Gas oil Gravity Drainage (GOGD) process is one of the highly efficient recovery mechanism in heavily fractured carbonate reservoirs. The driving force is the gravity, the oil is moving down from the matrix block through fractures until it reaches the producers. To ensure optimum offtake from the reservoir through the horizontal wells the oil rim need to be stabilized and very well managed through the gas injection, offtake/intake balance, aquifer pump off.
A key challenge of optimizing the gas injection as well as the offtake from the horizontal wells to achieve effective GOGD recovery is to ensure that a minimum oil rim thickness is maintained, and that oil rim is kept at the same depth as the oil production well. If not;
When the oil rim moves down, the well will produce gas instead of oil, therefore resulting in deferment and inefficient GOGD recovery.
When the oil rim moves up, the well will produce water from the fracture system, therefore resulting in deferment and significant time to recover the oil rim to the correct depth.
A typical monitoring method for the fracture fluid fill is to run "gradio" surveys in dedicated observation wells to measure the fGOC and fOWC (where "f" represent the fracture system contacts i.e. fractured gas oil contact and fractured oil water contact rather than the matrix contacts). Also the fluid movement in matrix can be monitored by Pulsed-Neutron logging, in combination with Open-hole logs that have been acquired at different times through the development.
In addition to the logging methods, GOGD Flow Unit characterization was introduced to depict the effect of the intake/offtake changes and oil rim movement within the GOGD system along with a surveillance strategy focusing on reservoir optimization. Fluid contact visualization plots were generated for each flow unit which give a clearer picture in how effective is the current GOGD system and the future reservoir development and optimization.
More than 5 flow units with almost 150 active wells,3 gradio wells and 5 pulsed neutron log data were reviewed, to generate fluid contact visualization plots and justify flow unit production behavior. This analysis initiative helped to depict the effect of the intake/offtake changes and oil rim movement within the GOGD system. This work also led to adding additional development opportunities like placement of future new wells or adding perfs /carrying out zone change in existing closed in wells either to produce oil or to pump off water. This eventually fed into the reservoir optimization and surveillance strategy.