A feasibility study for pore-pr pore-pressure essure prediction using seismic velocities in the offshore Nile Delta, Egypt M OHAMMED OHAMMED A. BADRI , Schlumberger Oilfield Services, Cairo, Egypt COLIN M. SAYERS , Schlumberger Reservoir Evaluation Seismic, Houston, Texas, Texas, U.S. U.S. RASHAD AWAD and ARDENGHI GRAZIANO , Belayim Petroleum Company, Company, Cairo, Egypt
verpressure sure in a formation, caused Overpres
by abnormally high fluid pressures, is a concern during all phases of oil field operations—exploration,, drilling, casoperations—exploration ing, completion, and reservoir evaluation. Accurate knowledge of formation pore pressure and fracture pressure is essential for drilling efficient and safe wells with optimum mud weights. Furthermore, knowledge of these pressures aids understanding of fluid migration pathways, sealing potential, and probability of fault leakage. Overpressuree by definition occurs Overpressur when pore pressure exceeds normal hydrostatic pressure and is related to certain environmental conditions in a given earth section. In the offshore Nile Delta, for example, low permeability shale in the Pliocene can trap fluids and cause overpressured shale as a result of undercompaction. Overpressured sediments also can be caused by fluid expansion mechanisms (e.g., heating, hydrocarbon maturation and expulsion of intergranular water during clay transformation). Local tectonic compression can also generate overpressured sediments. Given the young age and shallowness of overpressured sediments in the offshore Nile Delta, the observed pore pressure is largely attributed to undercompaction. Several methods for detecting/ estimating overpressured formations are based on interpretation of drilling data, wireline logs, and geophysical data. Drilling and wireline log data are obtained while the well is drilled. They cannot, therefore, be used for predrill pore-pressure prediction. This paper describes a feasibility study to predict pore pressure before drilling and the subsequent calibration of pore pressure and seismic velocities in a key well. Geologic and structural settings. Port Fouad Field was discovered in 1982 when exploratory well Port Fouad-1 penetrated gas-bearing formations in the late Miocene. The field is approximately 35 km northeast of Port Said 0000
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Figure 1. Map of Port Fouad Field.
Figure 2. Stratigraphy of Port Fouad Field.
(Figure 1). Gas is present at shallow and deep depths. Shallow zones, less than 2200 m, belong to the Kafr El Sheikh Formation of Pliocene age which is
characterized by high porosity and permeability. The deep zones, about 3500 m, belong to the Wakar Formation of Miocene age. The shallow gas-bearing sands tend to be small OCTOBER 2000
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Figure 3. 2-D seismic section extracted from 3-D volume containing PFM SE-2.
Figure 4. 2-D seismic section extracted from 3-D volume containing PFM SW-1.
Figure 5. Pressure gradients before and after drilling PFM SE-2. Red circles represent measured formation pore pressure. 1104
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Figure 6. Time-depth pairs recorded during a borehole velocity survey in PFM SE-2.
in area but large in thickness; the deep zones are thinner but cover a much larger area. The shallow gas-bearing sands are extremely soft and unconsolidated. They are evident in surface seismic data as large amplitude anomalies in a depositional sequence of sand-shale formations. As a result of tectonic activity during the late Miocene, the turbiditic period ended with repeated episodes of channel levee turbidite-type deposits embedded in finer graded, mostly silty-shaly succession. The channelized sands of turbidite systems were deposited in the proximity of a NW-SE structural high. Onlaps of the basal turbidite type sands are seen. These channels are are at depths of 3200 m. Figure 2 shows a generalized stratigraphy of the area. Several channel complexes have been encountered by wells in the field. The extension of the main reservoir covers approximately 25 km2. The hydrocarbon-bearing net sand thickness is 21 m. Figure 3 is a 2-D seismic section from a 3-D volume that contains well PFM SE-2. Note that the Rosetta evaporite complex (2.1 s TWT) is missing where the well was drilled. drill ed. However, However, it is present away from the well to the east and to the west at about 2.3 s (TWT). An amplitude strength map over the time window 2-2.1 s showed a high amplitude associated with the event where the well encountered an overpressured zone. In contrast, the Rosetta complex was observed on the seismic data to be fairly consistent where well PFM SW-1 was drilled (Figure 4). Based on recent drilling in this part of the offshore Nile Delta, the Miocene can be divided into: • ABurdigalian-Langhian period with OCTOBER 2000
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widespread deposition of the Qantara Formation Formatio n in a deep marine environment marked by welldefined seismic signature. • ALanghian-Serravallian period with widespread deposition of a turbidite sandstone system that tends to shale out toward the paleohighs. These lithologies represent the Sidi Salem Formation which is well defined on seismic data. • A Tortonian period with deposition of channelized sandstone (turbiditic in nature). These sands are mainly quartz, metamorphic, and volcanic fragments and cherts in silty-shaly fossiliferous section that comprises Wakar Formation. The discontinuity discontin uity of the seismic expression is consistent with drilling results and represents the lack of continuity of alternated lithologies. • A Messinian period that essentially represents shaly sedimentation of the Wakar Formation followed by the evaporite complex deposition marked at the bottom by a well defined seismic event. Methodology. Pore pressure can be estimated by appropriate transformation of seismic velocities. This implies that accurate determination of seismic interval velocity is essential for reliable results. The pore-pressure prediction technique in this paper is based upon predicting the effective stress from velocity data (see Appendix A). The technique, and the reason that such data are needed, will be illustrated with an example from the offshore Nile Delta. Several wells had been drilled within the 3-D survey area without encountering overpressure problems. However,, during drilling of an exploHowever ration well (PFM SE-2) to Miocene sands, a strong gas kick at 1700 170 0 m indicated an overpressured zone predicted from offset well data. This required increasing the mud weight. Figure 5 shows pressure gradients representing fracture gradient, overburden gradient, predrill pore-pressure gradient, and actual (postdrill) gradient for PFM SE-2. Wireline log data recorded over this interval were gamma ray, resistivity, density, compressional sonic velocities, and borehole seismic velocities. Formation pressure data were recorded at a few specific depths. A second well in the region, region, PFM SW-1, was drilled with a similar predicted pressure gradient. At the depth that PFM SE-2 encountered the overpressured zone, the mud weight was significantly increased. However, no 1106
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Figure 7. (a) Interval velocity obtained by inverting VSP traveltimes. (b) Traveltime residuals (t (t measured - t predicted).
Figure 8. Pore-pressure prediction compared with formation pressure in PFM SE-2 and the overburden stress obtained from equation A3.
overpressure was encountered, and significant fluid losses occurred where the mud weight exceeded the fracture gradient. This led to a further loss of rig time. The mud weight was reduced, and the well was completed. co mpleted. This situation was dramatic evidence that predrill identification of overpressure zones was needed to optimize the drilling strategy in this area. Thus, we initiated a feasibility study to examine the relation between seismic velocity and pore pressure and
generate a pore-pressure 2-D section. The overall objective was to test the sensitivity of seismic velocity to pore pressure. In order to predict pore pressure accurately, reliable interval velocities are required. Frequently, velocities obtained from surface seismic stacking velocity data are used, but these often of ten lack the resolution for accurate pore pressure prediction. However, seismic reflection tomography can give the needed resolution for more accurate OCTOBER 2000
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pore pressure prediction. Borehole seismic velocities recorded in wells PFM SE-2 and PFM SW-2 were used. Although these wells well s are deviated, data were acquired with the source vertically above the receivers so that vertical incidence traveltimes could be recorded. The borehole seismic velocities consisted of time-depth pairs at 750-3400 m. Figure 6 shows the time-depth pairs recorded in PFM SE-2. Figure 7a shows interval velocities obtained by inverting time-depth pairs from the borehole velocity survey in PFM SE-2 using the approach described in Appendix B. The traveltime residuals—t residuals—timeasured-tipredicted (Figure 7b)—are random and show no trend. Here timeasured is the picked travel time at depth zi, and tipredicted is the predicted traveltime using equation B1 and the slownesses obtained by inversion. A strong velocity reversal on the interval velocity versus depth plot (Figure 7a at about 1700 m) indicates overpressure. The velocity reversal agrees with mud weight data in Figure 5 where a sudden onset of overpressure is observed. The parameters in Bowers’ equation were determined based on the formation pressure data matched to the pore-pressure predicted using the interval borehole seismic velocities and the pore-pressure calibration developed for PFM SE-2. Bowers’ parameters were found to be A= A = 4.95 when B = 0.9. Figure 8 shows the pore-pressure predictions at PFM SE-2 and the overburden stress gradient, fracture gradient, and formation pore-pressure data. The pore-pressure prediction was computed using equation A2. The effective stress was computed using equation A6. It is clear from this figure that three pore pressure regimes are present. The match between the predicted pore pressure and formation pore pressure is generally good, demonstrating that seismic velocity has sufficient sensitivity to changes in pore pressure to be used for pore-pressure prediction in this region. Figure 9 shows pore-pressur pore-pressuree prediction along the seismic line passing through PFM SE-2. Determination of boundaries was guided by geologic knowledge of the area. It is concluded that the overpressured zone at 1790 m can be attributed to upward movement of gas through the sediments due to absence of the Rosetta evaporite seal where PFM SE-2 was drilled. This interpretation is supported by the fact that PFM SW-1 did not encounter any 0000
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Figure 9. Pore-pressure prediction for the seismic line in Figure 2, using the interval velocity pore-pressure transform and equation A2 with parameters determined by inverting the mud weight from two wells.
abnormal pore pressure system since the Rosetta evaporite is present (thickness = 55 m), providing a good seal to the upward movement of gas. This study implies that it is possible to predict pore pressure before drilling from 3-D seismic data. Appropriate interval velocities can be obtained using tomographic inversion which provides precise boundaries of interval velocity variations attributed to changes in pore pressure. Furthermore, determination of pore pressure allows analysis of hydraulic connectivity and effectiveness of seals such as faults. Conclusions. Seismic interval velocities can be used to generate pore pressure sections from surface seismic data and well data. Pore-pressure prediction provides critical information for the design of future wells and the understanding of fluid migration. Detection of overpressured zones in the offshore Nile Delta can be achieved through establishing an accurate seismic velocity-pore pressure transform. A seismic velocitypore pressure transform has been derived for the Port Fouad Marine gas field in the offshore Nile Delta.
The predicted pore pressure after calibration to formation pore pressure measurements indicated different pore pressure regimes at different depths. This could be adapted to a 3-D seismic volume to generate a 3-D pore-pressure cube provided accurate interval velocities are available. Inclusion of “while drilling” sonic, resisitivity,, and annular resisitivity annul ar pressure data can enhance the predrill pore-pressure model. The pore-pressure prediction approach requires integration of surface and borehole measurements to minimize drilling risks and reduce the cost of drilling. Suggestions for further reading. “Porepressure estimation from velocity data: Accounting for pore-pressure mechanisms besides undercompaction” by Bowers (SPE (SPE Drilling and Completion, Completion , 1995). “Pressure-prediction from seismic data: Implications for seal distribution and hydrocarbon exploration and exploitation in deepwater Gulf of Mexico” by Dutta ( NPF Special Publication No. 7, 7, Elsevier, 1007). “Shale compaction burial diagenesis, and geopressures: Adynamical model, solutions and some results” by Dutta (1st IFP (Continued on p. 1108) OCTOBER 2000
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Appendix A Determination of pore pressure from seismic velocity
The effective stress tensor, , is ij
defined to be the difference between the total stress tensor, tenso r, Sij, and the pore pressure, p: ij = Sij - p ij
(A1)
Denoting the vertical component of the effective stress tensor ij by , and the vertical component of the total stress tensor Sij by S, the vertical component of equation A1 may be written
For uniaxial compaction, it is usually assumed that the elastic wave velocity is a function only of the vertical effective stress . The vertical component of the total stress, S, at depth H represents H represents the combined weight of the fluids and the formation above H and can be computed if the sediment density is known as a function of depth above the location of interest. This may be calculated from an integral of density: H
S = g ∫ (z) dz,
(A3)
0
= S - p.
(A2)
where (z) is the density as a function of depth z.
Appendix B Determination of interval velocity from the borehole velocity survey
Interval velocities were computed
from the time-depth pairs shown using an inversion approach designed to minimize the effect of errors in traveltime picking. Let si =1/vi be the interval slowness of layer i, v i the interval velocity of layer i, and t i the measured traveltimes at receiver depths z i (i=1,..,N); ti, s i and zi are related via the linear relation
t z t z = M M t N z 1
1
2
1
1
0
z2
L L
M
z2
L
s s 0 (B1) M M z N sN 0
1
least-squares inversion employing a penalty function that used the second derivative of the estimated slowness as a smoothing criterion (Lizarralde and Swift, 1999). The interval velocity was found by inverting traveltimes from the borehole velocity survey by minimizing the value of the absolute value of 2-1 where 2
χ
2
t measured − t ipredicted (B2) = ∑ i N i = σ i 1
N
1
2
In order to minimize the effect of errors in traveltime picking, this system was solved using a damped
Here timeasured is the picked traveltime at depth zi, and tipredicted is the predicted traveltime using equation B1 and the slownesses obtained by inversion. The standard deviation i was taken to be 0.3 ms for all i. LE
(From p. 1107)
Exploration Research Conference, Caracans, France, 1986). “The equation for geopressure prediction from well logs” by Eaton (SPE 5544, 1975). “Estimation of formation pressures from log-derived shale properties” by Hottman and Johnson ( Journal of Petroleum Technology, Technology, 1956). “Smooth inversion of VSP traveltime data” by Lizarralde and Swift (GEOPHYSICS, 1999). “Predrill pore pressure prediction using seismic data” by Sayers et al. (IADC/SPE Drilling Conference, 2000). “Pore/fracture pressure determinations 1108
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in deep water” by Traugott (Deepwater (Deepwater Technology,, supplement to August 1997 Technology World Oil). Oil). “Seismic pressure-prediction method solves problem common in deepwater Gulf of Mexico” by Wilhelm E et al. (Oil (Oil & Gas Journal, Journal , 1998). L
In the absence of a density log, a frequently used method for computing the overburden stress is the Amoco equation (Traugott, 1977): avg(z) = 16.3 + [z/3125] 0.6
(A4)
where avg(z) is the average sediment density in ppg between the seafloor and depth z in feet from the seafloor. Several pore pressure techniques have been based on various mechanisms causing the pore pressure. For example, Bowers (1995) provides a method to determine effective stress that accounts for both undercompaction and fluid expansion through definition of the unloading curve. This technique has recently been used successfully in the Gulf of Mexico. The technique is based on the fact that during compaction (loading) a velocity increase occurs. During the unloading process, the effective stress is reduced due to fluid expansion. Fluid expansion zones are characterized as zones of reversal in velocity trend. The relation between the effective stress and velocity in normally pressured sediments suggested by Bowers is: V = V 0 +AB,
(A5)
Where V0 is the velocity of unconsolidated fluid-saturated sediments (taken to be 1480 m/s) and A and B describe the variation in velocity with increasing effective stress and can be derived from offset well data. The effective stress can be determined from this equation: = [(V - V0)/A)]1/B,
(A6)
The pore pressure can then be calculated from equation A2. Bowers obtained the values A=4.4567 and B=0.8168 for Gulf coast wells and A=28.3711 and B=0.6207 for deepwater Gulf regions. LE
Acknowledgments: The authors are grateful to Acknowledgments: Belayim Petroleum Company and Eni Egypt for permission to publish the data. We thank Steve Montgomery of Schlumberger IPM Egypt for useful discussion. Corresponding author:
[email protected] Corresponding
[email protected]. eoquest. slb.com OCTOBER 2000
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