Salt Steerability and Hard Rock Drilling Performance in Santos Basin
Authors: Amaro Renato , Moreira Joao Bosco , Forza Fabiola Terezinha 
Published: April 2012

The drilling operation in the pre-salt Santos basin faced great challengesfrom the beginning, so efforts were concentrated to obtain a better drillingperformance as a consequence of the high daily rates of drilling rigs. In thisway, we look for a combination of drill bits and drilling systems to increasethe drilling performance, both in drilling and path control in salt formations, also looking for solutions to increase the rates of penetration in thereservoir, once these rocks have extremely low rates of penetration. Toovercome these challenges some aspects were prioritized, such as control ofverticality in salt and in the case of directional wells, the path control, vibration monitoring and continuous improvements in drill bits selection. Thelow penetration rate in the reservoir can be considered the most importantproblem, the main constraint in a challenging scenario composed by ultra deepwater, very large layers of halite with anhydrite on the top and at the bottom, the greatest depth of the wells and particularly the low rates of penetrationin the carbonate rocks in the reservoir.

This paper presents the strategy of bottom hole assembly selection used tocontrol the well verticality, the work developed to make a good selection of amore efficient drilling system in such scenario. It presents the evolution ofdrilling performance in the reservoir and salt formations, and complement withthe work applied to mitigate the deleterious effects of vibration that affectthe rates of penetration and tool reliability. Finally, the paper offers somesuggestions for speeding up performance at those points that remain as anopportunity for improvements.

Introduction

After a period characterized by a drilling campaign of exploratory andappraisal wells, it was started the development plan with the drilling of wellsof gas and water production and injection. These wells, composed by verticaltrajectories, directional and horizontal wells, represent a new stage with newchallenges and, above all, a commitment to improve drilling performance, inorder to get better results in time and cost reduction.

Taking this approach into consideration, after the conclusion of drillingoperations in each well, decisions based on lessons learned were taken topromote the performance improvement.

Primarily focused on top sections, where it is supposed to have opportunitiesto maximize drilling performance, as a consequence of its long length anddiameter of the drill bit (currently being drilled at 26") there are a lot ofchallenges to be overcome, as control of verticality and management parametersto mitigate the effects of vibration, that is responsible to accelerate theprocess of fatigue in the components of the bottom hole assembly and drillstring. In addition, the length of the section, in most of the wells is notpossible to finish the section with a single tri cone bit and the main reasonfor that is the presence of a hard layer of anhydrite on the top of the halite. In the hard layer of anhydrite, the penetration rate is drastically reduced. After this hard layer is still necessary to drill a length of 150 m of halite, so that a good leak off test can be obtained and the salt section could bedrilled safely, avoiding the salt creeping and minimizing the problems withstucking pipe.

The salt interval is drilled in a combination of diameters of 17 ½" and 14 ¾". The challenges are focused on the control of the verticality and mitigation ofthe vibration effects in the vertical wells and the build-up construction, reaching the designed dog-legs, in directional wells. After drilling fourdirectional wells with build-up rates ranging 2º/30 m to 3º/30 m and reachingthe TD of the first horizontal well, it is possible to get a good idea aboutthe common problems regarding to build-up construction in the area, this is aconsequence of lessons learned in off set wells where similar procedures havebeen tested with the specific objective of acceleration in the learningcurve.