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Africa creating pressure is the collapse of the rock’s load-supporting framework (load transfer), potentially leading to high overpressures. Other mechanisms, believed to be minor in amount of overpressure generated, relate to water expansion on heating (aquathermal), selected mineral dehydration reactions and osmosis (where salinity differences are found). However, one or more of these other mechanisms can add to the overpressure already created by compaction disequilibrium and push the pressures toward the fracture pressure and tensile failure (a condition known as seal breach). In summary, multiple mechanisms are possible. The most effective is rapid burial of fine-grained rocks (when overpressure can commence within a few hundred meters of the sea-bed), but in deeper, hotter rocks many other mechanisms can add additional overpressure. Where does high pressure occur? It follows from the above description of the mechanisms that three main components control where overpressures occur: rate of sediment burial, temperature, and sediment permeability. Areas of high sedimentation rate will normally correlate, along a continental margin such as offshore West Africa, with high sediment input. Big river systems depositing large volumes of suspended sediment at their deltas are the most likely locations, the most visible of which are the Niger and Congo rivers exiting into the South Atlantic. The Niger Delta sediments are a mixture of sandstone (reservoirs) and shales (seals), deposited over time in a complex pattern of distributary systems running across the delta top, across the shoreface and into offshore deepwater environments where they interbed with marine sediments, largely shales. Tertiary deltas such as these typically exhibit a high sand content on the shelf and delta top, leading to near normal pressure conditions down to depths of 3000 m+ (10,000 ft), below which a sharp increase in overpressure (a rapid pressure-transition zone, Figure 1) leads to several drilling challenges. Firstly, the start of a rapid buildup in overpressure can be difficult to predict if the pressure change is accompanied by a facies change. Secondly, the rate of pressure increase can be high. In Brunei, for example, an increase in overpressure of 220 bar (3200 psi) is reported (Ellenor, 1984) across a shale only 17 m (55 ft) thick. Dickinson (1953) reports a > 230 bar (3300 psi) increase in overpressure over a 60-m vertical section at the base of the sand-rich facies in the Gulf of Mexico. Finally, the increase in overpressure can lead to a narrow drilling window (difference between pore pressure and fracture gradient, Figure 1). All these issues need to be assessed predrill and captured in the well-planning process for most likely and high-case pressure predictions and optimal positioning of casing to maintain wellbore stability, drilling safety, and minimize potential rig downtime. Further seaward, and especially down the continental slope, the sediments become more mud-rich and any sand reservoirs are confined to channel and fan systems, most likely enclosed in low-permeability shales. The pressure profile in more continuous shale sections is one of constant increase in overpressure, often with a gradient running parallel to the June 2011 The Leading Edge 683 <strong>Downloaded</strong> 08 Jul 2011 to 195.99.165.130. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
Africa overburden (Figure 2). From a drilling perspective, the rate of increase along the pressure-transition zone is gradual with less probability of a drilling surprise. However, the start of overpressure at relatively shallow depths of burial, typically 1000 m (3000 ft) below seabed, leads to a long and continuous narrow drilling window, requiring frequent casing strings to maintain borehole integrity. Some wells along the continental margin of West Africa have had to set total depth prior to reaching all their expected targets because of these extensive pressure-transition zones. Thermal processes will occur at depths where the temperatures allow selected reactions in suitable rocks. Source rocks are required for gas generation and the overpressure generated with the source rock will, over geological time, influence other sediments both above and below. Similarly, clay-mineral reactions occur when the kinetics (time and temperature conditions for reaction) are met at depth. Once a single reaction has occurred, no further overpressure will be generated unless other reactions occur. Detailed knowledge of clay-mineral diagenesis remains relatively elusive. How do we predict high pressure? Traditionally, the method to predict the pressure of shale-rich rocks is to analyze seismic and well data, working with the principles that (a) high pressure is associated with higher than expected porosity; (b) the parameter used to capture porosity is of good quality with high data density; and (c) the only reason for the overpressure is compaction disequilibrium. The two main relationships used to quantify pore pressure are the (a) Eaton ratio method, an empirical approach in which Eaton provided constants for sonic velocity, resistivity and drilling exponent (Dxc) data, and (b) equivalent depth method, a deterministic approach, which is most commonly used with density and sonic velocity data. Ideally, pore-pressure prediction practitioners will use both methods applied to several data types to compare prediction profiles. In that way a measure of the uncertainty in the prediction can be usefully assessed. In Figure 3, four types of log data (sonic, density, neutron, and resistivity) have been used from a single well to estimate the pressures in a thick shale section. In this example, from offshore West Africa, there is good consistency of patterns of pressure estimation in the shales using all four log types. Because shale pressure cannot be independently verified, except in shallow sediments (< 200 m burial), it is imperative to build confidence of the overpressure magnitude, because drilling any sediments with high permeability will lead to a wellbore influx (potential kick) if the mud weight is not high enough to balance the formation fluid pressures. High confidence comes with consistency of results, combined with understanding the reservoirs and their relation to interbedded shales (and other lithologies). Isolated reservoirs often share similar overpressures with surrounding shales (good calibration to shale-based models), whereas interconnected reservoirs can allow pressure transfer leading to both higher and lower pressures than the surrounding shales (poor calibration for shale-based models). Laterally drained reservoirs in which reservoir pressures are significantly lower than the shales above and below are known along the West African margin. The thick reservoir in Figure 3 has lower overpressure (from direct pressure measurements or MDTs) than the shales above, an indication of a laterally drained reservoir. Pressure prediction becomes more challenging and uncertain where pressure-generating mechanisms other than compaction disequilibrium are contributing to the overpressure and assessing which mechanisms may be contributors could benefit from plotting velocity against density, coded to depth (Chopra and Huffman, 2006). Figure 4 shows a schematic plot to illustrate trends of data with depth (arrows are from shallow to deep) to capture compaction disequilibrium, unloading (Bowers, 1994) associated with fluid expansion, such as gas generation, and the range of responses associated with chemical change in shales. The results of the plot help to guide the adaptation of the traditional approach to prediction, such as changes to the Eaton exponent, or application of a Bowers unloading model (Bowers, 1995). The confidence of any prediction in which multiple mechanisms are active is lower than for disequilibrium compaction alone, and careful calibration to any direct pressure measurements in offset wells becomes more critical. In addition, the effectiveness of seismic-based pore-pressure prediction (well suited to locations situated far from available well data) can be severely reduced. From a predrill prediction point of view, the inability to anticipate contributions to overpressure from fluid expansion and chemical processes and the associated modification of the rock properties used for pressure profiling can lead to substantial underprediction of reservoir pressures and hence add substantial risk to the safety of well operations. Realistic uncertainty estimates are important at this stage. Regional pressure studies Offset well data must always be examined carefully as the main calibration data for pressure prediction in undrilled areas. Where sufficient density of wells is available, compilation of the data basin-wide offers immense advantages. Successful Figure 3. Example of pressure-depth plot for a thick shale interval in which pressure is estimated from four log types (sonic, density, resistivity, and neutron). The reservoir beneath has lower overpressures on account of lateral drainage (see text). 684 The Leading Edge June 2011 <strong>Downloaded</strong> 08 Jul 2011 to 195.99.165.130. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
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