A Vertical Seismic Profile acquired while drilling and which utilized an a priori velocity model template to facilitate accurate well landing in a constrained drilling section is presented. The results were compared to original predictions based on surface seismic and actual formation depths taken from log data.
The approach used actual checkshot velocities acquired in real time using VSP lookahead imaging while drilling to reduce spatial and depth uncertainty. Projections ahead of the well landing utilized the checkshot data to perturb the a priori velocity templates in real time. It was also complemented by the borehole seismic image to check for sub-seismic faults and alternate interpretations. Based on these projections, trajectory corrections were made to optimize landing the well in a key reservoir sand. Initiating early directional changes were critical to land on a short, directionally-constrained open-hole section whilst ensuring the section was within the targeted fault block.
A comparison of the actual wellbore velocities against the predrill scenarios is provided along with corresponding vertical depth predictions. Lateral constraint was provided by the correlations of the VSP with the surface seismic image at key stages while drilling. Mapping of the drilling data to the velocity templates showed a deep case scenario for well placement. Details of the two resultant trajectory changes initiated after 2 and 5 stands of drilling respectively are explained. The approach allowed for accurate well placement, reducing depth uncertainty from 60-100 ft. predrill to within 5 ft from final while drilling prediction to actual depth. Final depth confirmation utilized Gamma Ray and Resistivity at Bit Inclination (GABI and RABI) for the key sand. The sand was found to be 18 ft. deeper than initially expected based on the pre-drill model. This method saved the drillers a potential side track.
Conventional Electromagnetic well placement techniques can be limited in short open-hole sections where early time information is required to facilitate trajectory changes to allow for correct spatial landings. By using VSP while drilling in conjunction with a priori modelling, data can be acquired early enough to successfully, address this challenge.