Different indications of abnormal pore pressures will be observed in clastic and impermeable formations:
Indications of abnormal pressures in clastic formations (e.g. unconsolidated sands):
- Actual kick
- Drop in circulating pressure (Good).
- Increased drilling rate, called a "drilling break", (Good).
- Increased pit level (Good, but poor if drilling in "ballooning" formations).
- Changes in differential flow rate (Good).
- General hole conditions, e.g. when overpulls are experienced during trips or making connections, and when drilling torque increases. This may be caused by sloughing shale (Fair).
- Decreasing values of the calculated modified 'd' exponent (Good in certain situations).
- Connection gas (Poor).
- Increasing trip gas or gas-cut mud (Fair).
- Change in mud properties, e.g. salinity (Marginal).
- MWD with GR readings (Good).
- Decreasing trend in values of shale density with depth (Marginal; requires very accurate well site measurement).
- Abnormal temperature at flow line (Very poor).
The phenomena described above may be observed immediately prior to, or just after penetrating a higher pressured zone. It is therefore imperative that crews are properly instructed and trained to recognise such phenomena. However, all indicators which are based on flowline measurements are inevitably delayed and their usefulness is correspondingly limited.
Indications of abnormal pressures in impermeable formations
There is, in most cases, no real indication of the bit approaching abnormally pressured formations. Sometimes the presence of sulphur salts in the mud returns and H 2S from the cuttings may give an indication prior to penetrating higher pressured hydrocarbon bearing zones.
Use of logs to predict abnormal pore pressures
Interpretation of intermediate logs may also assist in the prediction of abnormal pore pressures.