Based on three established commercial test cases, this paper documents their polymer flood process performance, revealing the impact of reservoir properties and heterogeneity on incremental recovery and dynamic performance. The key reservoir management measures for improving polymer flood sweeping efficiency are illustrated, which are particularly important for the successful application of chemical EOR techniques in heterogeneous sandstone reservoirs.
The three cases produced oil with similar fluid properties from sandstone reservoirs. They all adopted 5-spot well patterns, which provide excellent examples for studying the impact of reservoir property and heterogeneity on incremental recovery. The producing reservoir sands in the three pilot represent two types. The former is of high permeability and low heterogenity. The latter is characterized by low permeability, multiple flow units, thin individual sand layers and high inter-layer permeability contrast. The authors reviewed and compared changes in water-cut and production rate performance versus pore volume of polymer solution injected during the process, as well as the applied well spacing, polymer utilization, and ultimate incremental recoveries. The two high quality reservoir cases adopted 22 ac well spacing, achieved an incremental recovery of 12.5% and 13.7%, a low polymer utilization of 1.08 kg/BO and 1.79 kg/BO, respectively, and a maximum water-cut reduction of 24.4%. In the low permeability case, although a much smaller well spacing of 11 ac was adopted, it achieved a relatively low incremental recovery of 8.2% and a maximum water-cut drop of 5.2%, with a polymer utilization of 2.51 kg/BO. Incremental EUR of individual producer through polymer flood is 49 MBO in the low permeability case, compared with 130 MBO in the high permeability reservoir sands. The three cases proved the effectiveness of fit-for-purpose reservoir management and conformance improvement techniques in improving polymer flood sweep efficiency to achieve high incremental recovery. Such techniques include hydraulic fracturing of low permeability sands, optimization of polymer solution concentration and molecular weight to adapt to different sand units with varied permeability, optimization of injection process to maintain a uniform pressure system, adjustment of injection direction to sweep by-passed remaining oil, and, zonal injection to improve polymer solution intake in low permeability sand units.
The discussed cases proved the technical feasibility and economic viability of polymer flood in sandstone reservoirs with varied heterogeneities and revealed the impact of heterogeneity on performance and incremental recovery. The reservoir management and conformance improvement efforts during polymer flood process provided valuable lessons and best practices for similar applications.