Extreme conditions can cause wellbore instability, well control problems and ultimately loss of the well.High temperature: require the development of downhole tools capable of operating in that environmentHigh pressure reservoir present unique challenges in well controlSour gas such as hydrogen sulfide is a toxic gas. It can be a series threat to people’s life and environmental pollution. It is also a corrosive gas and can cause corrosion damage to metal equipment and tools. For example, high-strength pipe becomes embrittled and damage immediately in the presence of H2S.


After cementing operation, the drilling fluid has high gel strength and since the gel strength is proportional to the pressure reduction. Therefore, pipe withdrawal after cementing operation could cause big pressure reduction and can cause a kick.


Magnetometer is used to measure the Earth magnetic field. Mostly oil occurs in sedimentary rocks which are non-magnetic, therefore oil can be determined in rock layer that has low magnetic reading.


c. Coiled Tubing Drilling (CTD).