As means of primary and secondary recovery, the fractured reservoirs with strong aquifer can be developed either by water injection or by utilizing the natural energy of the aquifer itself. High permeability contrast between fracture and matrix blocks and preferably mixed to oil-wet nature of the naturally fractured reservoirs makes waterflooding and primary methods of production mostly inefficient leaving vast amount of oil unrecovered in the oil-bearing matrix blocks. Due to the absence of a sufficient pressure gradient, wettability of the reservoir rock determines the rate of the displacing fluid invasion into the matrix blocks by capillary and gravity forces. Chemical Enhanced Oil Recovery (cEOR) mechanisms are aimed to intensify oil recovery by affecting these forces. Laboratory and field pilot tests showed the application of surfactant to be a promising cEOR agent for increasing oil recovery from the matrix blocks, both in water-wet and oil-wet fractured reservoirs. For the case studied in this paper, analysis of the surfactant application in the fractured reservoir required the solution of the following challenges:
Interpretation and reproduction of the EOR mechanisms by mathematical modelling
Adaptation and integration of the EOR effects into the full field model
Development of the proper technology for surfactant injection under the given reservoir conditions
The aim of the paper is to present the discussions and the workflow for analyzing and identification of the surfactant application in the fractured reservoir with strong bottom driven aquifer.
Introduction of the trapping number (accounting the capillary and Bond numbers) as a scaling parameter enabled to evaluate the combined effect of capillary, viscous and gravity forces on oil desaturation. To integrate the trapping number into commercial simulator, a special interface was developed within the scope of this work. The EOR effects of surfactant were evaluated on the single porosity numerical models representing a discretized matrix block.
To upscale the specific recovery mechanism as a mass exchange term into full field dual porosity model a special coupling solution was introduced. A pseudo-capillary pressure is suggested as an intermediate function to translate the recovery mechanism from single to dual porosity model.
The developed innovative technology proposes a special injection and production strategy for more effective areal sweep efficiency as well as alteration of injection water chemistry to drive the surfactant into target areas without high losses into the aquifer.
This technology and the described workflow, both were employed for advanced estimation of surfactant EOR potential in a naturally fractured carbonate reservoir. The surfactant aided recovery mechanism based on transition from capillary to gravity dominated displacement showed enhanced effect on ultimate oil recovery from this reservoir.