In their quest to develop new oil and gas resources to feed the growing global appetite for energy, operators are challenged with drilling into reservoirs that are deeper, thinner and more geologically complex. Fortunately, new drilling technologies continue to be developed to increase the odds of drilling success in challenging hydrocarbon pockets that might have been technically or economically infeasible just a decade or two ago.

One technology area that has shown particular promise is the development of bed-boundary mapping tools to measure reservoir properties as a well is being drilled.

However, because these tools only provide a depth of measurement of a few feet around the wellbore, they have shown limited success in drilling applications where the operator needs reservoir information farther afield. This includes reservoirs with narrow pay zones or those that contain faults, unconformities, and injected or channel sands.

This limitation prompted the development of the Schlumberger GeoSphere reservoir mapping-while-drilling service. The service expands on the bed-boundary mapping potential of conventional tools using an LWD system consisting of a transmitter and an array of multiple subs in the bottomhole assembly. These subs can detect deep, directional electromagnetic (EM) measurements more than 30 m (100 ft) from the wellbore to reveal subsurface-bedding and fluid-contact details at a significantly greater distance than conventional bed-boundary tools (Figure 1).

FIGURE 1. A typical GeoSphere deployment consists of a transmitter (bottom) linked to two receiver strings containing subs (top). (Source: Schlumberger)

Well- and field-scale benefits

The reservoir-scale view afforded by the GeoSphere service provides the operator with vastly improved depth of investigation that can optimize landing, improve geosteering to stay in zone and refine geological maps in real time.

Landing on target

The ability to land the wellbore into the reservoir’s targeted zone of interest is critical to delivering a well that will achieve its long-term production potential. The service maps structural shifts at a reservoir scale, providing a precise true vertical depth of the top of the reservoir to reduce the risk of shallow or deep landings. It also affords a clear, real-time view of formation boundaries and fluid contacts, which reduces the risk of losing lateral exposure and creating sumps while exposing more of the lateral section of the wellbore to the reservoir.

Steering with certainty

The reservoir-imaging ability of the service improves steering in challenging downhole environments such as discontinued sand bodies. The service’s ability to estimate structural dips helps operators avoid unplanned reservoir exits and adjust drilling trajectories ahead of geological variations such as water zones and geological sidetracks. This eliminates sudden and erratic changes to the well’s trajectory, resulting in a smoother wellbore that is easier to complete.

FIGURE 2. In the first well drilled, data from the GeoSphere service (top) and a seismic acoustic impedance on the same level (bottom) led to reservoir exposure of 815-m MD, which provided a net-to-gross ratio of 0.98. (Source: Schlumberger)

Mapping the field

The 30-m-plus depth of investigation afforded by the service reveals subsurface beddings and fluid boundaries that allow for mapping of the reservoir top and base and provides data on the presence of lateral heterogeneities, subsurface unconformities and reservoir geometry. These data can be integrated with surface seismic data to refine reservoir models and improve the understanding of sweep efficiencies in horizontal wells.

The mapping data provided by the service can be integrated into 3-D reservoir models to enhance the evaluation of layered formations, which allows operators to refine completion design, regulate flow control management and enhance depletion profiles before completions are run. In addition, by indicating fluid contacts, the service helps optimize injector and production well development by influencing infill drilling decisions to eliminate bypassed pay zones.

Optimizing placement of North Sea wells

The GeoSphere reservoir mapping-while-drilling service proved useful to an operator in the North Sea that was challenged with steering two production wells within a glauconite-rich sandstone reservoir. The target reservoir sands were 2 m to 15 m (7 ft to 16 ft) thick and comprised post-depositionally remobilized sand capped by unstable shale. Although initially deposited as gravity flow sands, over time they were remobilized under burial, which led to sand fluidization and injection along weakness zones such as faults and low-competent stratigraphic levels. This made a determination of the exact stratigraphic location of the sand difficult.

These steering challenges, coupled with the reservoir’s low resistivity, made conventional landing methods and bed-boundary mapping tools less than ideal since they could only provide a depth of investigation of less than 2 m. As a result, the operator was not confident that timely trajectory adjustments could be made to account for the complex geology of the reservoir such as sudden structural dip changes.

FIGURE 3. Data from the second well (above) showed that a 260-m MD was achieved, with a 0.96 net-to-gross ratio. (Source: Schlumberger)

What’s more, the operator was concerned that if it could not accurately map the internal variations of the reservoir’s structure, then it would not be able to define geological interpretation to optimize future field developments.

The operator had a historical basis for its lack of confidence in conventional tools since previous wells drilled in similar reservoirs have been marginally economical, with net-to-pay ratios below 0.5 and an increased need for sidetracks.

Recognizing the need for accurate reservoir steering to optimize well positioning, the operator decided to deploy the GeoSphere service to steer into the two planned horizontal production wells. The deep directional EM measurements from this service and surface seismic measurements were overlain in real time while drilling to provide important look-ahead information for optimized landing.

These combined measurements made it possible to steer away from reservoir structure shifts well ahead of when they would be encountered. With more informed steering, the wells were placed within the thin reservoir away from unstable shale and drilled completely to total depth.

The first well was drilled in a single run, achieving a horizontal section of 815 m (2,674 ft) measured depth (MD) with a net-to-gross ratio of 0.98. Actual reservoir exposure exceeded planned exposure by 65 m (213 ft), and the tortuosity of the well was kept within requirements (Figure 2).

The second well was also drilled in one run, achieving 260 m (853 ft) MD with a 0.96 net-to-gross ratio (Figure 3).

The two horizontal wells were accurately steered in the reservoir—96% in zone—and with no sidetracks. Both wells were tested at up to 8,000 bbl/d with minimal drawdown.

Understanding of the reservoir and its heterogeneity was also greatly improved. The information from the GeoSphere service was critical in refining the reservoir model and, ultimately, in making decisions for the long-term management of the field.

Because of the successful well positioning and the valuable reservoir mapping data acquired during drilling, the operator plans to run the reservoir mapping-while-drilling service for future campaigns.

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