Frontier drilling in deepwater environments is challenging, with a wide range of risks that oil operators need to evaluate carefully because of high investment costs. The formation targets selected are typically depth-transformed using a velocity function obtained from conventional 3D surface-seismic data. The data are usually low resolution and may not have been processed with the best prestack-depth-migration (PSDM) techniques. This pitfall often leads to uncertainties in reaching drilling targets and completing the well on time. Common uncertainties faced by drillers include target confirmation ahead of an intermediate-depth section and the distance to these targets. One approach to reducing these uncertainties is to use borehole-seismic techniques to record a vertical seismic profile (VSP) at intermediate total depth (TD) to look ahead and estimate the target depths below the bit.
Oil and Natural Gas Corporation Limited (ONGC), India's largest oil operator, has been using this simple, effective technique in drilling deepwater wells. In one case, the look-ahead prediction resulted in stopping of drilling operations because ONGC needed to ascertain if there was reservoir rock below the volcanics. The unambiguous VSP result did not show any possible reservoir below 3987-m measured depth (MD), which saved 3.5 weeks of drilling to reach the predrilling planned TD of 4415–m MD. In another example, ONGC needed to complete a vertical well that extended to a basaltic basement with TD planned at 4324 m at a subsea water depth of 2135 m. Three look-ahead VSP runs were performed at various sections for guidance throughout the drilling process, setting the casing, and reaching the deeper final target. Final TD was 6205-m MD as opposed to the planned TD of 4324–m MD.
While VSP techniques have been widely used by the exploration communities, particularly geologists and geophysicists, they are also a look-ahead tool applicable for drilling-operations planning and ahead-of-bit drilling-risk mitigation.