Brazilian Pre-Salt Paradigm Shift: Drilling Reservoir with Exposed Salt for Open Hole Completion with Oil-Based Mud

The design of a Pre-Salt well with open hole completion typically involves cementing the production casing to cover the entire saline section. This approach facilitates drilling through the entire reservoir with minimal overpressure while using a Water Based Mud that is compatible with logging, completion and acid stimulation. However, due to difficult ties in geological interpretation on a Itapu well, it became necessary to drill the carbonate section with exposed salt using oil-based mud, while preserving the original completion design. This represents an unprecedented development in the Brazilian pre-salt. This article discusses the formulation of the oil-based mud and a method for removing the filter cake from the wellbore. It also presents the wellbore diameter, measured at the end of drilling, and some indicators of the well cleanliness after the completion fluid swap, and the damage ratio assessed following the stimulation operation. The oil-based mud prevented dissolution of the salt rock in the region that was drilled with water-based mud and further down. The conditioning string was pulled out of the well after the displacement operation with brine and a surfactant pill, with a remarkable condition of cleanliness and without trace of oil-based mud adhered. Laboratory tests with the surfactant pill have shown that it is possible to minimize, or even remove, the organic layer of the filter cake created by the oil-based mud on the wellbore by the drilling. After the acid stimulation the carbonatic reservoir exhibited high injectivity and high flow capacity. This drilling operation on the Itapu field represents a paradigm shift because demonstrated, in certain scenarios, that it is possible not only to drill the reservoir phase with exposed salt, but also to do so with oil-based mud while removing the filter cake from the wellbore. This technique can make possible more ambitious projects on the Pre-Salt scenario, such as the development of mature Pre Salt fields with a high degree of depletion pressure on the reservoir.