Some deepwater reservoirs in West Africa are located at relatively shallow depths beneath the seabed. Development drilling poses several challenges for directional drillers as complex, 3-D, and high-angle extended-reach wells are required to be placed in suitable locations around the reservoir for efficient drainage and recovery. Drilling these wells requires kicking off the well as early as possible in a large hole size and building angle reliably to ensure
the target can be reached. The inability to kick off or build angle at a sufficient rate puts the well at risk of not reaching the target. The hole must be drilled smoothly without dramatic local trajectory deflections and with hole quality that allows trouble-free casing and completion. These holes also must be logged for geologists, engineers, and scientists to enhance their understanding of the subsurface to improve the safety and efficiency of further field development.

In the late 1990s, directional drillers introduced rotary steerable technology and learned how it could be applied in a wide range of drilling environments. One of the first rotary steerable systems (RSSs) to be introduced was the Baker Hughes AutoTrak rotary closed-loop steerable (RCLS) system.

All RSSs use a different principle to achieve their steering responses. A benefit identified with this system was its hybrid steering mechanism, capable of reliably delivering precise and consistent steering control in weak rock.

extended-reach well, West Africa

An extended-reach well in deepwater West Africa accesses otherwise stranded reserves. (Images courtesy of Baker Hughes)

In deepwater West Africa, formations encountered often are weak, which causes problems in wells requiring aggressive steering to reach challenging reservoir targets from allocated subsea locations. Inability to drill wells of this nature places some deepwater fields at risk of being economically unviable – leaving valuable reserves stranded and costly exploration programs wasted.

ERD in West Africa
In 2000, the RCLS was deployed to deepwater West Africa for the first time to drill an undulating horizontal well targeting seven different targets in a weak sand reservoir. This job was successful, with the well being delivered precisely on target and within budget. This boosted the confidence of operators to start planning more challenging trajectories in deepwater West Africa to exploit reserves more fully. As a result, this system has been relied upon to drill the most challenging deepwater well trajectories offshore the continent. Coupled with advances in drillbit steerability and capture/transfer of best practices and lessons learned, this has allowed expansion of considerably achievable deepwater boundaries.

An extended-reach well recently drilled in deepwater West Africa targeted an area located 5,577 ft (1,700 m) true vertical depth beneath the seabed and 14,764 ft (4,500 m) laterally from the subsea wellhead. Reservoir targets were stacked one below the other, requiring the well to drop angle to hit successive targets. To drill this well, the 30-in. conductor was jetted in with the 26-in. drilling assembly latched inside. After jetting in and setting the 30-in. conductor, the 26-in. hole bottomhole assembly (BHA) was unlatched and used to kick the well off in the desired direction without the need for a trip.
After setting 20-in. casing, the 171?2-in. hole was drilled using the RCLS. This section was drilled in a single run, steering and building inclination at 4°/100 ft (4°/30 m) to 85° inclination, then holding that to section total depth (TD).

While drilling this section, formation pressure tests were acquired using the Baker Hughes TesTrak logging-while-drilling (LWD) system – a process that now is requested frequently due to its ability to mitigate drilling hazards and improve overall formation evaluation without the need for expensive wireline logging on pipe. The capability to do this while steering at aggressive rates in weak formations is another technique developed in deepwater West Africa to the benefit of global deepwater operations.

The 133?8-in. casing was run to TD and cemented without incident, indicating adequate physical hole quality. The following 121?4-in. hole section was simultaneously under-reamed to 131?2-in. while drilling. This section consisted of maintaining the now almost horizontal trajectory of 85° inclination to gain the required lateral displacement to the targets before dropping inclination toward TD as the targets were approached.

Again, the automated RCLS was used to drill this section in conjunction with a full suite of LWD tools to eliminate pipe-conveyed wireline logging, saving time and reducing risk. In addition, close monitoring of (and reaction to) downhole equivalent circulating density and drilling dynamics data allowed drilling performance to be optimized. This section again was drilled in a single BHA run to section TD and currently is believed to be the longest 121?4-in. section drilled in deepwater West Africa. The 95?8-in. casing was run to TD without incident, indicating adequate hole quality for this challenging casing run. The 81?2-in. section was then drilled to TD, dropping the well to precisely intersect each of the stacked targets.

The overall drilling cost savings attained through the application of the automated RCLS in conjunction with comprehensive LWD has been estimated to be in excess of US $3.4 million.

RCLS proved to be a key enabling system, allowing another complex well to be drilled successfully in deepwater West Africa. The 171?2-in. hole section required continuous and precise steering at 4.5°/100 ft from 10° inclination at the 20-in. shoe to horizontal. To achieve this, between 70% and 80% of the full steering capacity of the RCLS was used, leaving spare capacity should local formation tendencies attempt to force the well off plan. The 133?8-in. casing was run successfully and cemented in place. The long horizontal 121?4-in. section was drilled precisely by the automated RCLS set in automatic inclination hold mode where the well trajectory is maintained automatically within a small fraction of 1° without intervention from surface. This section was then steered down and to the left at 3.7°/100 ft using 70% of the RCLS’s steering capacity to line the well up precisely above the stacked targets. The 95?8-in. casing was run and cemented without incident, and the targets intersected in 81?2-in. hole size as planned. Again, this well was drilled safely and within budget.

These successes are possible to achieve because of the investment made in drilling technology by the service industry, close engineering cooperation with operators, and implementation of continual improvement processes with focus on HSE and attention to well planning, peer review, and after-action review. The techniques developed to allow these complex trajectories to be drilled reliably in deep water have been transferred to and applied in other deepwater locations.