--> An Address to Shallow Hazards and Pore Pressure Complexities of Indian Petroliferous Basins: Integrated Modelling Approach

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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An Address to Shallow Hazards and Pore Pressure Complexities of Indian Petroliferous Basins: Integrated Modelling Approach

Abstract

Even after paradigm shift in advanced technology implementation in E&P business, geoscientists are still straining their brains in derisking prospect from shallow hazards and pore-pressure abnormalities, for safe and cost-effective drilling. Current work highlights that drilling in different Indian basins, is endowed with problems of shallow geohazards (e.g. Unstable Seafloor, fault scarps, Shallow Water Flow-SWF, Gas chimney etc.) and/or pore pressure (PP) complexities; especially in Krishna-Godavari (KG) offshore at East coast, Bombay offshore in West and onland Assam-Arakan (A&AA) fold belt in North-East. Study confers a way forward to mitigate these drilling risks, through holistic analysis of basic seismic attributes (Coherence, Dip& Azimuth, Relative Acoustic Impedance-RAI, Curvature and AFE- fracture delineation), AVO-amplitude versus offset and rock-calibrated high resolution interval velocity integrated with seismic driven 3D PP & fracture pressure (FP) modeling. In complex terrains with meager seismic quality, PP through basin modeling approach has been adopted to add new verticals to mitigate drilling severities. Starting with the case study from Prolific KG offshore, wellbores targeting prospective Miocene structures, evidenced issues of sea-floor unstability, shallow gas kicks, mud loss etc. Coherence, Dip-Azimuth and AFE attributes manifest presence of pockmarks, fault scarps, active faults akin to unstable seafloor and highly-fractured areas in shallow section. RAI-Curvature crossplot has brought out highly stable hard rock zones within study area, which is also calibrating with cone penetration testing (CPT) data. Moreover, modeled 3D PP coupled with presence of typical Class-III AVO, has delineated spatial distribution of overpressured sand flows (SWF) with dissolved gas. Based on above findings, well trajectory - avoiding hazardous zones with mud weight optimized as per modeled PP - is recommended in future drilling. Coming to the case study from Bombay offshore, Eocene and Paleocene prospective targets could not be successfully probed; because of kicks/blowout associated to gas chimney breaching the overlying seals and reaching upto shallow horizons through faults and fractures. Structural mapping, Chimney and AFE attributes, in calibration with wellbore data characterize densely fractured areas and presence of shallow gas accumulation through faults rooted to chimney zone. Pore pressure study suggests disequilibrium compaction, tectonic stress due to compression and gas buoyancy as probable causes for overpressure generation. Study recommends that wellbore away from gas cloud and sidetracking to deeper Paleocene reservoirs- in consensus with geomechanical understanding - is probably the best optimization for safe drilling. Last case study is from Assam-Arakan fold belt, putting forth the daunting challenges of specular overpressure (due to high sedimentation rate and tectonic unloading) encountered in wells penetrating Early-Mid Miocene formations. Highly shoddy seismic quality of this complex terrain leads to unreliable pore pressure estimates from seismic velocity. To overcome these limitations, genetic parameters of PP for different stratigraphic columns - porosity, compaction, specific surface area, lateral transmissibility and generation potential- have been inverted though basin modeling technology using input of 1D interpreted PP. Optimized parameters are calibrating well with SEM and XRD results. At blind well, PP modeled through estimated parameters honors well data, and also capture PP abnormalities of Early-Middle Miocene formations. In light of modeled pore pressure, wellbore admiring geomechanical study; will open a new corridor in drilling safe and cost-effective clean boreholes in future. Basin specific integrated strategy through well and seismic data - bridging the gaps between different geological, geophysical and geotechnical domains- has brought out vital success in drilling, especially in above mentioned puzzling sectors of different Indian basins.