Oil-based muds traditionally have been used by Petrobras to drill wells in the Upper Magdalena Valley of Colombia to address wellbore instability problems associated with the Villeta shale. Recently the operator faced additional challenges while drilling an exploratory well due to 1) environmental concerns in a highly sensitive area, and 2) formation evaluation problems associated with oil-based fluids. Significant advances in mud design in the last few years have allowed water-based mud performance to approach the performance of oil-based mud. Therefore, the operator and fluids company created a task force to evaluate the application of water-based mud for this well. The planning stage included laboratory tests, past history review and an evaluation of the service company's experiences when drilling similar shales in the area. A high performance water-based mud (HPWBM) containing clay and shale stabilizers, a ROP enhancer, and sealing agents was selected to drill the well.
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