Surfactant-enhanced oil recovery (EOR) in fracture-dominated naturally fractured reservoirs and very low-permeability Bakken type reservoirs are less known. Therefore, to predict their performance, improvement of the reservoir simulation tools is necessary to account for the fluid flow mechanisms as much as possible.
We present a dual-porosity numerical simulation model and algorithm (improved model) in which matrix-fracture fluid transfer function was improved by implementing a proper viscous displacement mechanism. This mechanism was added to the existed fluid expansion, gravity drainage, and capillary pressure mechanisms. Current dual-porosity reservoir simulators generally do not account for the viscous displacement mechanism. To validate both the accuracy and efficacy of the improved model, results were compared with the results from a variable permeability-porosity, single-continuum, fine-grid model (fine-grid model).
Simulation results of improved model were in agreement with the results of the fine-grid model as the reference case. In a one-dimensional numerical model, water flood cumulative oil production increased about 5% compared to the conventional dual-porosity model. Also, incremental oil production increased over 5% for 1 wt% surfactant concentration. In water flood stage the matrix grid oil production rate started at 0.25 bbl/day in improved model compared to 0.053 bbl/day in conventional dual-porosity model. This amount was 0.124 bbl/day against the 0.03 bbl/day at the start of chemical injection. Similar results were obtained in a 2-D numerical model. Improved model was computationally very efficient and it was much faster than the computation time of fine-grid model.
For a practical application, the improved model was used to design and assess the viability of an EOR pilot-test using a single-well, multiple-completion protocol in a fractured carbonate reservoir. This reservoir has a matrix permeability of 10 md and matrix porosity of 0.05, and fracture permeability of 10000 md. Similar result was obtained using improved and fine-grid models. Also sensitivity on various fracture spacing of 5 to 20 ft was performed. As a result, the smaller the fracture spacing is the higher the effect of viscous displacement.