters to determine any deviation from the normal pressure trend line. The objective of this exercise is to observe over pressure situation while drilling to avoid major accident such as high pressure kick or blow out which could be disastrous and fatal. Drilling parametrs are used  for D exponent are, rate of penetration (ROP) Weight on bit (WOB), Rotary speed (RPM) and bit size.
The general formula of D exponent is as follows;

d = log (R 60N) log (12W 1000D)Where; R = penetration rate in feet per hour, d = exponent in drilling equation, dimensionless, N = rotary speed in rpm , W = weight on bit in kilo pound
D = bit size in inch

Any abnormal pressure change will increase or decrease calculated D exponent from the normal trend line and action to be taken to control the potential problem.

Leak of test is a method which helps to determine the formation pressure limit where it actually breaks. Drillers run the drillpipe and circulate in open hole below the newly set casing and increase pressure to the point when formation breaks and mud starts to flow inside the formation. In Formation Integrity test (FIT)pressure increases to certain know level without actually breaking the formation. For the safety reason drillers usually do not perform the Leak off test but run FIT and use the pressure data to optimize mud weight for drilling the new section.

Hope this answer helps.
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Syed Tariq Hasany


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