pore pressure prediction in the niger delta â lessons ... - Ikon Science

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Pore Pressure Prediction in the Niger Delta - lessons learnt from regional analysis Figure 1 Profile A : Shale-rich lithology where reservoirs and shales are in pressure equilibrium. Red (resisitivity), brown (density) and blue (sonic) shale pressure interpretation. Red triangles are sand pressure tests, pink squares are leak-off test results, estimating fracture strength of rock. Figure 2 Profile B : High net/gross. Using methodology outlined in Figure 3, prediction of the shale pressures (and hence avoidance of kicks) was possible pre-drill assuming ages of seismic markers were known (see text for explanation). Red circle is the Fluid Retention Depth or “FRD”.are leak-off test results, estimating fracture strength of rock.
Pore Pressure Prediction in the Niger Delta - lessons learnt from regional analysis In this paper, we describe another method to calculate these shale pressures, one based on sedimentation rates and loading. The top of overpressure is the point at which the pore pressure starts to deviate from the hydrostatic line. In terms of process, as fluids are inhibited from escaping as sedimentation increases, porosity loss is slowed, and overpressure builds – there is a departure from the normal compaction curve. The depth of top of overpressure is controlled by the permeability, rock compressibility and sedimentation rate. The Fluid Retention Depth (or FRD) is the point at which rocks completely cease to dewater due to low sediment permeability (Swarbrick et al., 2002; Gluyas and Swarbrick, 2004). The FRD is determined by an extrapolation from inferred shale pressures, to intersect the hydrostatic gradient – the trend should be approximately overburden parallel. Swarbrick et al. (2002) showed that there is a correlation between sedimentation rate and FRD using data from shale dominated regions (Figures 3 and 4). Data in Swarbrick et al. (2002) is presented from the Gulf of Mexico, Trinidad, Nile Delta and other world-wide basins. Using this trend for pore pressure allows an assessment of shale pressures to be made. This technique is highlighted below. Figure 3 Methodology for prediction of shale pressure using Swarbrick et al. (2002) applied to data from Nile Delta. Using data from Mann and Mackenzie (1990) from the Nile Delta, where sedimentation or depositional rate is calculated to be 810m/Ma, using the relationship in Swarbrick et al. (2002) suggests an FRD of 2,620 feet TVDbml (where bml is below mud-line or seabed). Therefore, with foreknowledge of sedimentation rate, one can estimate the FRD depth using data compiled by Swarbrick et al. (2002). With this in mind the pressure at any depth can be estimated on a Pressure-Depth plot by plotting a gradient parallel to the overburden gradient, starting at the FRD. This method assumes that overpressure is generated entirely by disequilibrium compaction which for the Niger Delta seems reasonable since the sediments are young, shale/sand dominated and the velocity-density cross-plots reveal little or no secondary mechanisms to be present. In Figure 2, we present the results of testing this approach on a well taken from the East of the study area where water depth is 2433 feet. In this well, the Late Miocene, although relatively shale-rich, has numerous sand units present. Pressure tests in these sand record minor overpressures, with pressures
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