11:40 〜 12:00
[BH-04] Geophysical Imaging in Real Time to Reduce Uncertainties in Well Placement
★ Invited
In 2019, MOL Norge AS drilled two deviated exploration wells in the Norwegian North Sea. Each well presented different types of risk but both wells had considerable uncertainties and well placement challenges that could be mitigated with geophysical imaging during drilling.
The first 3/7-11 S well was planned to assess the Vinstra and Otta prospects and presented the following major challenges:
A large 140 bar pore pressure uncertainty below the base cretaceous unconformity (BCU) The BCU top pre-drill depth uncertainty of +/- 75m
The combination of pore pressure uncertainty and depth uncertainty has the potential for major drilling hazards, compromising the safety of the well. A technique was needed to drill 12.25” hole and place the 9 5/8” casing shoe close to the top of the BCU to maximize well control safety but without a risk of accidentally penetrating the target before the casing was in place.
Borehole seismic drilling technology enables measurement of seismic velocities, which is the main culprit in seismic depth uncertainty. Measuring the time taken for a seismic pulse from a surface airgun to reach a downhole sensor provides velocity information which can provide a significant reduction in depth uncertainty while drilling. The closer the seismic sensor is to the formation of interest then the more accurate the depth prediction of the formation top is. In addition, seismic reflections from the formation top can provide a lookahead capability extending to hundreds of meters.
The first 3/7-11 S well was planned to assess the Vinstra and Otta prospects and presented the following major challenges:
A large 140 bar pore pressure uncertainty below the base cretaceous unconformity (BCU) The BCU top pre-drill depth uncertainty of +/- 75m
The combination of pore pressure uncertainty and depth uncertainty has the potential for major drilling hazards, compromising the safety of the well. A technique was needed to drill 12.25” hole and place the 9 5/8” casing shoe close to the top of the BCU to maximize well control safety but without a risk of accidentally penetrating the target before the casing was in place.
Borehole seismic drilling technology enables measurement of seismic velocities, which is the main culprit in seismic depth uncertainty. Measuring the time taken for a seismic pulse from a surface airgun to reach a downhole sensor provides velocity information which can provide a significant reduction in depth uncertainty while drilling. The closer the seismic sensor is to the formation of interest then the more accurate the depth prediction of the formation top is. In addition, seismic reflections from the formation top can provide a lookahead capability extending to hundreds of meters.
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