Abstract
Wellbore instability problems in some wells located in the deepwater portion of the Campos Basin presented a serious challenge to the economical development of the huge oil reserves already discovered. Identifying the real causes of the instability was essential to better design the wells. This paper describes the comprehensive laboratory test study for wellbore stability purpose conducted on preserved downhole shale cores. The paper presents the rationale for the tests, the most important results, and also the new tests developed to better investigate the reactions and characterize the rocks. The results show, as opposed to the initial hypothesis, the problem was mainly due to mechanical failure, rather than due to a chemical interaction between the rock and drilling fluid. This broad investigation has allowed a better understanding of the interaction mechanisms between the shale and the drilling fluid, indicating the need to reevaluate some of the existing concepts and procedures.
