New Integrated Seismic Technology Improves Well Placement

The primary inputs to drilling planning and execution are from seismic and well data. An earth model consisting of structural information with geological targets and faults and with relevant formation properties such as pore pressure and fracture gradient guide drilling decisions to help place the well in the right “geological” target and to avoid drilling hazards. The best possible accuracy and resolution at the well location is required; therefore, all available information needs to be optimally combined and used with the latest model-building and imaging technologies.

The lack of adequate technologies and measurements, along with turnaround time limitations, have made this type of optimum use of seismic and well data (predrill and while drilling) impractical until now. Recent developments in model building, new rapid and more accurate imaging technologies, and the availability of new well measurements are making this optimum combination more of a reality. This new approach, seismic-guided drilling (SGD), has been used in a Gulf of Mexico (GoM) well, demonstrating the potential value it can provide to drillers.

Components Of A Seismic Project For Drilling

Three key components of this approach differentiate it from other applications. The first is a small volume of interest. An earth model relevant for drilling a particular well covers a volume that includes the planned well trajectory and possibly neighboring locations with well or geological information – the drilling volume of interest (DVI). The size of the DVI typically is much smaller than that used for other seismic applications such asregional imaging. The relatively small data volume allows the rapid use of sophisticated techniques to build and update the best possible high-resolution earth model in the DVI.

The SGD project execution consists of three phases: feasibility, drilling baseline model, and while-drilling updates. (Images courtesy of WesternGeco)

The second component is integrating well information. In a drilling project, there is at least one well – i.e., the one being drilled – in addition to any offset wells. Availability of new well measurements, such as checkshots and vertical seismic profiles (VSPs) while drilling, and technologies that can optimally integrate them with seismic information to constrain the local earth model, such as well-constrained tomography, are critically important.

The final approach is a fast turnaround time – “in time” for drilling decisions. The turnaround time of earth model building/updating and reimaging needs to be performed in the driller’s time scale, in time for decision-making for both baseline and while-drilling updates. New high-end imaging algorithms such as Gaussian packet migration that can handle complicated geology and produce rapid remigrations are required.

Drilling Challenges

Seismic information cannot address all of the challenges drillers face, but seismic data can help with a number of important ones. One area is better well placement. This includes:

-Reducing uncertainty in the location of drilling targets, including casing points, faults, and target reservoir(s);

-Quantifying uncertainty of drilling target locations; and

In a GoM well (Well C), a primary challenge was to place the 13/8-in. casing below a secondary fault. This was necessary for the hole size requirements in the final well completion. Locating both primary and secondary faults accurately was deemed critical. Large uncertainties were expected in the positioning of events using the existing seismic image. It was important to improve the velocity model and reimage while drilling to reduce the positional uncertainty of the fault locations.

Improving reservoir property and structure definition through inversion. Seismic data also can help avoid drilling hazards through shallow hazard identification improvements, pore pressure and fracture gradient estimates, reducing and quantifying uncertainties in estimates, and identifying other regional hazards such as tar and gas hydrates.

Project Execution

An SGD project starts with identifying challenges and risks expected when drilling a particular well. A baseline earth model is constructed, and the model is updated while drilling. There are three phases of project execution. During the feasibility phase, candidate solutions and technologies (e.g., the type of required migration/inversion techniques) for the expected drilling challenges are identified. Existing seismic, offset well data (if available), and other reference information are analyzed to determine if they are appropriate for the proposed techniques. Illumination studies are conducted for the key drilling targets. Uncertainty measures regarding the structures, fault locations, and drilling hazards are evaluated. This phase ends with a go/no-go recommendation based on whether project objectives can be met.

The panel on the left shows the legacy image that existed prior to the GoM project. An anisotropic velocity model was built using the offset well (Well B) data, and seismic data were depth-migrated. The panel on the right shows the result giving the best possible predrill image at the baseline stage. There was a significant (>750-ft or 230-m) depth shift in the new image compared with the legacy image.

The best possible earth model with all predrill information available is constructed in the drilling baseline phase. This model serves as the starting point for while-drilling updates in the next phase. The drilling baseline earth model includes an anisotropic local velocity model, a depth-migrated high-resolution image, interpreted horizons and faults, and drilling hazard estimates.

The final phase is the while-drilling updates phase. As the well is being drilled, new local information becomes available for updating the earth model in the DVI. This is particularly valuable in an exploration case, where limited or no nearby offset well data are available.

Updates, including pore pressure and fracture gradient estimates, are done on a predetermined schedule or as required. Real-time LWD data (checkshots and logs) are used for while-drilling updates. Recorded-mode LWD and intermediate wireline VSPs and logs are used to update the model and the image between bit runs. Local anisotropic velocity updates are obtained usingreflection tomography with check-shots and/or well tops used as constraints so the model is consistent with well data and seismic information. The structural depth image is updated by rapid remigration enabled by massively parallel computing resources. Recent developments in algorithms such as Gaussian packet migration have further decreased the turnaround time for these remigrations.

The panel on the left is the legacy image that existed prior to the GoM project. The panel on the right is the seismic image after the final update.

In one example, LWD, check-shot, and wireline data were acquired up to the mudline to complement the offset well for a good velocity model. Anisotropic velocity models were created in several stages by seismic tomography where the vertical velocities were constrained by well data. The volume for velocity models included the offset well to ensure a proper tie to that well in addition to the new well.

For each updated model, surface seismic data were reimaged with an updated model during drilling, enhancing fault location accuracy to impact drilling and casing decisions. The desired casinglocation was predicted within +/- 50 ft (15 m). The prediction was performed using reimaging performed while the drill bit was approximately 1,500 ft (458 m) above the planned casing depth.

Drilling planning and execution can benefit greatly from an accurate high-resolution earth model obtained from seismic data integrated with well information. Using recent developments in rapid and accurate imaging technologies and the availability of new well measurements, SGD allows this earth model to be regularly updatedwhile drilling the well, enabling optimum well placement and drilling hazard management.