Velocities derived from the seismic data provide indirect estimation of the formation pressure prior to drilling. The uncertainties in velocity estimation increase with the geological complexity and depth which in turn amplify the margin of error in the pore pressure prediction. Such uncertainties can be reduced by adopting suitable velocity to carry out predrill pore pressure prediction. Several advanced techniques for velocity analysis have been used in this study with varying degree of confidence for pore pressure estimation in a deep water HTHP well.The well was designed to drill to a depth of 5000 m with an overpressured Cretaceous clastic sediment column of 2000 m before reaching the reservoir (water depth 600m, maximum prognosed pressure ~11,000 psi and temperature ~190ºC). In deepwater Krishna Godavari basin, the conventional seismic velocity (Handpicked and stacking velocities) based pore pressure prediction resulted in considerable uncertainties in the older Cretaceous sediments as seen in the earlier drilled wells. This called for an advanced velocity analysis (AVO based and Inversion velocities) to reduce the margin of uncertainties for this study well. Such analysis added values to our understanding of the impedance contrast, temporal and spatial variations of velocity in terms of reservoir and non reservoir inter-relationship. A definitive predrill pore pressure curve, taking into account these geological and geophysical factors was the best estimate for well planning. HTHP well drilling challenges can be constrained by depth of top of overpressure, narrow pore-frac window and large ESD-ECD variations due to high temperature gradient. The definitive pore pressure curve catered to limiting all the three critical parameters as comparison with the post drill pore pressure analysis showed a variation of ±0.5ppg. Future deepwater HTHP prospects can be planned by the similar work flow as the drilling experience of this prospect was satisfactory. 2 SPE 153764

Overpressures (abnormally high fluid pressures) represent a significant geohazard and drilling problem. Prediction of overpressures is very important for well planning and safe drilling. However, accurate and reliable prediction requires an understanding of the origins and distribution of such overpressures. Petrophysical properties of the sediments are affected by different overpressure generation mechanisms and in turn help in understanding the types of such mechanisms. There are two distinct overpressure generating mechanisms, namely compaction disequilibrium (undercompaction) and fluid expansion (unloading), each of which have different petrophysical signatures and hence different prediction methodologies. The most common cause of overpressure generation in the majority of the sedimentary basins in the world is undercompaction, in which pressure increases due to rapid burial/loading of the sediments in an effectively sealed impermeable environment. This type of overpressure is normally associated with abnormally high porosities and shows up in changes in velocities. The secondary type of overpressure mechanism is fluid expansion. Thermal induced overpressure is the most common fluid expansion mechanism. This mechanism is very common in areas of high geothermal gradient and can result in significant overpressures. This mechanism, however, is not always present. Thermally induced overpressures result in decreasing effective stress in contrast to overpressure due to undercompaction where a constant effective stress is observed. Thermally induced overpressures are difficult to predict and require a different prediction methodology. Improved knowledge of overpressure generating mechanisms and distribution of pore pressure in a basin provides critical supporting information for the asset team in hydrocarbon exploration and production. This information not only has an immediate impact on drilling cost and safety but also provides insight to key elements in petroleum system analysis. This paper presents a study showcasing the geological control on origin and distribution of overpressure in a HPHT (high pressure, high temperature) field from offshore (water depth ~100-150m) South East Asia. Historically, the offset wells in the field were drilled through complex geological settings including high overpressure (~17-18 ppg), high temperature (170-185 deg C) and variable stress fields. The lithology is dominated by shales and most of the wells drilled in the area encountered drilling challenges with respect to high overpressure development. An initiative for a pore pressure prediction study was undertaken in a semi-regional scale involving ten offset wells in the study area. The main focus was to understand the overpressure mechanism and distribution in the study area vis-à-vis the geological setting and control. This was followed by predrill prediction for the planned wells, as one of the objectives of this study was also to aid in future development well drilling. Well planning based on the study results were done for two prospect wells which were located in similar shallow water.

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A geomechanical model calibrated to field data can be used to analyse the potentially severe impact of reservoir compaction on production. However, field data acquisition programmes can be expensive, and optimal reservoir monitoring design necessitates an understanding of reservoir dynamics. Forward geomechanical models can help establishing the appropriate field data gathering approach while reducing expenses and maximising value for model calibration. Significant reservoir compaction and surface subsidence have potential risks for fault reactivation, integrity of wells and surface facilities. This paper presents an integrated approach and workflow that combines geomechanically derived data, reservoir geometry and production data to predict reservoir compaction and surface subsidence throughout the life of the field. The results provide an essential understanding on the dynamics of production induced changes in effective stresses and formation mechanical properties and their impacts on the field development planning, risks mitigation and provision of contingencies for well construction and downhole and surface field monitoring requirements.

Pore pressure prediction and geomechanical modelling play a very important role in well planning. The exploration focus worldwide is moving more and more into the challenging environments. Well planning and design in high pressure high temperature (HPHT) environments comprise numerous challenges such as play identification and prospect de-risking to drillability and development to production. Overpressure prediction is one of the principal challenges facing the oil industry today, as exploration focus worldwide moves further into the HPHT environments. Pressure related problems in HPHT wells include well control incidents, lost circulation, formation breathing, differential sticking, reduced rates of penetration, and reservoir damage, many which can potentially lead to expensive sidetracks, well abandonments and underground blowouts. A better understanding of the prevalent pore pressure regimes including generating mechanisms, pressure maintenance and dissipation through geologic time enables invaluable insight into these challenges and the ability to mitigate or minimize them. It is important to analyse the challenges prior to drilling so that various plans and systems can be established. Once the pore pressure regime is well understood, the next step is to understand the stress regime. Thus building a geomechanical model is the next key element for well design. The stress model can be used for wellbore stability analysis to understand wellbore failures and help in thedesignof optimum mud weights. Additionally in HPHT environments, thermal-induced stresses and their impacts on stability must also be considered. This understanding helps in modelling wellbore failures due to heating and cooling arising from mud circulation. Other considerations including drilling through faults can set another aspect of wellbore failure which could further complicate the already difficult drilling situation. This paper presents a case study on using geomechanical evaluations to reduce drilling risks and costs in a high-pressure/high-temperature (HP/HT) well located in offshore Asia Pacific. The major risk anticipated for this well, which was drilled to explore a deep-play (>3.5 Km), was high-pressure (>10,000 psi) and high-temperature (>2000C) with narrow margin drilling conditions. The geomechanical study provided inputs for an effective well design.