Shallow Gas Reservoir Case Study

K&M Technical Engineering Services proved feasibility of drilling a relief well that would re-write the record for the shallowest horizontal well drilled in deep water, and uncovered and solved some other show stopping issues along the way.

K&M Technical Engineering Services proved feasibility of drilling a relief well that would re-write the record for the shallowest horizontal well drilled in deep water, and uncovered and solved some other show stopping issues along the way

Challenge

In order to develop a deepwater shallow gas field, the feasibility of drilling a relief well, to intersect the planned vertical producing wells, had to be proven. The relief well would need to build to at least 90° inclination to achieve the required surface location separation. The relief well would need to reach horizontal far shallower from the seafloor than had ever been achieved worldwide, and high doglegs would be needed in ultra-soft formation.

Solution

K&M Technology Group conducted extensive research of documented cases, and was able to collect sufficient clues that, although never achieved previously, the high doglegs in the ultra-soft overburden were feasible to both drill, and install casing through. Unique equipment and practices were proposed to achieve the objectives, and which were all supported by documented case studies.
During the study, it was discovered that due to the necessary perpendicular intersection, the relief well was expected to collapse closed upon intersection. A special string and operational sequence was devised to accommodate this outcome, by delivering the entire kill through the string.

Results

Using the solutions and operational strategy proposed by K&M, the Operator will now embark on a test well, to be able to definitely prove this World Record challenge can be achieved.

Are Either of these Two World-Record Wells Feasible?
K&M was contacted to assess the feasibility of achieving the directional objectives of both a high-dogleg and a fish-hook trajectory, and generate a design for each, selecting the casing points, hole and casing sizes, and fluid parameters. The intention of the fish-hook trajectory was to decrease the target dogleg to a more achievable value.


The shallow gas reservoir was in ~400m water depth, and only 164m below the mudline. Vertical producing wells were planned, and ≥500m surface separation was required between the relief well location and the hypothetical blowing well. An intersection point 10m above the reservoir was selected, meaning horizontal had to be achieved at just 154m below the mudline. This would far outperform the current World Record for a deepwater horizontal, of 250m below mudline.


Amplifying the challenge was the ultra-soft overburden the directional work would need to be achieved in. Prior geotechnical appraisal wells and studies indicated the overburden was composed of glacially deposited silt, with an initial UCS of just 20 psi, increasing to only 180 psi by intersection depth. There was no known experience of ever achieving anything close to the required doglegs in formation so unconsolidated.

The High-Dogleg Trajectory is the Clear Winner
A shortened fish-hook trajectory was technically feasible, but introduced considerably more operational complexity and risk compared to a high-dogleg design. The fish-hook would require multiple hole opening passes in a single section to achieve the dogleg requirements using smaller pilot-hole sizes then opening, before reverting to a different directional tool, and would deliver a much smaller MW window at the intersection point.
Extensive research of both publicly available and internal case studies provided clues that, although never achieved previously, the ~13°/30m build rates required by the high-dogleg design should be achievable, and casing installation was feasible. The research extended even as far as consulting experts from the HDD Industry for input, since similar Oilfield experience was limited.
Some of the unique proposed solutions were: using articulated motors, capable of at least 50°/30m in competent rock, to assure the planned ~13°/30m was achieved in the ultra-soft overburden, with directional jetting as the proposed back-up plan; under-sized directional pilot holes would be used, with hole openers rotated below a mud-motor while the string above was slid into the hole, to manage MWD dogleg limitations; and the use of flexible casing shoes.

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