Development challenges in a medium-heavy oil field, subject to water production: A study case of Opale field in Central Africa

The Opale field, onshore Gabon, was discovered by Perenco in 2005 and brought on stream the same year. Development focussed initially on the Azile and Cap Lopez reservoirs that produced light oil (44deg API and a few viscosities of a few centipoise). Oil production reached a plateau of around 10,000 bopd, and production was maintained at that level thanks to the drilling of infill wells and the implementation of peripherical water injection. In total, 47 vertical wells were drilled to produce the Cap Lopez and Azile reservoirs. Although good recovery factor was achieved for those reservoirs, the production started to decline in 2017. In order to compensate this decline, Interest was therefore shifted to a shallower reservoir, the Milongo, that produced a much heavier oil of 18deg API with 253 Centipoise viscosity. The table below recap the STOIIP and Recovery Factor of each reservoir in 2018, showing high RF for Cap Lopez and Azile, and only 3% RF for Milongo reservoir.