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Case Study: Real-Time Depth Determination in Deep Water Using Seismic-While-Drilling Checkshots

Real-time checkshots on a three-well/three-block exploration play help accurately determine depth of key formation marker in Myanmar

Challenge: Quickly predict the depth of a key formation marker within 20 m to reduce risk and save time in a deepwater three-block/three-well exploration campaign.
Solution: Gather checkshots while drilling using the seismicVISION seismic-while-drilling service to acquire a real-time update of the time-depth relationship, to accurately know the depth of the target ahead of time, and to accordingly make key drilling decisions.
Result: Predicted the key formation to within 10 m for all three wells, reducing the ±100-m uncertainty given by the original surface seismic.

Reducing uncertainty

Operating in Myanmar, Daewoo International needed to determine the depth of a key formation marker on three separate extension blocks that were being evaluated in deep water. With an initial depth uncertainty of ±100 m from the surface seismic, the company wanted to reduce this number to less than 20 m so that casing could be set safely and accurately. The formation had possible pressure ramp, well instability, and packoffs if the hole remained open for too long. Additionally, as two of the three wells were in deep water, the need to make accurate real-time drilling decisions was even more imperative.

seismicVISION seismic while drilling

The seismicVISION service was chosen for the project because of its ability to acquire checkshots while drilling, taking up minimal rig time. Windowed real-time waveforms are transmitted uphole from the LWD seismic tool using mud pulse telemetry after acquiring the seismic station at connection, an acoustically quiet period. The source is a standard three-air-gun cluster at surface deployed from the rig. The checkshots are then processed from the waveform at the wellsite, and real-time depth updates are calculated while drilling.

Detailed prejob modeling between Daewoo International and Schlumberger, along with checkshot acquisition close to the target, enabled highly accurate updated depth prediction using real-time answer products at the wellsite after checkshot processing.

Target depth confirmation

For all three vertical exploration wells in three different blocks, the target depth was predicted to within 10 m each time, allowing for successful drilling and casing set at the correct point. These depths were confirmed by subsequent wireline logging runs. Well TDs were set during drilling on the basis of the hydrocarbon sand depth prediction.

By employing the unique Schlumberger seismic-while-drilling technology, Daewoo International was able to accurately establish the depth of a key formation marker during drilling, thereby reducing risk and uncertainty for this three-block/three-well deepwater exploration campaign.

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