Mapping-While-Drilling System Improves Well Placement and Field Development
Authors: Ezioba Uche , Denichou Jean-Michel 
Published: February 2015

Technology Update
Safety, efficiency, and accuracy are fundamental goals of well construction. But as the search for new oil and gas resources pushes into deeper waters and increasingly complex reservoirs, meeting these goals has become more challenging.
A better understanding of the subsurface is one of the most efficient ways to mitigate drilling risk and optimize operations’ performances. The ability to map the reservoir in real time, while drilling, contributes to step change, such as understanding sweep efficiency in horizontal wells, landing and maximizing the length of drain within the optimal zone of the reservoir, and avoiding timeconsuming sidetracks or pilot holes.
While the industry has several bedboundary mapping tools and services to delineate the reservoir as a well is drilled, their depth of investigation is limited. The best of these systems can map to a distance of 15 to 20 ft (4.6 to 6.1 m) from the borehole. These limitations have made it difficult to improve directional drilling within narrow pay zones or complex reservoirs containing faults, unconformities, and injected or channel sands. As a result, wellbore positioning may be suboptimal and drilling may be less efficient.
Real-Time Reservoir Mapping
Schlumberger’s GeoSphere reservoir mapping-while-drilling technology was developed to improve the operator’s understanding of the reservoir beyond the first few feet from the wellbore. The system employs an array of multiple subs in the bottomhole assembly to transmit deep directional resistivity measurements that map multiple reservoir layers with resistivity contrasts in real time (Fig. 1). The multispaced receiver array extends the radial depth of investigation to 100 ft (30 m) from the tool, revealing subsurface bedding and fluid-contact details at a true reservoir scale.
The significant improvement in depth of investigation provides a reservoir- scale view, enabling operators to optimize landing, reduce drilling risk, and maximize reservoir exposure. By integrating real-time reservoir maps with seismic surveys, the interpretation of reservoir structure and geometry can be confirmed and refined, enabling a step transformation of field development strategy.
The system provides a wealth of realtime reservoir knowledge that improves well construction by helping the operator accurately land a well in its target zone and steer the drill bit to keep the wellbore away from reservoir boundaries, dips, and fluid contacts. The real-time mapping data obtained also helps the operator refine the structural and geological reservoir models, thus optimizing plans for recovery and improving development strategies for the entire field.
A Smooth Landing
The ability to land the wellbore accurately into the reservoir’s target zone of interest is a critical first step in delivering a well that achieves its long-term production potential. Although pilot holes may provide good local information about the reservoir geology, they are often ineffective in predicting lateral geological variability—a critical factor when drilling into long horizontal pay zones. Similarly, well-to-well correlation alone cannot accommodate various structural shifts inherent in many downhole environments.
The reservoir mapping-while-drilling technology’s extended depth of investigation, which is enabled by deep directional electromagnetic measurements, helps mitigate the risk of shallow or deep landings. The system supports the identification of large- or local-scale depth shift of the reservoir, providing a precise localization in true vertical depth (TVD) of the top of the reservoir and thus eliminating the cost of drilling a pilot hole. By providing a clear, real-time view of formation boundaries and fluid contacts, the system also avoids the risk of losing lateral exposure and creating sumps.