Working with naturally fractured reservoirs (NFRs) can be challenging. Inadequate understanding of the enhanced oil recovery (EOR) driving forces in these reservoirs may result in serious conformance issues due to excessive water production. As a result, this work investigates and numerically validates some fundamental flow mechanisms in heterogeneous reservoirs, particularly capillary-dominant ones, to highlight the best EOR strategy for this specific case.
Consequently, a two-dimensional lab-scale reservoir model with injection and production ports was designed, fabricated, and tested in single-phase and two-phase flow scenarios, simulating a water-wet fractured system. First, a single-phase flow waterflood baseline was studied, compared to the literature, verified by commercial reservoir simulation software, and eventually considered to calibrate the porosity and permeability model in the simulation model where the controlling variables are limited. Based on this work, the same procedures were experimentally repeated and verified by simulation, where waterflooding and polymer injection were used to displace oil with more governing variables.
The single-phase scenarios aided in distinguishing between the waterflood and polymer flood cases. Water prefers to channel through high permeable streaks when injected into a fractured water-wet reservoir, resulting in poor volumetric sweep and significant bypassed zones. Whereas the controlling variables in two-phase flow were increased, capillarity and mobility ratio were dominant in the simulation. During waterflooding, flow divergence was observed faster toward the matrix medium, overriding the high permeability front in the fracture due to the strong capillarity contrast between the matrix and fracture media. Even when capillarity is strongly present, polymer flooding demonstrated a better volumetric sweep in all scenarios.
The unique demonstration of fluid flow inside the two-dimensional lab-scale reservoir model, as well as numerical simulation, shed light on the efficacy of these EOR strategies in fractured reservoirs. Furthermore, for the first time, the behavior of capillary-dominant reservoirs with an advancing flow path within smaller pores compared to larger ones within the reservoir media has been experimentally captured. Understanding reservoir characteristics and having the know-how to implement the best recovery scenario can, in fact, maximize the field's life cycle and increase the Recovery Factor (RF).