Abstract
As part of ongoing study for CO2 sequestration in Sarawak basin, offshore Malaysia, storing of CO2 in depleted gas reservoirs is one of the options besides deep saline aquifers and dry structures. These single options have their own advantages and challenges. Most of the depleted giant gas fields in Sarawak basin experienced pore collapse during later period of production life. The magnitude and the severity of pore collapse depend on effective stress changes and reservoir compaction behavior. Most of these carbonate reservoirs in Sarawak have been producing for more than 10 years and have undergo pore collapse to various degrees at depleted reservoir pressures. Core studies have indicated possible pore collapse to happen from 900psi to 1590 psi of abandonment field pressure. It was observed from the core studies that carbonate facies with predominantly vuggy and mouldic porosities varying in the range of 20 – 35 % that are more susceptible to collapse. The pore collapse consequently resulted in reduction pore volume, destruction of permeability, reservoir compaction and subsequently surface subsidence. This could further impact the existing surface facilities (platform sinking), casing integrity in existing wells and possible fault reactivation jeopardising structural integrity.
Nevertheless, some of the carbonate reefal gas reservoirs in Sarawak basin have sizeable capacity presenting attractive opportunity for utilizing them as potential storage sites once they reach the abandonment stage. However due to the above enumerated risks associated with the possible pore collapse scenarios, a thorough screening and selection of storage reservoirs is required. This paper attempts to present various selection criteria evolved for screening, selection of potential storage sites and a case study, focusing on overall 3D/4D Geomechanics workflow for reservoir compaction and seabed subsidence, data calibration and other criteria that helps for final development program of CO2 geological storage site.
