Closure of Natural Fractures Caused by Increased Effective Stress, A Case Study: Reservoir Robore III, Bulo Bulo Field, Bolivia

Abstract

Abstract The gas condensate reservoir Robore III, in the Bulo Bulo field, is comprised by a set of naturally fractured, interbedded shales and sandstones. Recently, this reservoir observed a notorious reduction of its gas reserves and production rates. This reduction in reserves was found to be caused by the geomechanical effect of collapse/closure of its natural fractures. The effect was generated by the sudden increase of effective stress upon the formation. This publication documents the study and investigation of such phenomenon based on core analysis studies, pressure measurements and production data. Sustained gas production and pressure decline lead to closure of natural fractures as both increase the effective stress and confinement pressure. Consider the fracture network lacks the sufficient natural support agent to counteract increasing pressure, it eventually collapses altering the reservoir quality in the near wellbore region and turning it into a simple matrix permeability reservoir with very low magnitudes of measured permeability (0.001 – 0.01 md). As a consequence, production is severely diminished or even lost. The present study is the first of its kind in Bolivia as it documents the complete production shut down of a naturally fractured reservoir caused by this phenomenon. A large amount of reserves are stored in gas reservoirs with similar conditions to those observed at the Robore III reservoir, not only in Bolivia but also in the Latin-America region. This peculiarity gives a special significance to this matter. The effect has to be considered in this type of reservoirs when performing reserves estimations, investment decisions or gas production forecasts for contractual matters. The present study is strongly based on production data from wells BBL-11 and BBL-9D, wellhead and bottomhole pressure information from all completed wells at the reservoir of study and a detailed core analysis report for samples taken form well BBL-11, at depths corresponding to the Robore III reservoir. After two years of building up pressure, well BBL-11 was opened and its wellhead pressure measured. Pressure was found higher than normal pressure gradient and it was concluded that gas moved into the depleted zone, building pressure up as a result. This well is showing again a steep wellhead pressure and bottomhole pressure decline, as fractures are closing again. Recently drilled well BBL-10 is also showing a steep decline on wellhead pressure. This well is still productive due to reservoir pressure was found to be higher than the normal gradient in this zone and because the fracture network are not yet closing.