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Drilling engineers require estimates of the fluid pressures that they are likely to encounter in any given well to anticipate mud weights required to maintain optimal drilling rates and safety. Because seismic velocities correlate with effective pressure in the formation, sufficiently precise estimates of velocity obtained from seismic observations can be used to determine pore pressure. In the absence of dense well control, interval velocities derived from stacking velocities are used to estimate pore pressure. These interval velocities are compared with a general trend of velocities in the region (Figure 1), and a pore pressure volume is developed for use by drilling engineers, as shown in Figure 1. Acoustic impedance volumes obtained from seismic trace inversion can also be used to identify and detect anomalous pore pressure regions.
Abnormal pressure (overpressure) conditions in the subsurface can pose significant drilling hazards if not detected. This article discusses how acoustic logs can help to identify overpressure situations. Abnormal pressure is defined as any departure from normal hydrostatic pressure at a given depth. Abnormal subsurface pressures, either overpressure (geopressure) or underpressure, are encountered in hydrocarbon basins throughout the world in all lithologies, from all geologic ages, and at all depths. Early and reliable detection of geopressure is vital to avoid or mitigate potential drilling and safety hazards, e.g.: During drilling, advanced warning of approaching geopressuring enables the mud weight to be adjusted to avoid well and reservoir damage and to determine casing points. This is a particular concern in deepwater wells in which the pressure difference; i.e., the operating window, between the hydrostatic gradient and the fracture gradient can be very narrow.
In underbalanced drilling (UBD), as soon as the bit penetrates the reservoir, reservoir fluids start to flow into the wellbore. At this stage, the stabilized multiphase flow regime in the well prior to reservoir fluid entry must be adjusted to account for inflow and minimize the impact to the circulation system or moving out of the UBD window already established. The rate of reservoir fluid inflow depends, in part, on the drawdown and reservoir rock properties (the differential pressure between circulating bottomhole pressure (BHP) and reservoir pressure). There are a number of models that can be used to estimate the reservoir fluid inflow based on the rock and fluid parameters. However, the reservoir rock properties are fixed, and the only variable is the drawdown to control reservoir fluid inflow.