Abnormal pressure is a pressure higher than normal pressure. The techniques which are used to predict abnormal pressure (before drilling), detect (whilst drilling) and confirm (after drilling) over pressures are summarized in table below.
Predictive Techniques Of abnormal Pressure
The predictive techniques are based on measurements that can be made at surface, such a geophysical measurements, or by analysing data from wells that have been drilled in nearby locations (offset wells). Geophysical measurements are generally used to identify geological conditions which might indicate the potential for abnormal pressure over pressures such as salt domes which may have associated overpressured zones. Seismic data has been used successfully to identify transition zones and fluid content such as the presence of gas. Offset well histories may contain information on mud weights used, problems with stuck pipe, lost circulation or kicks. Any wireline logs or mud logging information is also valuable when attempting to predict abnormal pressure or over pressures.
Detection Techniques Of Abnormal Pressure
Detection techniques are used whilst drilling the well. They are basically used to detect an increase in pressure in the transition zone. They are based on three forms of data:
• Drilling parameters – observing drilling parameters (e.g.ROP) and applying empirical equations to produce a term which is dependent on pore pressure.
• Drilling mud – monitoring the effect of an over pressured zone on the mud (e.g. in temperature, influx of oil or gas).
• Drilled cuttings – examining cuttings, trying to identify cuttings from the sealing zone.
Detection Abnormal Pressure Based on Drilling Parameters
The theory behind using drilling parameters to detect over pressured zones is based on the fact that:
• Compaction of formations increases with depth. ROP will therefore, all other things being constant, decrease with depth
• In the transition zone the rock will be more porous (less compacted) than that in a normally compacted formation and this will result in an increase in ROP. Also, as drilling proceeds, the differential pressure between the mud hydrostatic and formation pore pressure in the transition zone will reduce, resulting in a much greater ROP.
The use of the ROP to detect transition and therefore overpressured zones is a simple concept, but difficult to apply in practice. This is due to the fact that many factors affect the ROP, apart from formation pressure (e.g. rotary speed and WOB). Since these other effects cannot be held constant, they must be considered so that a direct relationship between ROP and formation pressure can be established. This is achieved by applying empirical equations to produce a “normalised” ROP, which can then be used as a detection tool.
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