DRILL BIT SEISMIC—DRILLING OPTIMISATION CASE STUDY: CRUX–1

Abstract

Drill Bit Seismic (DBSeis) technique utilises the acoustic energy generated during the drilling process to provide vital information about the subsurface structure. This information, produced in real time at the wellsite, is used to optimise the drilling process, leading to significant cost savings and enhanced safety.

When a working drill bit destroys the rock at the bottom of the hole, it radiates acoustic energy into the surrounding formation. This acoustic energy is recorded by sensors both at the top of the drill-string and placed on the sea floor in the vicinity of the rig. Travel times recorded by the sensors on the ground are corrected for drill-string travel times to provide the time-depth information. This accurate time-depth information can then be used to continuously update the depth of drilling hazards by converting the surface seismic markers from time into depth domain.

After experiencing excessive loss of circulation in the Johnson Formation in a nearby well, DBSeis was run in the Crux–1 well to help predict the depth of the top Johnson Formation while drilling to the 13 3/8” casing shoe depth. The well program called for the casing shoe to be set as close to, but above this formation. The two way time of the Johnson formation had been estimated from the surface seismic to be 0.994 s. DBSeis was used to provide real time time-depth information to convert the two way time to depth.

The estimated top Johnson Formation at 0.994 s two way time (TWT) corresponded to a depth of 1,231 m SS using pre-drill velocity information. Using the DBSeis time-depth data, this depth was reduced to 1,192 m SS and the casing shoe was set at 1,164 m SS. The actual depth of the top Johnson Formation was later estimated at 1,178 m SS from ROP/WOB.