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Technical Paper: Deepwater Drilling in Both Hard and Abrasive Formations: The Continuing Challenge of Bit Optimization

Society: SPE
Paper Number: 151787
Presentation Date: 2012
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Abstract

Discovery of the Jubilee field, offshore West Africa presented a new challenge to the drilling industry when it had to contend with hard sandstone drilling in a deepwater environment. A solution to the key 12 1/4-in section was developed utilizing a 4D modeling system, which accelerated optimization in the large hole section. This resulted in one bottom hole assembly (BHA) run to total depth (TD), compared with an average of four BHA runs previously. Additional modeling achieved a doubling of rate of penetration (ROP). 

Further improvement to the one-run success would involve risk, and engineers had to weigh the prospective gain against the potential loss. In deepwater, particularly with a successful solution in place, high operating costs generally reduce the motivation to embrace change even if a potential cost-reducing solution could produce a significant economic impact. Achieving additional gains required an engineered approach to reduce the risks of deploying new solutions.

During the initial campaign the simulation models were constructed around standard rock types. However, the acquisition of reservoir core allowed direct testing on actual formation. This valuable new information became the basis for a new 12 1/4-in PDC bit design and provided the justification for field testing.  Additionally, different well plans called for a more challenging 17 1/2-in hole section on a new development. To solve the challenge, the operator considered using a different bit type and/or smaller hole size. Based on simulation results, and an analysis of global offset data of both sizes, the smaller hole size option was selected.

The initiatives have been extremely successful, culminating in two wells drilled in record time, with one of them reducing drilling time by 40%. This work has demonstrated that continuous improvement is possible, but where the cost of failure is high, a thorough engineering analysis is required to justify each change.

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