Abstract
This paper presents the design of a polymer flooding pilot test for an extra heavy oil reservoir located in the Orinoco Oil Belt, Venezuela. We describe a workflow consisting of extensive experimental work, numerical simulation and upscaling that led to an optimized polymer flooding. Experimental results show both oil recovery and polymer consumption can be further improved when using low salinity water for polymer mixing. In addition to this, the design of the pilot project forecasts the economic viability of the process.
We studied the impact of salinity in polymer rheology and oil recovery. Recovery factors were measured in core-flooding experiments performed using live extra heavy oil (API 8.7, Rs of 95 scf/bbl and viscosity of 2 000 cP at reservoir conditions) with high content of basic and acidic compounds (6.27 and 2.3 mg potassium hydroxide/g of oil, respectively). Rheology data showed that 50% less polymer is required to achieve the desired mobility ratio when polymer solution was prepared using low salinity water. Polymer flooding with a salinity of 1000 ppm resulted in a residual oil saturation (Sor) of 25%. This was further reduced to 17.4% when low-salinity polymer flooding was implemented. Numerical simulation of the pilot area indicated that using the optimal injection rate of 4 000 BWPD per well and a polymer solution-oil mobility ratio of approximately 10, lead to an optimum and attractive technical-economical injection scheme when using horizontal injector and producer wells pattern.
We attribute the incremental oil recovery to wettability alterations and ion exchange taking place between low salinity polymer flooding and the rock surface. The basic and acidic components of the crude oil of the Orinoco Oil Belt have a tendency to wet reservoir rocks when clays are present. We believe these compounds are detached from the rock during low salinity floods altering wettability into a more water-wet condition, therefore reducing Sor.
Low salinity polymer flooding is an attractive choice for EOR in heavy and extra-heavy oil reservoirs additional to thermal methods. The lower polymer requirements and the higher incremental recovery achieved by low salinity polymer flooding improves economic performance making its application attractive in reservoirs where primary recovery is normally below 10% of OOIP.
