pore pressure offshore nile delta

Overpressure in a formation, causedby abnormally high fluid pressures, isa concern during all phases of oil fieldoperations—exploration, drilling, cas-ing, completion, and reservoir evalu-ation. Accurate knowledge offormation pore pressure and fracturepressure is essential for drilling effi-cient and safe wells with optimummud weights. Furthermore, knowl-edge of these pressures aids under-standing of fluid migration pathways,sealing potential, and probability offault leakage.

Overpressure by definition occurswhen pore pressure exceeds normalhydrostatic pressure and is related tocertain environmental conditions in agiven earth section. In the offshore NileDelta, for example, low permeabilityshale in the Pliocene can trap fluidsand cause overpressured shale as aresult of undercompaction. Overpres-sured sediments also can be caused byfluid expansion mechanisms (e.g.,heating, hydrocarbon maturation andexpulsion of intergranular water dur-ing clay transformation). Local tectoniccompression can also generate over-pressured sediments. Given the youngage and shallowness of overpressuredsediments in the offshore Nile Delta,the observed pore pressure is largelyattributed to undercompaction.

Several methods for detecting/estimating overpressured formationsare based on interpretation of drillingdata, wireline logs, and geophysicaldata. Drilling and wireline log dataare obtained while the well is drilled.They cannot, therefore, be used forpredrill pore-pressure prediction.

This paper describes a feasibilitystudy to predict pore pressure beforedrilling and the subsequent calibra-tion of pore pressure and seismicvelocities in a key well.

Geologic and structural settings. PortFouad Field was discovered in 1982when exploratory well Port Fouad-1penetrated gas-bearing formations inthe late Miocene. The field is approx-imately 35 km northeast of Port Said

(Figure 1).Gas is present at shallow and

deep depths. Shallow zones, less than2200 m, belong to the Kafr El SheikhFormation of Pliocene age which is

characterized by high porosity andpermeability. The deep zones, about3500 m, belong to the WakarFormation of Miocene age. The shal-low gas-bearing sands tend to be small

0000 THE LEADING EDGE OCTOBER 2000 OCTOBER 2000 THE LEADING EDGE 1103

A feasibility study for pore-pressure prediction using seismic velocities in the offshore Nile Delta, Egypt

MOHAMMED A. BADRI, Schlumberger Oilfield Services, Cairo, EgyptCOLIN M. SAYERS, Schlumberger Reservoir Evaluation Seismic, Houston, Texas, U.S.RASHAD AWAD and ARDENGHI GRAZIANO, Belayim Petroleum Company, Cairo, Egypt

Figure 1. Map of Port Fouad Field.

Figure 2. Stratigraphy of Port Fouad Field.

in area but large in thickness; the deepzones are thinner but cover a muchlarger area. The shallow gas-bearingsands are extremely soft and uncon-solidated. They are evident in surfaceseismic data as large amplitude anom-alies in a depositional sequence ofsand-shale formations.

As a result of tectonic activity dur-ing the late Miocene, the turbiditicperiod ended with repeated episodesof channel levee turbidite-typedeposits embedded in finer graded,mostly silty-shaly succession. Thechannelized sands of turbidite systemswere deposited in the proximity of aNW-SE structural high. Onlaps of thebasal turbidite type sands are seen.These channels are at depths of 3200m. Figure 2 shows a generalizedstratigraphy of the area. Several chan-nel complexes have been encounteredby wells in the field. The extension ofthe main reservoir covers approxi-mately 25 km2. The hydrocarbon-bear-ing net sand thickness is 21 m.

Figure 3 is a 2-D seismic sectionfrom a 3-D volume that contains wellPFM SE-2. Note that the Rosetta evap-orite complex (2.1 s TWT) is missingwhere the well was drilled. However,it is present away from the well to theeast and to the west at about 2.3 s(TWT). An amplitude strength mapover the time window 2-2.1 s showeda high amplitude associated with theevent where the well encountered anoverpressured zone. In contrast, theRosetta complex was observed on theseismic data to be fairly consistentwhere well PFM SW-1 was drilled(Figure 4).

Based on recent drilling in this part ofthe offshore Nile Delta, the Miocenecan be divided into:

• ABurdigalian-Langhian period with


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 circlesrepresent measured formation pore pressure.

Figure 6. Time-depth pairs recordedduring a borehole velocity survey inPFM SE-2.

widespread deposition of theQantara Formation in a deep marineenvironment marked by well-defined seismic signature.

• ALanghian-Serravallian period withwidespread deposition of a turbiditesandstone system that tends to shaleout toward the paleohighs. Theselithologies represent the Sidi SalemFormation which is well defined onseismic data.

• A Tortonian period with depositionof channelized sandstone (turbiditicin nature). These sands are mainlyquartz, metamorphic, and volcanicfragments and cherts in silty-shalyfossiliferous section that comprisesWakar Formation. The discontinuityof the seismic expression is consis-tent with drilling results and repre-sents the lack of continuity ofalternated lithologies.

• A Messinian period that essentiallyrepresents shaly sedimentation ofthe Wakar Formation followed bythe evaporite complex depositionmarked at the bottom by a welldefined seismic event.

Methodology. Pore pressure can beestimated by appropriate transforma-tion of seismic velocities. This impliesthat accurate determination of seismicinterval velocity is essential for reli-able results. The pore-pressure pre-diction technique in this paper is basedupon predicting the effective stressfrom velocity data (see Appendix A).The technique, and the reason thatsuch data are needed, will be illus-trated with an example from the off-shore Nile Delta.

Several wells had been drilledwithin the 3-D survey area withoutencountering overpressure problems.However, during drilling of an explo-ration well (PFM SE-2) to Miocenesands, a strong gas kick at 1700 m indi-cated an overpressured zone predictedfrom offset well data. This requiredincreasing the mud weight. Figure 5shows pressure gradients represent-ing fracture gradient, overburden gra-dient, predrill pore-pressure gradient,and actual (postdrill) gradient for PFMSE-2. Wireline log data recorded overthis interval were gamma ray, resis-tivity, density, compressional sonicvelocities, and borehole seismic veloc-ities. Formation pressure data wererecorded at a few specific depths.

A second well in the region, PFMSW-1, was drilled with a similar pre-dicted pressure gradient. At the depththat PFM SE-2 encountered the over-pressured zone, the mud weight wassignificantly increased. However, no

overpressure was encountered, andsignificant fluid losses occurred wherethe mud weight exceeded the fracturegradient. This led to a further loss ofrig time. The mud weight wasreduced, and the well was completed.

This situation was dramatic evi-dence that predrill identification ofoverpressure zones was needed tooptimize the drilling strategy in thisarea. Thus, we initiated a feasibilitystudy to examine the relation betweenseismic velocity and pore pressure and

generate a pore-pressure 2-D section.The overall objective was to test thesensitivity of seismic velocity to porepressure.

In order to predict pore pressureaccurately, reliable interval velocitiesare required. Frequently, velocitiesobtained from surface seismic stackingvelocity data are used, but these oftenlack the resolution for accurate porepressure prediction. However, seismicreflection tomography can give theneeded resolution for more accurate


Figure 7. (a) Interval velocity obtained by inverting VSP traveltimes. (b) Traveltime residuals (tmeasured - tpredicted).

Figure 8. Pore-pres-sure predictioncompared withformation pressurein PFM SE-2 andthe overburdenstress obtainedfrom equation A3.

pore pressure prediction.Borehole seismic velocities

recorded in wells PFM SE-2 and PFMSW-2 were used. Although these wellsare deviated, data were acquired withthe source vertically above thereceivers so that vertical incidencetraveltimes could be recorded. Theborehole seismic velocities consisted oftime-depth pairs at 750-3400 m. Figure6 shows the time-depth pairs recordedin PFM SE-2.

Figure 7a shows interval velocitiesobtained by inverting time-depth pairsfrom the borehole velocity survey inPFM SE-2 using the approachdescribed in Appendix B. The travel-time residuals—ti

measured-tipredicted (Figure

7b)—are random and show no trend.Here ti

measured is the picked travel timeat depth zi, and ti

predicted is the predictedtraveltime using equation B1 and theslownesses obtained by inversion.

A strong velocity reversal on theinterval velocity versus depth plot(Figure 7a at about 1700 m) indicatesoverpressure. The velocity reversalagrees with mud weight data in Figure5 where a sudden onset of overpres-sure is observed.

The parameters in Bowers’ equa-tion were determined based on the for-mation pressure data matched to thepore-pressure predicted using theinterval borehole seismic velocities andthe pore-pressure calibration devel-oped for PFM SE-2. Bowers’ parame-ters were found to be A = 4.95 when B= 0.9.

Figure 8 shows the pore-pressurepredictions at PFM SE-2 and the over-burden stress gradient, fracture gradi-ent, and formation pore-pressure data.The pore-pressure prediction wascomputed using equation A2. Theeffective stress was computed usingequation A6. It is clear from this fig-ure that three pore pressure regimesare present. The match between thepredicted pore pressure and forma-tion pore pressure is generally good,demonstrating that seismic velocityhas sufficient sensitivity to changes inpore pressure to be used for pore-pres-sure prediction in this region.

Figure 9 shows pore-pressure pre-diction along the seismic line passingthrough PFM SE-2. Determination ofboundaries was guided by geologicknowledge of the area. It is concludedthat the overpressured zone at 1790 mcan be attributed to upward move-ment of gas through the sediments dueto absence of the Rosetta evaporite sealwhere PFM SE-2 was drilled. Thisinterpretation is supported by the factthat PFM SW-1 did not encounter any

abnormal pore pressure system sincethe Rosetta evaporite is present (thick-ness = 55 m), providing a good seal tothe upward movement of gas.

This study implies that it is possi-ble to predict pore pressure beforedrilling from 3-D seismic data.Appropriate interval velocities can beobtained using tomographic inversionwhich provides precise boundaries ofinterval velocity variations attributedto changes in pore pressure.Furthermore, determination of porepressure allows analysis of hydraulicconnectivity and effectiveness of sealssuch as faults.

Conclusions. Seismic interval veloc-ities can be used to generate porepressure sections from surface seis-mic data and well data. Pore-pres-sure prediction provides criticalinformation for the design of futurewells and the understanding of fluidmigration.

Detection of overpressured zonesin the offshore Nile Delta can beachieved through establishing anaccurate seismic velocity-pore pres-sure transform. A seismic velocity-pore pressure transform has beenderived for the Port Fouad Marinegas field in the offshore Nile Delta.

The predicted pore pressure after cal-ibration to formation pore pressuremeasurements indicated differentpore pressure regimes at differentdepths.

This could be adapted to a 3-Dseismic volume to generate a 3-Dpore-pressure cube provided accu-rate interval velocities are available.Inclusion of “while drilling” sonic,resisitivity, and annular pressure datacan enhance the predrill pore-pres-sure model. The pore-pressure pre-diction approach requires integrationof surface and borehole measure-ments to minimize drilling risks andreduce the cost of drilling.

Suggestions for further reading. “Pore-pressure estimation from velocity data:Accounting for pore-pressure mecha-nisms besides undercompaction” byBowers (SPE Drilling and Completion,1995). “Pressure-prediction from seismicdata: Implications for seal distributionand hydrocarbon exploration andexploitation in deepwater Gulf ofMexico” by Dutta (NPF SpecialPublication No. 7, Elsevier, 1007). “Shalecompaction burial diagenesis, and geo-pressures: Adynamical model, solutionsand some results” by Dutta (1st IFP


Figure 9. Pore-pressure prediction for the seismic line in Figure 2, usingthe interval velocity pore-pressure transform and equation A2 with para-meters determined by inverting the mud weight from two wells.

(Continued on p. 1108)

The effective stress tensor, �ij, isdefined to be the difference betweenthe total stress tensor, Sij, and the porepressure, p:

�ij = Sij - p�ij (A1)

Denoting the vertical component ofthe effective stress tensor �ij by �, andthe vertical component of the totalstress tensor Sij by S, the vertical com-ponent of equation A1 may be writ-ten

� = S - p. (A2)

For uniaxial compaction, it is usuallyassumed that the elastic wave veloc-ity is a function only of the verticaleffective stress �. The vertical com-ponent of the total stress, S, at depthH represents the combined weight ofthe fluids and the formation above Hand can be computed if the sedimentdensity is known as a function ofdepth above the location of interest.This may be calculated from an inte-gral of density:

S = g H∫0

�(z) dz, (A3)

where �(z) is the density as a func-tion of depth z.

In the absence of a density log, a fre-quently used method for computingthe overburden stress is the Amocoequation (Traugott, 1977):

�avg(z) = 16.3 + [z/3125]0.6 (A4)

where �avg(z) is the average sedimentdensity in ppg between the seafloorand depth z in feet from the seafloor.

Several pore pressure techniqueshave been based on various mecha-nisms causing the pore pressure. Forexample, Bowers (1995) provides amethod to determine effective stressthat accounts for both undercom-paction and fluid expansion throughdefinition of the unloading curve. Thistechnique has recently been used suc-cessfully in the Gulf of Mexico. Thetechnique is based on the fact that dur-ing compaction (loading) a velocityincrease occurs. During the unloadingprocess, the effective stress is reduceddue to fluid expansion. Fluid expan-sion zones are characterized as zonesof reversal in velocity trend.

The relation between the effectivestress and velocity in normally pres-sured sediments suggested by Bowersis:

V = V0 +A�B, (A5)

Where V0 is the velocity of uncon-solidated fluid-saturated sediments(taken to be 1480 m/s) and A and Bdescribe the variation in velocity withincreasing effective stress and can bederived from offset well data. Theeffective stress can be determinedfrom this equation:

� = [(V - V0)/A)]1/B, (A6)

The pore pressure can then be calcu-lated from equation A2. Bowersobtained the values A=4.4567 andB=0.8168 for Gulf coast wells andA=28.3711 and B=0.6207 for deep-water Gulf regions. LE

Interval velocities were computedfrom the time-depth pairs shownusing an inversion approachdesigned to minimize the effect oferrors in traveltime picking.

Let si =1/vi be the interval slow-ness of layer i, vi the interval veloc-ity of layer i, and ti the measuredtraveltimes at receiver depths zi(i=1,..,N); ti, si and zi are related viathe linear relation

(B1)

In order to minimize the effect oferrors in traveltime picking, this sys-tem was solved using a damped

least-squares inversion employing apenalty function that used the secondderivative of the estimated slownessas a smoothing criterion (Lizarraldeand Swift, 1999).

The interval velocity was foundby inverting traveltimes from theborehole velocity survey by mini-mizing the value of the absolutevalue of �2-1 where

(B2)

Here timeasured is the picked traveltime

at depth zi, and tipredicted is the pre-

dicted traveltime using equation B1and the slownesses obtained byinversion. The standard deviation �iwas taken to be 0.3 ms for all i. LE


Appendix ADetermination of pore pressure from seismic velocity

Appendix BDetermination of interval velocity from the boreholevelocity survey

(From p. 1107)Exploration Research Conference,Caracans, France, 1986). “The equationfor geopressure prediction from welllogs” by Eaton (SPE 5544, 1975).“Estimation of formation pressures fromlog-derived shale properties” byHottman and Johnson (Journal ofPetroleum Technology, 1956). “Smoothinversion of VSP traveltime data” byLizarralde and Swift (GEOPHYSICS, 1999).“Predrill pore pressure prediction usingseismic data” by Sayers et al.(IADC/SPE Drilling Conference, 2000).“Pore/fracture pressure determinations

in deep water” by Traugott (DeepwaterTechnology, supplement to August 1997World Oil). “Seismic pressure-predictionmethod solves problem common indeepwater Gulf of Mexico” by Wilhelmet al. (Oil & Gas Journal, 1998). LE

Acknowledgments: The authors are grateful toBelayim Petroleum Company and Eni Egyptfor permission to publish the data. We thankSteve Montgomery of Schlumberger IPMEgypt for useful discussion.

Corresponding author: [email protected]

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pore pressure offshore nile delta

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