Impact of Pore Volume Compressibility on Recovery from Depletion Drive Miscible Gas Injection in South Oman

摘要

Rock Pore Volume Compressibility (PVC) data can be misinterpreted during the early life of reservoir development due to the fact that there are minimal amounts of this data acquired during early reservoir life. This data is typically obtained from uniaxial or hydrostatic tests using conventional core acquired during the appraisal phase of the reservoir. This article presents a case study from a cluster of reservoirs, in Southern Oman that highlights the importance of using PVC to determine reserves associated with both the primary depletion and miscible gas injection. The cluster is being developed in a phased approach. The key objective of each phase is to gather data from the different reservoirs to assess if a miscible gas injection project would be feasible. Permanent downhole pressure gauges have been utilized to monitor reservoir performance from the depletion phase and to aid in the forecasting of oil recovery for the miscible gas injection projects. The reservoir pressure in one of the reservoirs producing in the depletion phase has declined faster than expected and can be attributable to either lower than expected oil in place volume or a lower the expected PVC. Obviously, having lower oil volumes in place would greatly impact the economics of a miscible gasflood development. Therefore, renewed focus on proper evaluation of the PVC from the latest emerging core data from appraisal wells in a this reservoir has indicated that although the originally assumed PVC was within the uncertainty range, it was at the high range of the data and some of the measured data was skewing the average. A new look at the material balance and simulation results verified that PVC and not a reduction in OOIP was the root cause of the difference in performance estimates and the observed reservoir performance. By using a new lower average PVC, the observed reservoir pressure is found to be consistent with new material balance and reservoir simulation results. This approach has clearly, provided vital information to underpin the recoverable reserves associated with the miscible gas injection.