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A key element in any pore pressure prediction study is the comparison between the predicted pore pressure at the prospect location and the formation pressure measurements in offset wells. In order to understand, and predict, pressure distribution within a basin, pressure data and the sediments within which pressure regimes are confined need to be analyzed within a robust and consistent stratigraphic framework. This comparison is primarily required for: a) calibration for forward modelling and b) to have an idea of the extremes in the various pore pressure scenarios in a particular basin. The formation pressure measurements are acquired in permeable units like sands and are assumed to be in pressure equilibrium with adjacent shales. Thus, the predicted estimates in shales are compared with pore pressure measurements from sands in offset wells. This comparison is imprecise until and unless the offset pressure measurement is normalized to the same frame of reference at the prospect. The normalization process of offset pressure data can be broadly classified into two types: a) the simplistic depth normalization and b) the more detailed stratigraphic normalization. The simplistic depth normalization involves mere normalization of offset pressure measurements with respect to change in the positioning of the datum reference in the prospect location; this method precludes consideration of the structural difference (current plumbing) of each geological unit or in-depth analyses of depositional setup in offset and prospect locations presuming connected sands with constant overpressure. The second method entails the understanding of the regional stratigraphy, the structural differences in positioning of each stratigraphic unit in the prospect location and offset wells, depositional setting including sand connectivity and overpressure-generating mechanisms. This method can be invoked through appropriate approaches; a) constant overpressure (OP) method, b) constant vertical effective stress (VES) method and c) ratio method (ratio of either constant OP or VES to the normal effective stress i.e. NES). Stratigraphic normalization also includes the correct positioning of the offset pressure data in each stratigraphic unit at the prospect. Whilst the above normalization methods are extremely useful concepts, they form only part of the geological definition of the pressure system for predicting pore pressure profiles in an undrilled prospect. On the basis of geological understanding, stratigraphic normalization method may be preferred over the simplistic depth normalization method. An optimized normalization process for offset well data integrated with predicted pore pressure estimates at prospect locations illustrates a realistic scenario in terms of bounds of uncertainty. This paper brings forth a new dimension in predicting pore pressures at new prospects by describing scenarios within the framework of the conceived geological model. Also, the methods discussed provide the scope of capturing a valid uncertainty envelope that is imperative for a robust well design to define possible risks and foresee mitigation measures.
A key element in any pore pressure prediction study is the comparison between the predicted pore pressure at the prospect location and the formation pressure measurements in offset wells. In order to understand, and predict, pressure distribution within a basin, pressure data and the sediments within which pressure regimes are confined need to be analyzed within a robust and consistent stratigraphic framework. This comparison is primarily required for: a) calibration for forward modelling and b) to have an idea of the extremes in the various pore pressure scenarios in a particular basin. The formation pressure measurements are acquired in permeable units like sands and are assumed to be in pressure equilibrium with adjacent shales. Thus, the predicted estimates in shales are compared with pore pressure measurements from sands in offset wells. This comparison is imprecise until and unless the offset pressure measurement is normalized to the same frame of reference at the prospect. The normalization process of offset pressure data can be broadly classified into two types: a) the simplistic depth normalization and b) the more detailed stratigraphic normalization. The simplistic depth normalization involves mere normalization of offset pressure measurements with respect to change in the positioning of the datum reference in the prospect location; this method precludes consideration of the structural difference (current plumbing) of each geological unit or in-depth analyses of depositional setup in offset and prospect locations presuming connected sands with constant overpressure. The second method entails the understanding of the regional stratigraphy, the structural differences in positioning of each stratigraphic unit in the prospect location and offset wells, depositional setting including sand connectivity and overpressure-generating mechanisms. This method can be invoked through appropriate approaches; a) constant overpressure (OP) method, b) constant vertical effective stress (VES) method and c) ratio method (ratio of either constant OP or VES to the normal effective stress i.e. NES). Stratigraphic normalization also includes the correct positioning of the offset pressure data in each stratigraphic unit at the prospect. Whilst the above normalization methods are extremely useful concepts, they form only part of the geological definition of the pressure system for predicting pore pressure profiles in an undrilled prospect. On the basis of geological understanding, stratigraphic normalization method may be preferred over the simplistic depth normalization method. An optimized normalization process for offset well data integrated with predicted pore pressure estimates at prospect locations illustrates a realistic scenario in terms of bounds of uncertainty. This paper brings forth a new dimension in predicting pore pressures at new prospects by describing scenarios within the framework of the conceived geological model. Also, the methods discussed provide the scope of capturing a valid uncertainty envelope that is imperative for a robust well design to define possible risks and foresee mitigation measures.
Knowledge of the in-situ stress state and how it varies with reservoir depletion is important for the design and execution of in-fill drilling. This paper highlights the key geomechanical aspects and their usage in planning of wells through severely depleted (up to 25 MPa) and overpressured zones within a very short depth interval (few 10s of m), in an onshore gas field in Brunei. With focus shifting from oil to deep-gas development, drilling complications include risks of wellbore instability, excessive mud loss and internal blowouts, as well as differential sticking in the depleted reservoirs. Moreover, fracturing of the depleted sands while drilling infill wells carries the risk of jeopardizing production at nearby producing wells because of locally altered flow paths. The risks were evaluated by application of empirical and analytical geomechanical models of stress changes with depletion, and by elasto-plastic finite element models of borehole instability (collapse) due to shear failure. Our results show that for an average depletion rate of 1 MPa/year, the drilling window (difference between maximum allowable mud weight controlled by fracture pressure and minimum mud weight controlled by formation pore pressure or borehole collapse pressure, whichever is greater) is likely to remain open for the coming 12 years. Minifrac or extended leak-off tests at different stages of field development should be taken to monitor stress changes within the reservoirs and provide updates for calibration of the geomechanical model. Next to showing the geomechanical model results and their application to drilling, we demonstrate the refinement of pore pressure/fracture pressure predictions (i.e. narrowing down the uncertainty in the drilling window) for mature fields where producing "from the bottom up" has not been feasible. We also indicate how risks associated with drilling through depleted/undepleted reservoir sequences in a single hole section can be managed to as low as reasonably practicable with the help of geomechanical input. These results "open the door" for accessing deeper potential pay zones by drilling through severely depleted formations.
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