Abstract

The development of Brazilian Pre-Salt reserves will require a large numbers of wells to be drilled. Once the operational associated costs are extremely high, operational practices that can save drilling time are very welcome. The salt layers are composed mainly of halite, but intercalations of carnallite and tachyhydrite are also present. The use of water based mud to drill salt has been presented as a feasible option to increase the rate of penetration (ROP) and to avoid the operational problems related to the use of oil based muds. The critical point in this case is to control the dissolution of salt due to fluid contact.

This work presents the development of a dissolution simulator, its calibration using well drilling data and also the best determined strategies related to operational parameters and fluid composition to increase the operational safety and performance. The calibration was performed by a detailed review of a real drilling operation of 2.000 m salt intercalations. The results show that rheological parameters play an important role as they not only improve the solids transportation but also control the rate of salt dissolution. The evaluated parameters include ROP, flow rate, non-productive time (static dissolution), salt concentration and layer extension and formation temperature.

Introduction

Since the beginning of the Brazilian Pre-Salt exploration until the late drilling operations, oil based muds have been rather used to drill salt layers. The main reason of this choice is to avoid problems regarding severe enlargement of the wellbore caliper at the salt layers, which may lead to well instability problems and poor quality of casing cementation. Nevertheless, some drawbacks result of this practice:

  • Oil based mud has a no biodegradable characteristic and may cause serious environmental damage in case of leakage to the sea.

  • The use of this type of mud requires a drying process of the generated cuttings in order to avoid environmental impacts and attend to the Brazilian environmental legislation.

  • This use of cuttings drier ends up on a limitation of the ROP according to the maximum processing capacity of the cuttings drier. This is a huge bottleneck on the drilling performance.

  • Oil based mud is more expensive than water based mud.

  • Oil based mud requires a more complex logistic and fluid shifts, which increase the operation time.

It is vastly known that the operational costs of the Brazilian Pre-Salt development are extremely high, so that there is a huge search for operational practices that can save drilling time. The usage of water based mud to drill salt may be a feasible option to increase ROP on the salt layers drilling. As this type of fluid is less aggressive to the environment, the use of cuttings drier is not a concern in this case, so that higher ROP can be potentially achieved.

At the same time, the critic point of this approach is that water based fluid saturated with salt under ambient conditions becomes sub-saturated under bottomhole conditions, due to the high temperatures. Thus, completely avoid wellbore salt dissolution is a very hard task. The wellbore salt dissolution however can be increased or attenuated depending on the operational parameters and the flow state of the fluid in the annulus space (between the drillstring and the wellbore), so that a detailed study is required in order to minimize the issues related to salt dissolution and increase performance.

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