As part of the development planning of an alternating water and gas injection scheme for Enchance Oil Recovery in the Bokor Field, a risk evaluation study was carried out to assess the likelihood of injection gas migration from the reservoir to the seabed. This hazard was identified as a principal HSE risk to the platform installations by the Bokor Field development team. A work flow was constructed which involved the integration of geological and petrophysical based seal evaluations with coupled fluid-stress geomechanical modeling of the depletion and injection processes over the life of the field to determine reservoir shale seal efficiency and gas leakage risks due to both hydraulic fracturing and fault reactivation. The coupled geomechanical modeling process incorporated, not only the resulting pressure changes arising due to depletion and injection within the reservoir, but also utilized a consolidation scheme to model the imposed pore pressure changes within the overburden arising due to stress transfer, and the subsequent dissipation of these excess pore pressures with time.
Evidence of fault related gas migration in the geological past is seen by the presence of a shallow gas cloud identified on seismic data sitting above the crest of the structure. The scope of the study included the evaluation of this cloud on various vintages of seismic from 1976 to 2011 to determine whether the size and geometry of the gas cloud was related to previous field development.
The study clearly demonstrates the application of coupled geomechanical modeling in assessing surface and subsurface risks resulting from field development.