Abstract

In the Niger Delta in Nigeria, some fields are classified as tough drilling environment due to numerous challenges encountered during the drilling phase of well construction process. In the Gbaran field in the Niger Delta, the greatest challenge is encountered in top hole drilling in 16” hole size and above. From a stratigraphical standpoint, the surface hole formation down to 6000ft in this field and other fields of similar drilling challenges shared common characteristics, with the lithology ranging from soft and loose to medium and abrasive sand inter-bedded with stringers of lignites, gumbo shale, and hard streaks (cemented sand). Bottom hole assembly (BHA) failures such as wash outs, cracks and twist-offs caused by erratic torques, excessive lateral, axial and torsional vibration are regular occurrence while drilling top hole at depths between 3000ft and 6000ft. From offset well records, more than 75% of 16” and 17-1/2” surface hole drilled in the Gbaran field ended up with unplanned side-tracks due to twist-off and lost hole. In some wells, the non productive time arising from these failures is up to 100% of planned AFE time for the section with the attendant cost impact on the wells in addition to lost-in-hole charges.

To address these challenges, steps were taken to analyse the failures in the offset wells and come up with BHA design capable of withstanding the inherent down-hole vibrations and shock. This paper discusses this engineered approach which is centred on two things: Fit-for-purpose BHA design and optimized drilling parameters. It also highlights the success story of drilling the deepest 16” hole so far in the fields without BHA failure as experienced in other wells drilled in the same field. The result from field application showed a tremendous reduction in drilling days from average of 27days per well in the last three offset wells to average of 9.5 days per well. Furthermore, the MWD output of down-hole vibration chart also showed a 50% reduction in severity compared to offset wells in the field.

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