Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco

Reservoir Characterization: Integrating Technology and Business Practices 1233

Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco

Van Dyke, Staffan*Vanco Energy Co.Houston, Texas* Current address: Aera Energy LLCBakersfield, California

Abstract

Analysis for placement of a well in deep watertypically begins with a thorough study of the seafloor.This is followed by a shallow geohazard report, whichis used to identify zones of instability in the shallowsubsurface (faulting, over-pressured zones, etc.).

An example from a 415.55 km2 (160.44 mi2) 3Dseismic survey, offshore Morocco, is presented. Ampli-tude extraction and stratal slice maps were generatedwithin the focus area of the Ras Tafelney 3D seismicdata-set volume. Three horizons have been mapped inthe subsurface to track reflection events that showedbright positive amplitudes. In the survey area, the mainpotential hazards appear to be active sediment path-ways (gullies) and shallow sands, both of which can bethe site for shallow water-flow conditions. Minor fault-ing is present through different stratigraphic intervals

but is relatively insignificant and therefore not consid-ered to be a potential geohazard.

Gullies and canyons are the most prominent fea-tures in the study area. They include active modernsediment pathways, which may be subject to slumpsand slides and therefore may negatively impact nearbyseabed structures. Older groups of buried channels thatmay be sand prone and/or associated with pore pressureanomalies were also mapped.

Sand-rich facies in the near seafloor sedimentcolumn are not in themselves hazards but should becharacterized because of the potential for problemsrelated to setting casing points. Sandy facies are alsohost to shallow water-flow conditions, shallow gas res-ervoirs, and hydrates.

Improvements in estimating drilling risk andcosts that could be carried out include the analysis of

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offset logs, velocity data, sediment properties, and pres-sure data. In concert with the existing seismic data,these data can be used to create pore pressure cross sec-

tions and other displays that may reduce drilling riskand costs.

Introduction

Emerging technologies, such as 3D modeling andvisualization, can be used in conjunction with funda-mental geoscience principles to locate potential zonesof interest for deep- water reservoir targets. Potentialdrilling locations are determined after regional geologi-cal studies, sedimentological and stratigraphic studies,possibly field work, basin modeling, and all interpretedgeophysical data. Typically in frontier areas, the workis highly interpretative due to the lack of data availabil-

ity and content. Thus, the subsequent interpretationsplay a major role in determining potential timing andmigration of the hydrocarbons, as well as determiningthe locations for acceptable source and reservoir rocks.Despite the lack of hard data in these frontier areas,when sound geological principles are applied, the finalinterpretations can make a target very attractive fordrilling.

Methodology

The 3D model that was developed for this studycompared the character of the images in a shallow geo-hazards seismic dataset to that of deeper seismicreflections records, to form a more complete under-standing of the underlying strata and to identify

possible reservoir targets. This comparison is importantbecause shallow seismic records contain a larger com-ponent of high-frequency data than do the underlyingexploration seismic records, thus providing improvedinterpretation of the deeper data.

Study area

The first deep-water well drilled offshoreMorocco, the Shark B-1 was located at latitude31.00846º N and longitude 11.18078ºW, 5n a waterdepth of 2,120 meters (based on 1,493 m/sec watervelocity), and had a TD of 4,162 meters. The well was

drilled in May, 2004, and was operated by VancoEnergy Company in its Ras Tafelney block (Fig. 1).

Analysis for placement of the Shark B-1 wellcommenced with a thorough study of the seafloor. Thiswas accomplished through a shallow geohazard 3D

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seismic survey, which was used to identify zones ofinstability in the shallow subsurface (faulting, over-pressured zones, etc.). A pore-pressure analysis wasdone at the same time to complement interpretationsfrom the shallow hazard study. These studies were thenused in tandem to construct a robust 3D geological/geo-physical model to help determine the character andquality of the potential underlying reservoir strata.

The following figures and content contained inthis paper are based on the shallow geohazard surveyand the pore pressure study from the Shark B-1 wellarea. The area of interest surrounding Shark B-1 is

located offshore northwest Africa in waters adminis-tered by the country of Morocco. The live data area is

approximately 3,000 km2 or about 1,160 mi2. Ampli-tude extraction and stratal slice maps were generatedwithin the focus area of the Ras Tafelney 3D seismicdataset, delineated within the southern region of the fullseismic volume, bounded to the east by inline 100, tothe west by inline 1000, to the south by crossline 1000,to the north by crossline 2600, and covers an area of

415.55 km2 or 160.44 mi2 (Fig. 2).

Pore pressure analysis and shallow geohazard survey

The pore pressure analysis was carried out on a

smaller data set, having an area of 25 km2, and centeredover the well location. The initial seismic velocity vol-ume contained 25 migration velocity functions over the

25 km2 target area. These functions were corrected forwater velocity variations, interpolated and smoothed tocreate an interval depth velocity volume. Ultimately,40,401 velocity functions were analyzed for pore pres-sure anomalies. Each function in the volume wasanalyzed independently for overburden gradient, nor-mal compaction trend, and pore pressure gradient. Thevelocity function at the proposed location was extractedfrom the volume and was analyzed for fracture gradi-ent, kick tolerance, and maximum sustainable columnheight.

Jointly, the pore pressure analysis and the 3Dseismic volume analysis summarized slope instabilityissues related to bathymetry, shallow sediment strengthand coherency, and shallow sediment stratigraphy,including identification of sand prone intervals. In addi-tion, the study provided evidence for shallow waterflow conditions and issues related to sand control. Theshallow geohazard study focused primarily on visualiz-ing seismic anomalies and mapping sedimentaryfeatures related to one or more of the issues listed above(Fig. 3). Specifically, the geohazards study presentedmaps and slices over the first several hundred meters ofthe sedimentary column as well as interpretation andcommentary.

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Geologic settingFigure 4 is a composite seismic line of 2D and 3D

data transecting part of the slope environment. Thecomposite line shows substantial vertical exaggeration.The 3D seismic dataset rests in the lower slope/rise

region of the continental margin. The angle of the slopeof the water bottom in this area averages about 2degrees. Updip, 95 km from the study area, is the shelf/slope break.

Environments of deposition and facies description Figure 5 shows an arbitrary northwest-southeast

seismic line. The top picture represents the uninter-preted line, while the lower picture shows the sameseismic line with interpreted seismic-depositionalfacies overlain. Five main channel-belt facies are recog-nized in the dataset; they are channel-fill facies,proximal levee facies, distal levee facies, sheet fanfacies, and a debrite facies. The interpretation showsaggradationally stacked channel-fills encased within arelatively dim section interpreted to represent shaledeposited by hemipelagic settling.

The channel-fill facies is the most prominent inthe study area (Fig. 6). Channels may be active modernseafloor sediment pathways and may contain channel-fill subfacies, such as slumps and slides, and thereforemay negatively impact nearby seabed structures. Older

groups of buried channels that may be sand prone and/or associated with pore pressure anomalies also havebeen mapped. These buried channels are groupedwithin three major channel-belt systems. Small-scalefans and overbank features are associated with some ofthe gullies. These regions can be the site of rapid depo-sition of coarse clastics. Rapid deposition of the sandand burial of an impermeable shale is a condition asso-ciated with enhanced risk of shallow water flow.

The numerous modern and buried channel-filldeposits also are the site of strong negative amplitudevalues that appear to be caused by sand-rich fans, chan-nel axes, and overbank deposits. This paper documentsthe facies distribution and depositional environment ofthe entire study area.

Seismic interpretationFigure 7 shows a close-up view of the water bot-

tom amplitude extraction; the location of the proposedShark B well is indicated. Three slumps are clearly vis-ible on the surface; one lies northwest of the well

location, another lies south-southeast, and the third liesdirectly northeast of the well location. Sediment wavescan be clearly seen on the seafloor. An amplitudeextraction map is useful for revealing hazards at that

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interface, but it is designed also to see beneath the sur-face, at least for a few hundred meters. Clearly, a timeslice at 1.0 seconds would not follow a stratal surface.Therefore, the four horizons mapped for the shallowgeohazard survey have been used as datums to flattenthe cube volume at these horizons in order to comparewith deeper horizons (discussed below). These four

horizons are: H100 horizon (blue); H160 horizon

(green); H180 horizon (pink); and H200 horizon (yel-

low) (Fig. 6). Also, it should be noted that the faint sub-

vertical lines seen on most amplitude extraction maps

and stratal slices are the acquisition footprint from the

3D seismic survey.

Depositional history of shallow subsurface strata via stratal slices and amplitude extractions

All of the amplitude extractions in this papershow sedimentological features that help to describe thedepositional history of the area. Color bars have beenchosen that are meant to heighten the contrast betweenanomalous amplitudes and the background reflectivity.Strong negative reflection values indicate coarser mate-rial. The area around the channels is typicallyquiescent, therefore positive reflection values areshown in white (for monochrome extractions) and pur-ple (for full colored extractions) and strong positiveamplitudes are in blue (for monochrome extractions)and red (for full colored extractions).

After flattening, time slices reveal much aboutthe structure and properties of the shallow sediments.Figure 8 shows the result at 52 milliseconds below theflattened water bottom. Here, channels cut the seafloor,and sediment transport direction is shown by redarrows. The three slumps previously described are stillpresent, however, one has completely encroached thewell location. Their direction of mass transport is

denoted by green arrows, while the proposed well loca-tion is marked by the yellow dot. At this depth,unconsolidated and unstable conditions should beexpected while drilling.

Figure 9 is an amplitude extraction map gener-ated on the H100 horizon. The slump feature in thenorthwest now encroaches onto the wellsite location.This region may once again cause problems for drillingas the sediments may still be unconsolidated and unsta-ble at such shallow subsurface depths. All three channelsystems are present; stronger amplitudes within theseregions infer they are filled with coarse clastics. Sys-tems 1 and 2 (Fig. 6) are abruptly cut in the northernregions of the study area due to an interpreted slumpblock trending from the northwest to the southeast,moving in a southerly direction.

Figure 10 is an amplitude extraction mapbetween the seafloor and H100 horizon. It shows a pro-nounced slump feature in the northwest region of thestudy area. System 3 shows bifurcation of its channel

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directly south of the well location. The region immedi-ately surrounding the well location shows a debris-flowcharacter in the seismic response. Sediment waves alsoappear to be present. This region is once again inter-preted to contain a relatively thick package of debritesthat may cause instability while drilling.

Figure 11 shows a time slice from the flattenedcube volume generated from the H160 horizon. Figure11 is located 52 milliseconds below the flattened hori-zon and shows the prominent northwest-southeasttrending slump feature terminating directly west of thewellsite. A large expression of a sheet fan is also inter-preted to be fed from the mouth of System 3 into thesouthwestern part of the study area.

Figure 12 is an amplitude extraction mapbetween the H160 and H180 horizons. Channel Sys-tems 1 and 2 can no longer be seen because the conduitthat fed these systems is not present at this time. Chan-nel System 3 dominates the map by its strong positivereflection character; it has a broader geometry thanbefore, particularly in the southwest regions where faintfan-like geometry is visible. A northwest to southeasttrending debris flow deposit dominates the relativelyquiescent area outside of System 3’s environment. Theregion immediately surrounding the well locationshows stronger reflectors than previous maps and isthus interpreted as a debris-flow. This interval may be a

potential hazard for drilling because the sediments maystill be unconsolidated and unstable at these shallowsubsea depths.

Figure 13 is an amplitude extraction mapbetween the H180 and H200 horizons. Channel Sys-tems 1 and 2 are not present within this time interval,but Channel 3 is present. The dominant feature, how-ever, is the paleo-Agadir Canyon located in thesouthern region of the study area. The region immedi-ately surrounding the well location shows very weakreflection events, interpreted to represent a thick,argillaceous sequence deposited during this timeinterval.

Figure 14 is a stratal slice located 100 millisec-onds above the H200 horizon that was generated fromthe flattened cube volume from that horizon. The mainfeature is the east-west trending paleo-Agadir Canyon,which exhibits extremely strong negative reflectionswithin the canyon boundaries. Another large debris-flow deposit occurs in the northern channel margin.

Figure 15 shows a gated window of 40 millisec-onds at about the H200 horizon (20 ms above and 20ms below). The main feature manifested on the map is aseries of extremely strong negative reflections confinedwithin the paleo-Agadir Canyon, but the region aroundthe wellsite remains quiescent.

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Economics of shallow hazard survey

Most shallow hazard surveys are accompanied bya pore-pressure analysis. Pore pressure imaging isbased on mapping seismic velocity to pore pressureusing a petrophysical model. Any uncertainty in thevelocity analysis will impact the pore pressure predic-tion, as well as the calibration of the pore pressuremodel. Thus, pore pressure prediction is heavily relianton a well developed, and accurate, velocity model. Typ-ically this is not exact, but it can be close to exact if ahigh-quality seismic dataset is available and the propertime is spent developing the velocity model. Particu-larly in frontier regions, where little, if any, true datasuch as well logs are available, a pore pressure analysiswill lead to a better understanding of the shallow sedi-ments and their associated geohazard risk potential.

In order to grasp the economic impact thataccompanies a potential hazard disclosed by the shal-low hazard survey or pore pressure prediction analysis,a dendrogram can be drawn to gather a more completeunderstanding of each hazard. This diagram can beaccompanied by a spreadsheet that relates a quantitativecost to each hazard represented in the diagram; an over-simplified dendrogram can be seen in Figure 16. Eachbranch, or pathway, on the dendrogram represents apotential hazard and associated cost based on its riskpercentage; therefore there can be numerous brancheson each diagram. Depending on the size and detail ofthe shallow hazard survey and the pore pressure predic-tion analysis, nearly infinite branches can exist.

Typically, a branch on the diagram is representative ofa particular horizon (geologic time or event) and itsassociated hazard (e.g., an over-pressured zone).

The best way to quantify a numerical value forthe economic exposure to each potential geohazard is tomultiply the likelihood of the risk’s existence versus thetotal cost of alleviating the problem (e.g., 20% risk ofencountering an over-pressured zone in the Oligocene/Miocene boundary at a 200m depth below the mud lineat location X,Y). Historically, no oil blowout hasoccurred in deep water, but the industry has spent hun-dreds of millions of dollars actively preventing shallowwater flows and underground blowouts. Thus far, how-ever, three broached mud line gas flows and one BOPfailure resulting in a gas blow-out have occurred in thedeep water. The estimated costs of these four eventstotaled $40MM. During a gas or oil blowout, the loss ofthe ~$1MM tool assembly and the additional 10 days tosidetrack and recover the lost hole, can cost the operatora significant amount of money to get back to normaldrilling operations; e.g., a minor underground blowoutcan result in as much as an $8MM claim. It should benoted that many of these well control problems areunique to deep-water operations.

Many other potential risks exist, such as encoun-tering a shallow gas pocket that might be breached, thusreleasing the gas upward, engulfing the drill ship orsemi-submersible, and in a worst case scenario, sinkingthe vessel. The costs involved with this type of disaster

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can be many hundred of millions of dollars, not includ-ing the potential for loss in human life. It should benoted, that nowadays, total costs for some deepwaterwells can be as high as $95MM, and daily rig costs canbe >$500,000/day.

Evaluating the economic impact that these poten-tial disasters can cause is a helpful and debatablynecessary exercise that should accompany all shallow

hazard reports in any region of the world where a deep-water well is to be drilled. In the example for this paper,the shallow Moroccan sediments studied are consideredto be relatively tight and there is little risk for over-pressured zones or other anomalies that are generallyencountered in other deep-water regions of the world,such as those commonly found in the Gulf of Mexico.

ConclusionsIn summation, these studies suffered from a lack

of well control, shallow geo-boring data, and drillingand wireline data from deeper boreholes. If such datacould have been obtained, then many of the assump-tions as to sediment and fluid characteristics of the area

could perhaps be verified. However, all of the maps andstratal slices that were generated from these studieshelp to show the basic geometry of the channel systemspresent, which can be inferred to deeper reservoir tar-gets for their improved characterization.

AcknowledgementsThe author would like to thank Dr. Roger Slatt,

Vanco Energy Company, and Aera Energy LLC forproviding the opportunity to present the material withinthis paper.


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Figure 1. Ras Tafelney block, offshore Morocco.

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Figure 2. Ras Tafelney 3D seismic dataset with area of interest highlighted. Proposed well location is shown by yellowcircle.

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Figure 3. Example of amplitude extractions performed within shallow subsurface between H100 and H160 (see text forexplanation of symbols).

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Figure 4. West-east composite seismic transect showing shelf-slope break. Vertical exaggeration is 3X.

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Figure 5. Uninterpreted and interpreted seismic line showing channel subenvironments.

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Figure 6. Seismic line showing the 3 major channel-belt systems. Crossline 1830.

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Figure 7. Maximum peak amplitude extraction of seafloor showing slumps and slides (outlined in yellow); greenarrows point in the direction of down-slope movement, sediment waves, and channels; and red arrows point in thedirection of presumed sediment transport.

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Figure 8. Flattened time slice 52 ms below the seafloor showing the features illustrated in Figure 7.

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Figure 9. Maximum peak amplitude extraction for H100 Horizon showing slumps/slides and sediment transport direc-tions.

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Figure 10. RMS amplitude extraction between the seafloor and H100 horizon.

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f

Figure 11. Flattened time slice 52 ms below the H160 horizon showing slumps, channels, and sheet fan.

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Figure 12. RMS amplitude extraction between H160 and H180 horizons.

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Figure 13. RMS amplitude extraction between H180 and H200 horizons showing the paleo Agidar Canyon.

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Figure 14. Flattened time slice 100ms above the H200 horizon showing paleo Agadar Canyon and slump scar.

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Figure 15. H200 horizon amplitude extraction with 40ms gated window (20ms above and below) showing the paleoAgadir Canyon.

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Figure 16. Simplified representation of a typical dendrogram.

PotentialGeohazard

GeologicBoundary Rock Type Probability Projected

Cost

PossibleOil

Blowout

PossibleGas

Blowout

LowerPliocene

Plio-MioceneBoundary

UpperMiocene

LowerMiocene

SS

Sh

LS

0%

10%

25%

50%

$5 MM

$8 MM

$10 MM

$15 MM