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Geological Challenges in the Development of a Colombian Waterflood Using Horizontal Wells


Since 2005, Occidental Andina and Ecopetrol have been redeveloping the La Cira Infantas field, the oldest oilfield in Colombia, as a waterflood to increase the reservoir pressure and oil recovery factor. In December 2013 and 2014, horizontal wells were proposed to develop the C zone (Mugrosa Formation) in the low part of the structure, where the thickness of the pay zone is reduced due to its proximity to the oil/water contact. Drilling horizontal wells appeared to be a good alternative to increase the drainage area, productivity index, and recovery factor in zones where vertical wells were not economically attractive. The planning stage involved: (1) building a geological sector model of the structural configuration and rock properties; (2) designing the directional well plan with the restriction of using the available surface platform; (3) planning the horizontal section to adjust the plan to the model; and (4) creating a simulation model to define the appropriate method for steering the horizontal wells. The geological complexity, which includes lateral variations due to the fluvial environment, azimuth variations, a high formation dip of nearly 35°, a thin target zone, and low resistivity contrast between sandstones and mudstones, proved to be a big challenge. Such complexity was evaluated during planning, as it would significantly impact the execution of the horizontal section. To make sure the horizontal wells landed in the narrow target zone, azimuth was carefully maintained with respect to the plan, and a complex correlation was developed that required the inclusion of additional formation tops. Running deep directional resistivity logs with density images plus gamma ray at the bit was crucial to being able to respond to formation changes observed while steering the horizontal wells. Additionally, the structural model had to be adjusted in real time in order to follow the strike of the target beds. While drilling these horizontal wells, more variations than expected were observed in both the structure and the stratigraphy. Nevertheless, these wells were effectively steered using the azimuthal gamma ray tool placed close to the bit (6.5 ft), which enabled to react quickly to unexpected formation conditions encountered during horizontal drilling. The horizontal wells were steered through 8–15 ft TVD of net pay thickness for distances of 2,500 to 2,800 ft, resulting in productivity index 3–5 times greater than a vertical well in the same sand.