Study of Produced CO₂ Storage into Aquifer in an Offshore Field, Malaysia

To meet the ever increasing gas demand in Malaysia, many gas fields with high CO₂ content are being considered for development. Located in offshore Peninsular Malaysia, B field is one of those gas fields. It is vital for the field development plan to focus on the issues of associated CO₂ gas production disposal in view of the global warming effects of releasing CO₂ to the atmosphere. The offshore separation and removal of CO₂ has to be carried out to meet the gas sales specifications from an average CO₂ of 29 mol % in the Full Well Steam (FWS) down to 8%, thus raising the issue of managing this huge volume of CO₂ greenhouse gas production. The study on how to manage the disposal and storage of the CO₂ separated from the gas stream was made as an integral part of the B field development plan, since a disposal site and an infrastructure for injection operations would be needed to store the produced CO₂.

This paper presents the workflow and methods of evaluating the potential formation for subsurface disposal of produced CO₂ into a saline aquifer over the entire life of the field and its impact in the field development plan. The main tasks associated with the study are the geological evaluation of the shallow subsurface and subsequent reservoir engineering and CO₂ injection modeling challenges due to limited reservoir data. The water filled aquifer zones that were identified to be potential sequestration site are in the Group D with depth surfaces starting from 1170 m subsea and ending at 1300 m subsea, 5 km south of the A-Platform (main). All CO₂ gas re-injection wells will be drilled from the satellite platform-B. All reservoirs which include potential aquifer zones below 1800 m subsea depth have been found to be over-pressured and are not suitable for CO₂ sequestration.

A faulted geo-cellular model was built and subsequently characterized by stochastic facies and petrophysical distributions based on exploration/appraisal wells data and 3D seismic interpretation. Different sensitivity studies were carried out to have a good understanding of the key parameters that would control the re-injection process.

Based on the results of the model, the aquifer is adequate to store the volume of the re-injected CO₂ using four injection wells. The model results show that by lengthening the CO₂ migration path, the CO₂ dissolution in the formation water can be maximized and thus minimizing the build-up of aquifer pressure. It is achieved by targeting the injection further down-dip of the aquifer. Furthermore, the low salinity of the aquifer water is also an important factor in helping to maximize the amount of CO₂ dissolved. High injectivity wells are essential to minimize the bottom-hole pressure required to inject the gas, in order to stay below the fracture pressure limit. This factor becomes very important as the aquifer pressure builds up.