Overview of overpressure in Bengal Basin,India |
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Authors: | D K Roy G K Ray A K Biswas |
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Institution: | 1.MBA Basin,ONGC,Kolkata,India |
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Abstract: | Abnormally high formation pressures are encountered worldwide, ranging in geological age from Cenozoic to Paleozoic, within
a depth range of few hundred meters to as deep as six thousand meters while carrying out exploratory drilling by E and P companies.
Several causes can increase formation fluid pressure i.e. rapid loading of sediments results compaction disequilibrium, thermal
expansion of fluids, compression and/or upliftment of strata by tectonic forces, generation of oil and gas from organic matter
and its volume expansion due to high thermal stress within the restricted pore volume in subsurface condition. Few global
examples on overpressure occurrences have been compiled in the paper with special reference to Bengal Basin. Emphasis has
been given on methodology and interpretation on abnormal pressure detection in Bengal Basin with a compiled data package on
generated curves (Geologs), charts, tables in a systematic way to understand the depth/stratigraphic horizons proved/interpreted
as proved or likely to be within transition and overpressure regime. The integrated analysis indicates that the wells drilled
in the east of Eocene hinge zone in the onshore and offshore parts of Bengal Basin have penetrated overpressure formation
within Miocene in the depth range of 2800 m to 5340 m and the mud weight used to control this overpressure zone was more than
2.0 sp gr mud. The generated Geologs can be used as reference to understand the regime of transition and overpressure, as
a valuable document for exploration drilling planning and monitoring. The generated model curve (modified using available
data after Hottman and Johnson, 1956 curve) using sonic departure (i.e. Δtob(sh) −Δtn(sh)) from drilled wells may be used as an additional tool to find out the expected formation pressure gradient and equivalent
mud weight in all future wells. The correlation of wells based on the trend of dcs and σ logs will be useful for predicting
transition and overpressure top provided all the parameters required for calculating dcs and σ log recorded smoothly during
drilling phase. The study has brought out the detail procedure to generate the pressure profile in the future wells. The generation
of pressure profile of a well prior to drilling has got immense importance in oil industry. The drilling of the well should
be done by maintaining the optimum mud weight generated from the pressure profile. In case, during drilling, formation pressure
is more than the mud pressure, resulted gas kicks or worse, blowouts of the well. Excessively high mud pressure can fracture
the formation and cause lost circulation. The oil and gas companies, worldwide, attributed 15% losses due to various problems
associated with drilling complications, mostly related to improper pressure prediction of a well. The losses include loss
of material as well as drilling process continuity, called non-productive time (NPT). The generation of accurate pressure
profile reduces drilling problems, cuts exploration and development costs and allows billions of dollars now spent on losses
to be better spent-building and replacing reserves. |
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