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

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

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Analysis for placement of a well in deep water typically begins with a thorough study of the seafloor. This is followed by a shallow geohazard report, which is used to identify zones of instability in ...

Analysis for placement of a well in deep water typically begins with a thorough study of the seafloor. This is followed by a shallow geohazard report, which is used to identify zones of instability in the shallow subsurface (faulting, over-pressured zones, etc.). An example from a 415.55 km2 (160.44 mi2) 3D seismic survey, offshore Morocco, is presented. Amplitude extraction and stratal slice maps were generated within the focus area of the Ras Tafelney 3D seismic data-set volume. Three horizons have been mapped in the subsurface to track reflection events that showed bright positive amplitudes. In the survey area, the main potential hazards appear to be active sediment pathways (gullies) and shallow sands, both of which can be
the site for shallow water-flow conditions. Minor faulting is present through different stratigraphic intervals but is relatively insignificant and therefore not considered to be a potential geohazard. Gullies and canyons are the most prominent features in the study area. They include active modern sediment pathways, which may be subject to slumps and slides and therefore may negatively impact nearby seabed structures. Older groups of buried channels that may be sand prone and/or associated with pore pressure anomalies were also mapped. Sand-rich facies in the near seafloor sediment column are not in themselves hazards but should be characterized because of the potential for problems related to setting casing points. Sandy facies are also host to shallow water-flow conditions, shallow gas reservoirs, and hydrates.
Improvements in estimating drilling risk and costs that could be carried out include the analysis of offset logs, velocity data, sediment properties, and pressure data. In concert with the existing seismic data, these data can be used to create pore pressure cross sections and other displays that may reduce drilling risk and costs.

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

    • 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 LLC Bakersfield, California Abstract Analysis for placement of a well in deep water typically begins with a thorough study of the seafloor. This is followed by a shallow geohazard report, which is used to identify zones of instability in the shallow subsurface (faulting, over-pressured zones, etc.). An example from a 415.55 km2 (160.44 mi2 ) 3D seismic survey, offshore Morocco, is presented. Ampli- tude extraction and stratal slice maps were generated within the focus area of the Ras Tafelney 3D seismic data-set volume. Three horizons have been mapped in the subsurface to track reflection events that showed bright positive amplitudes. In the survey area, the main potential hazards appear to be active sediment path- ways (gullies) and shallow sands, both of which can be the 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 modern sediment pathways, which may be subject to slumps and slides and therefore may negatively impact nearby seabed structures. Older groups of buried channels that may be sand prone and/or associated with pore pressure anomalies were also mapped. Sand-rich facies in the near seafloor sediment column are not in themselves hazards but should be characterized because of the potential for problems related to setting casing points. Sandy facies are also host to shallow water-flow conditions, shallow gas res- ervoirs, and hydrates. Improvements in estimating drilling risk and costs that could be carried out include the analysis of 4 3 Papers Start Author Search Help Print 8.5 x 11
    • Van Dyke 1234 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 risk and costs. Introduction Emerging technologies, such as 3D modeling and visualization, can be used in conjunction with funda- mental geoscience principles to locate potential zones of interest for deep- water reservoir targets. Potential drilling locations are determined after regional geologi- cal studies, sedimentological and stratigraphic studies, possibly field work, basin modeling, and all interpreted geophysical data. Typically in frontier areas, the work is highly interpretative due to the lack of data availabil- ity and content. Thus, the subsequent interpretations play a major role in determining potential timing and migration of the hydrocarbons, as well as determining the locations for acceptable source and reservoir rocks. Despite the lack of hard data in these frontier areas, when sound geological principles are applied, the final interpretations can make a target very attractive for drilling. Methodology The 3D model that was developed for this study compared the character of the images in a shallow geo- hazards seismic dataset to that of deeper seismic reflections records, to form a more complete under- standing of the underlying strata and to identify possible reservoir targets. This comparison is important because shallow seismic records contain a larger com- ponent of high-frequency data than do the underlying exploration seismic records, thus providing improved interpretation of the deeper data. Study area The first deep-water well drilled offshore Morocco, the Shark B-1 was located at latitude 31.00846º N and longitude 11.18078ºW, 5n a water depth of 2,120 meters (based on 1,493 m/sec water velocity), and had a TD of 4,162 meters. The well was drilled in May, 2004, and was operated by Vanco Energy Company in its Ras Tafelney block (Fig. 1). Analysis for placement of the Shark B-1 well commenced with a thorough study of the seafloor. This was accomplished through a shallow geohazard 3D 4 3 7 Papers Start Author Search Help
    • 1235 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco seismic survey, which was used to identify zones of instability in the shallow subsurface (faulting, over- pressured zones, etc.). A pore-pressure analysis was done at the same time to complement interpretations from the shallow hazard study. These studies were then used in tandem to construct a robust 3D geological/geo- physical model to help determine the character and quality of the potential underlying reservoir strata. The following figures and content contained in this paper are based on the shallow geohazard survey and the pore pressure study from the Shark B-1 well area. 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 generated within the focus area of the Ras Tafelney 3D seismic dataset, delineated within the southern region of the full seismic volume, bounded to the east by inline 100, to the 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 centered over the well location. The initial seismic velocity vol- ume contained 25 migration velocity functions over the 25 km2 target area. These functions were corrected for water velocity variations, interpolated and smoothed to create an interval depth velocity volume. Ultimately, 40,401 velocity functions were analyzed for pore pres- sure anomalies. Each function in the volume was analyzed independently for overburden gradient, nor- mal compaction trend, and pore pressure gradient. The velocity function at the proposed location was extracted from the volume and was analyzed for fracture gradi- ent, kick tolerance, and maximum sustainable column height. Jointly, the pore pressure analysis and the 3D seismic volume analysis summarized slope instability issues related to bathymetry, shallow sediment strength and coherency, and shallow sediment stratigraphy, including identification of sand prone intervals. In addi- tion, the study provided evidence for shallow water flow conditions and issues related to sand control. The shallow geohazard study focused primarily on visualiz- ing seismic anomalies and mapping sedimentary features related to one or more of the issues listed above (Fig. 3). Specifically, the geohazards study presented maps and slices over the first several hundred meters of the sedimentary column as well as interpretation and commentary. 4 3 7 Papers Start Author Search Help
    • Van Dyke 1236 Geologic setting Figure 4 is a composite seismic line of 2D and 3D data transecting part of the slope environment. The composite line shows substantial vertical exaggeration. The 3D seismic dataset rests in the lower slope/rise region of the continental margin. The angle of the slope of the water bottom in this area averages about 2 degrees. 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 same seismic line with interpreted seismic-depositional facies overlain. Five main channel-belt facies are recog- nized in the dataset; they are channel-fill facies, proximal levee facies, distal levee facies, sheet fan facies, and a debrite facies. The interpretation shows aggradationally stacked channel-fills encased within a relatively dim section interpreted to represent shale deposited by hemipelagic settling. The channel-fill facies is the most prominent in the study area (Fig. 6). Channels may be active modern seafloor sediment pathways and may contain channel- fill subfacies, such as slumps and slides, and therefore may negatively impact nearby seabed structures. Older groups of buried channels that may be sand prone and/ or associated with pore pressure anomalies also have been mapped. These buried channels are grouped within three major channel-belt systems. Small-scale fans and overbank features are associated with some of the gullies. These regions can be the site of rapid depo- sition of coarse clastics. Rapid deposition of the sand and burial of an impermeable shale is a condition asso- ciated with enhanced risk of shallow water flow. The numerous modern and buried channel-fill deposits also are the site of strong negative amplitude values that appear to be caused by sand-rich fans, chan- nel axes, and overbank deposits. This paper documents the facies distribution and depositional environment of the entire study area. Seismic interpretation Figure 7 shows a close-up view of the water bot- tom amplitude extraction; the location of the proposed Shark 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 lies directly northeast of the well location. Sediment waves can be clearly seen on the seafloor. An amplitude extraction map is useful for revealing hazards at that 4 3 7 Papers Start Author Search Help
    • 1237 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco interface, but it is designed also to see beneath the sur- face, at least for a few hundred meters. Clearly, a time slice at 1.0 seconds would not follow a stratal surface. Therefore, the four horizons mapped for the shallow geohazard survey have been used as datums to flatten the cube volume at these horizons in order to compare with 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 paper show sedimentological features that help to describe the depositional history of the area. Color bars have been chosen that are meant to heighten the contrast between anomalous amplitudes and the background reflectivity. Strong negative reflection values indicate coarser mate- rial. The area around the channels is typically quiescent, therefore positive reflection values are shown in white (for monochrome extractions) and pur- ple (for full colored extractions) and strong positive amplitudes are in blue (for monochrome extractions) and red (for full colored extractions). After flattening, time slices reveal much about the structure and properties of the shallow sediments. Figure 8 shows the result at 52 milliseconds below the flattened water bottom. Here, channels cut the seafloor, and sediment transport direction is shown by red arrows. The three slumps previously described are still present, however, one has completely encroached the well 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 be expected while drilling. Figure 9 is an amplitude extraction map gener- ated on the H100 horizon. The slump feature in the northwest now encroaches onto the wellsite location. This region may once again cause problems for drilling as the sediments may still be unconsolidated and unsta- ble at such shallow subsurface depths. All three channel systems are present; stronger amplitudes within these regions infer they are filled with coarse clastics. Sys- tems 1 and 2 (Fig. 6) are abruptly cut in the northern regions of the study area due to an interpreted slump block trending from the northwest to the southeast, moving in a southerly direction. Figure 10 is an amplitude extraction map between the seafloor and H100 horizon. It shows a pro- nounced slump feature in the northwest region of the study area. System 3 shows bifurcation of its channel 4 3 7 Papers Start Author Search Help
    • Van Dyke 1238 directly south of the well location. The region immedi- ately surrounding the well location shows a debris-flow character in the seismic response. Sediment waves also appear to be present. This region is once again inter- preted to contain a relatively thick package of debrites that may cause instability while drilling. Figure 11 shows a time slice from the flattened cube volume generated from the H160 horizon. Figure 11 is located 52 milliseconds below the flattened hori- zon and shows the prominent northwest-southeast trending slump feature terminating directly west of the wellsite. A large expression of a sheet fan is also inter- preted to be fed from the mouth of System 3 into the southwestern part of the study area. Figure 12 is an amplitude extraction map between the H160 and H180 horizons. Channel Sys- tems 1 and 2 can no longer be seen because the conduit that fed these systems is not present at this time. Chan- nel System 3 dominates the map by its strong positive reflection character; it has a broader geometry than before, particularly in the southwest regions where faint fan-like geometry is visible. A northwest to southeast trending debris flow deposit dominates the relatively quiescent area outside of System 3’s environment. The region immediately surrounding the well location shows stronger reflectors than previous maps and is thus interpreted as a debris-flow. This interval may be a potential hazard for drilling because the sediments may still be unconsolidated and unstable at these shallow subsea depths. Figure 13 is an amplitude extraction map between 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 the southern region of the study area. The region immedi- ately surrounding the well location shows very weak reflection events, interpreted to represent a thick, argillaceous sequence deposited during this time interval. Figure 14 is a stratal slice located 100 millisec- onds above the H200 horizon that was generated from the flattened cube volume from that horizon. The main feature is the east-west trending paleo-Agadir Canyon, which exhibits extremely strong negative reflections within 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 20 ms below). The main feature manifested on the map is a series of extremely strong negative reflections confined within the paleo-Agadir Canyon, but the region around the wellsite remains quiescent. 4 3 7 Papers Start Author Search Help
    • 1239 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco Economics of shallow hazard survey Most shallow hazard surveys are accompanied by a pore-pressure analysis. Pore pressure imaging is based on mapping seismic velocity to pore pressure using a petrophysical model. Any uncertainty in the velocity analysis will impact the pore pressure predic- tion, as well as the calibration of the pore pressure model. Thus, pore pressure prediction is heavily reliant on a well developed, and accurate, velocity model. Typ- ically this is not exact, but it can be close to exact if a high-quality seismic dataset is available and the proper time is spent developing the velocity model. Particu- larly in frontier regions, where little, if any, true data such as well logs are available, a pore pressure analysis will lead to a better understanding of the shallow sedi- ments and their associated geohazard risk potential. In order to grasp the economic impact that accompanies a potential hazard disclosed by the shal- low hazard survey or pore pressure prediction analysis, a dendrogram can be drawn to gather a more complete understanding of each hazard. This diagram can be accompanied by a spreadsheet that relates a quantitative cost to each hazard represented in the diagram; an over- simplified dendrogram can be seen in Figure 16. Each branch, or pathway, on the dendrogram represents a potential hazard and associated cost based on its risk percentage; therefore there can be numerous branches on each diagram. Depending on the size and detail of the shallow hazard survey and the pore pressure predic- tion analysis, nearly infinite branches can exist. Typically, a branch on the diagram is representative of a particular horizon (geologic time or event) and its associated hazard (e.g., an over-pressured zone). The best way to quantify a numerical value for the economic exposure to each potential geohazard is to multiply the likelihood of the risk’s existence versus the total cost of alleviating the problem (e.g., 20% risk of encountering an over-pressured zone in the Oligocene/ Miocene boundary at a 200m depth below the mud line at location X,Y). Historically, no oil blowout has occurred in deep water, but the industry has spent hun- dreds of millions of dollars actively preventing shallow water flows and underground blowouts. Thus far, how- ever, three broached mud line gas flows and one BOP failure resulting in a gas blow-out have occurred in the deep water. The estimated costs of these four events totaled $40MM. During a gas or oil blowout, the loss of the ~$1MM tool assembly and the additional 10 days to sidetrack and recover the lost hole, can cost the operator a significant amount of money to get back to normal drilling operations; e.g., a minor underground blowout can result in as much as an $8MM claim. It should be noted that many of these well control problems are unique to deep-water operations. Many other potential risks exist, such as encoun- tering a shallow gas pocket that might be breached, thus releasing the gas upward, engulfing the drill ship or semi-submersible, and in a worst case scenario, sinking the vessel. The costs involved with this type of disaster 4 3 7 Papers Start Author Search Help
    • Van Dyke 1240 can be many hundred of millions of dollars, not includ- ing the potential for loss in human life. It should be noted, that nowadays, total costs for some deepwater wells can be as high as $95MM, and daily rig costs can be >$500,000/day. Evaluating the economic impact that these poten- tial disasters can cause is a helpful and debatably necessary 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 considered to be relatively tight and there is little risk for over- pressured zones or other anomalies that are generally encountered in other deep-water regions of the world, such as those commonly found in the Gulf of Mexico. Conclusions In summation, these studies suffered from a lack of well control, shallow geo-boring data, and drilling and wireline data from deeper boreholes. If such data could 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 and stratal slices that were generated from these studies help to show the basic geometry of the channel systems present, which can be inferred to deeper reservoir tar- gets for their improved characterization. Acknowledgements The author would like to thank Dr. Roger Slatt, Vanco Energy Company, and Aera Energy LLC for providing the opportunity to present the material within this paper. 4 3 7 Papers Start Author Search Help
    • Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco 1241 Figure 1. Ras Tafelney block, offshore Morocco. 4 3 7 Papers Start Author Search Help k
    • Van Dyke 1242 Figure 2. Ras Tafelney 3D seismic dataset with area of interest highlighted. Proposed well location is shown by yellow circle. k 4 3 7 Papers Start Author Search Help
    • 1243 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco Figure 3. Example of amplitude extractions performed within shallow subsurface between H100 and H160 (see text for explanation of symbols). k 4 3 7 Papers Start Author Search Help
    • Van Dyke 1244 Figure 4. West-east composite seismic transect showing shelf-slope break. Vertical exaggeration is 3X. k 4 3 7 Papers Start Author Search Help
    • 1245 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco Figure 5. Uninterpreted and interpreted seismic line showing channel subenvironments. k 4 3 7 Papers Start Author Search Help
    • Van Dyke 1246 Figure 6. Seismic line showing the 3 major channel-belt systems. Crossline 1830. k 4 3 7 Papers Start Author Search Help
    • 1247 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco Figure 7. Maximum peak amplitude extraction of seafloor showing slumps and slides (outlined in yellow); green arrows point in the direction of down-slope movement, sediment waves, and channels; and red arrows point in the direction of presumed sediment transport. k 4 3 7 Papers Start Author Search Help
    • Van Dyke 1248 Figure 8. Flattened time slice 52 ms below the seafloor showing the features illustrated in Figure 7. 4 3 7 Papers Start Author Search Help k
    • 1249 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco Figure 9. Maximum peak amplitude extraction for H100 Horizon showing slumps/slides and sediment transport direc- tions. k 4 3 7 Papers Start Author Search Help
    • Van Dyke 1250 Figure 10. RMS amplitude extraction between the seafloor and H100 horizon. 4 3 7 Papers Start Author Search Help k
    • 1251 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco f Figure 11. Flattened time slice 52 ms below the H160 horizon showing slumps, channels, and sheet fan. 4 3 7 Papers Start Author Search Help k
    • Van Dyke 1252 Figure 12. RMS amplitude extraction between H160 and H180 horizons. 4 3 7 Papers Start Author Search Help k
    • 1253 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco Figure 13. RMS amplitude extraction between H180 and H200 horizons showing the paleo Agidar Canyon. 4 3 7 Papers Start Author Search Help k
    • Van Dyke 1254 Figure 14. Flattened time slice 100ms above the H200 horizon showing paleo Agadar Canyon and slump scar. 4 3 7 Papers Start Author Search Help k
    • 1255 Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco Figure 15. H200 horizon amplitude extraction with 40ms gated window (20ms above and below) showing the paleo Agadir Canyon. 4 3 7 Papers Start Author Search Help k
    • Van Dyke 1256 Figure 16. Simplified representation of a typical dendrogram. Potential Geohazard Geologic Boundary Rock Type Probability Projected Cost Possible Oil Blowout Possible Gas Blowout Lower Pliocene Plio- Miocene Boundary Upper Miocene Lower Miocene SS Sh LS 0% 10% 25% 50% $5 MM $8 MM $10 MM $15 MM 4 3 7 Papers Start Author Search Help k