Overpressure
Overpressure Under normal condition fluid pressure of a permeable rock at depth will be hydrostatic i.e. in equilibrium with the weight of a water column extending from the permeable rock level to surface. This pressure is termed as normal pressure. Hydrostatic pressure increases in a normal gradient with increasing burial depth. In certain cases pressure higher than normal hydrostatic pressure is generated. This is called overpressure. In other word, fluid pressure in the subsurface rock that exceeds the hydrostatic pressure is known as overpressure.
In Bangladesh overpressure was encountered in all the deep wells drilled in the eastern part of the country i.e; geosynclinal part of the basin. The table below lists well names and depths to top overpressure in the wells in Bangladesh. However, wells drilled on the western part i.e. stable platform (Bogra-Rajshahi) did not encounter any overpressure zone. The occurrence of overpressure is related to the thick shale sequence of Miocene Bhuban formation deposited in a deltaic environment.
Table Some of the overpressured wells in Bangladesh.
| Name of well | Year of drilling | Depth to top of overpressure (meter) | Total overpressure depth (meter) |
| Patiya-1 | 1952 | 2575 | 3102 |
| Sylhet-2 | 1956 | 2700 | 2820 |
| Chattak-1 | 1959 | 1980 | 2135 |
| Lalmai-2 | 1960 | 4052 | 4119 |
| Rashidpur-2 | 1960 | 4120 | 4593 |
| Kailashtila-1 | 1961 | 3560 | 4141 |
| Titas-1 | 1962 | 3650 | 3759 |
| Semutang-1 | 1967 | 3200 | 4083 |
| Muladi-1 | 1975 | 4500 | 4732 |
| Begumganj-1 | 1976 | 3300 | 3656 |
| Beanibazar-1 | 1981 | 4016 | 4097 |
| Sitakund-5 | 1983 | 1620 | 4005 |
| Fenchuganj-2 | 1985 | 3124 | 4977 |
| Sitapahar-1 | 1988 | 1100 | 1560 |
| Patharia-5 | 1989 | 460 | 3346 |
| Sangu | 1999 | 3420 | - |
Source Petrobangla.
Comparison of the sedimentary history of thick Tertiary deltaic sequence of Bangladesh with those of other parts of the world reveals that the most likely cause of overpressure in Bengal Basin is compaction disequilibrium due to rapid loading. Generation of hydrocarbon gas in deep subsurface sediments may also cause overpressure. Shale samples from drilled wells show indication of illitization and smectite clay dehydration. Clay dehydration and dewatering may also contribute to the generation of overpressure in Bangladesh.'
Overpressure zone may host large hydrocarbon accumulation within itself. Moreover it provides mechanism of generation and migration of gas in the normal pressure sediment lying above. However, overpressure zone is a potential hazard for exploration drillers. In case of drilling exploratory wells overpressured zone may account for blow out of rigs taking even lives of the drillers. In order to run a safe exploration program it is mandatory to understand the nature of overpressure. [Mir Moinul Huq and Sanzida Murshed]
