Making proactive decisions to manage structural uncertainty and well trajectory

An onshore field operated in Sicily, Italy, produces from heavy oil–bearing formations. The upper reservoir formation consists of interbedded shales and carbonates, and the lower reservoir formation is mostly fractured dolomite. The operator wanted to produce oil from a structural high—as identified from seismic data—with the lower reservoir as the main target formation.

Because of possible subseismic faults, a highly fractured composition, and a water zone at the bottom of the formation, drilling into the lower reservoir would be difficult. The operator planned to drill within the upper reservoir and approach the top of the lower reservoir sufficiently high on the structure with an inclination that would quickly level off the trajectory at 90°. Drilling at 90° in the lower reservoir would allow the operator to optimize production by maximizing the distance from the water contact below.

The trajectory would need to be modified based on well correlation, LWD images, and reservoir mapping information. The operator planned to discriminate between open and closed fractures, as well as identify producible zones via primary permeability. Because of its high depth of investigation, reservoir mapping with GeoSphere 475 service would play a key role in helping the operator make proactive decisions in managing the structural uncertainty and well trajectory.

Integrating the BHA to make real-time, informed trajectory changes

The operator planned a sidetrack operation by setting a whipstock to mill a 9 5/8-in casing window, followed by 8 1/2-in and 6-in sections of directional drilling. After the 8 1/2-in section was drilled 3,267-m [10,719-ft] MD, the 6-in section—the more challenging of the two sections—was used to drill through the two target reservoir formations.

As part of the formation evaluation plan, the geoVISION imaging-while-drilling service was used to assess the structural setting in the 8 1/2-in section, and the real-time information enabled updating the structural setting; this helped to plan a change both in the azimuth and inclination of the trajectory for the 6-in section.

Because the 6-in section required a turn and build, the operator used the PowerDrive Orbit system to deliver up to 5°/100 ft [5°/30 m] of dogleg severity (DLS), maintaining operational efficiency and withstanding several days of drilling under severe shock and vibration conditions.

To discriminate between open and closed fractures, the operator used the SonicScope service and MicroScope service on the BHA while the GeoSphere 475 service geosteered the section to confirm directional drilling did not cross relevant faults and the trajectory would not exit the lower reservoir as the main target. With the integration of these services, the Schlumberger well placement team collaborated with the operator to optimize the trajectory to compensate for the structural change, enabling steering the well into the lower reservoir at 90° by performing an azimuthal trajectory change while maintaining inclination.

The SonicScope sonic service and MicroScope resistivity service were used to identify the most productive reservoir intervals, as validated by the mud logging data.

Completing geosteering ahead of schedule and providing large depth of investigation footprint

The GeoSphere 475 service used real-time data to map the top of the lower reservoir with a depth of investigation of 22 m [72 ft] around the wellbore to successfully geosteer the well for 178 m [584 ft] along the lateral. The Petrel software platform used inversion results from the GeoSphere 475 service to provide 3D visualizations that enhanced formation evaluation. Because of the comprehensive formation evaluation—an integrated solution using the GeoSphere 475 service with a fit-for-purpose BHA—the drilling operation was completed two days ahead of schedule.