PREDICTION AND DETECTION OF Abnormal PRESSuRES
The techniques which are used to predict (before drilling), detect (whilst drilling) and confirm (after drilling) overpressures are summarised in Table 1.
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 overpressures 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 mudlogging information is also valuable when attempting to predict overpressures.
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 overpressured 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.
|
Source of Data |
Parameters |
Time of Recording |
|
Geophysical methods |
Formation velocity (Seismic) Gravity Magnetics Electrical prospecting Methods |
Prior to spudding well |
|
Drilling Mud |
Gas Content Flowline Mudweight "kicks" Flowline Temperature Chlorine variation Drillpipe pressure Pit volume Flowrate Hole Fillup |
While drilling |
|
Drilling parameters |
Drilling rate d. dc exponent Drilling rate equations Torque Drag Drilling |
While drilling Delayed by the time required for mud return |
|
Drill Cuttings |
Shale cuttings Bulk density Shale factor Electrical resistivity Volume Shape and Size Novel geochemical, physical techniques |
While drilling Delayed by time required for sample return |
|
Well Logging |
Electrical survey Resistivity Conductivity Shale formation factor Salinity variations Interval transit time bulk density hydrogen index Thermal neutron cam capture cross section Nuclear Magnetic Resonance Downhole gravity data |
After drilling |
|
Direct Pressure Measuring Devices |
Pressure bombs Drill stem test Wire line formation test |
When well is tested or completed |
|
Table 1 Methods for predicting and detecting abnormal pressures |