Abstract
In Absheron appraisal well, a short section (16½″ × 20″) was dedicated to isolate the "Creeping Claystone" troublemaking interval.
The creeping shales correspond to a 6 meters thick shale layer compressed between two competent layers of anhydrite. This layer is saturated with water as it is impossible for the fluid to be expelled due to the enclosing anhydrite. The behavior of the shale layer is not considered as over pressured but only as a rheological effect.
Only 250 meters were drilled in 16½″ × 20″ section that included "Creeping Claystone" interval. The section was drilled in steps due to previous experienced borehole instabilities. Consequently, a 16½″ pilot hole was first planned to be drilled to TD (Total Depth), then being enlarged to 20″ size. Furthermore, the section was planned to be drilled with 1.67sg NABM (Non Aqueous Based Mud) mud, nevertheless, due to encountered problems with borehole stability, the mud weight was increased by steps up to 1.73sg.
Lots of problems were encountered while drilling, such as:
several mud overflows from header box in the shaker room after experiencing pack-offs;
two SonicScope failures (tool could not withstand such borehole conditions);
huge volume (3964.5 t) of cavings, cuttings and mud were recovered, filling 849 skips (4.3 t/skip average).
Eventually, 16″ liner was set and cemented at well section TD.
