We predict stresses and strains in the Tarfaya salt basin on the West African Coast using a 3D static geomechanical model and compare the results against a simplified 2D plane-strain model. Both models are based on present-day basin geometries, are drained and use a poro-elastic description for the sediments and visco-plastic description for salt. We focus on a salt diapir, where an exploratory well has been drilled crossing a major fault. The 3D model shows a significant horizontal stress reduction in sediments at the top of the diapir, validated with measured data later obtained from the well. The 2D model predicts comparable stress reduction in sediments at the crest of the diapir. However, it shows a broader area affected by the stress reduction, overestimating its magnitude by as much as 1.5MPa. Both models predict a similar pattern of differential displacement in sediments along both sides of the major fault, above the diapir. These displacements are the main cause of horizontal stress reduction detected at the crest of the diapir. Sensitivity analysis in both models show that the elastic parameters of the sediments have minimal effect on the stress-strain behavior. In addition, the 2D sensitivity analysis concludes that the main factors controlling stress and strain changes are the geometry of the salt and the difference in rock properties between encasing sediments and salt. Overall, our study demonstrates that carefully built 2D models at the exploration stage can provide stress information and useful insights comparable to those from more complex 3D geometries.

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This paper presents field results obtained while drilling difficult offshore wells in Azerbaijan and Brazil using a new service that delivers conventional check-shot data while drilling. The operation is similar to a conventional wireline operation in that it uses a surface-deployed source and downhole receiver. The key difference between this service and wireline seismic services is that the receiver is included in the drilling assembly; therefore, drilling does not have to be stopped to take measurements. The first case study presents results from the A-1 deviated wildcat well in the South Caspian Sea, where the service was used to reduce depth uncertainty while drilling. The tool delivered real-time check-shot data that were accurate to within 3 ms of a conventional vertical seismic profile (VSP) over the same interval (equivalent to a 10-m depth error at a 4-km depth). In the second case study, seismic data from the B-2 well in Brazil were used to set casing in a narrow window between a fault and the primary reservoir target. The data enabled safe and successful installation of the casing. Both operations were completed with zero rig downtime. Data accuracy was suitable for real-time decisions and eliminated the need to stop drilling to run intermediate wireline VSP surveys. The service offers the industry new options for reducing uncertainty in the well construction process, and it can therefore make a significant impact on cost and safety while drilling a well. Introduction Drillers are often confronted by large depth uncertainties on key targets while drilling exploration wells. These uncertainties arise from inaccurate velocity models that may have been derived solely from the processing of the surface seismic data. As a result, depth errors may be as high as thousands of feet when there are no nearby wells to calibrate the velocity models. Wireline borehole seismic surveys are frequently run to reduce the uncertainty, but the information may come too late to have an impact on the well construction process. In addition, the drilling operation must be interrupted to acquire the wireline data, and significant additional costs may be incurred, particularly in deepwater environments where rig rates are high. Drilling operations have been shown to benefit from real-time seismic techniques that can deliver the necessary time-depth or check-shot information to place the well on the seismic map in real time.1 Aside from the immediate savings of rig time, bigger value may come from improved wellbore construction. Key applications of real-time borehole seismic information include: casing/coring point selection, target depth identification and optimal landing in the reservoir. Large savings may be realized through avoidance of contingent casing strings and sidetracks. Production may be improved by entering the reservoir in the "sweet spot." Time-depth information can easily be converted into formation velocity to improve velocity and pore-pressure models. When available in real time, the velocity information contributes to pore-pressure management and improves the safety of the drilling operation. Real-time velocities may also be used to improve the seismic image through reprocessing of the surface seismic data. Until recently, the only real-time seismic technique available used the noise generated by the drill bit while drilling as a seismic source and receivers placed on the surface to record borehole seismic data. The technique known as Drill Bit Seismic has been tested in most environments, and the operating limitations are now well understood.2 However, this technique is generally ineffective in soft sediments, horizontal wells, and while drilling with polycrystalline diamond compact (PDC) bits.