A mechanistic model for the fines-migration during the modified-salinity waterflooding in carbonate reservoirs

The fines migration is one of the most reported mechanisms for the improved oil recovery during low salinity waterflooding in sandstone reservoirs. However, the release of particles and its effect on the recovery of oil from carbonates has received less attention and in this work, we emphasize its role.

When injecting a brine incompatible with the formation water, different phases can precipitate which can also lead to more calcite dissolution, releasing some particles from the surface. These particles, together with the detachment of organic layers from the rock surface or the migration of clay minerals present in the formation, can be retained blocking some pore throats and diverting the flow of water to different zones, increasing the sweep efficiency.

In the present paper, we first study the mechanical equilibrium of a particle by considering DLVO, drag, lift and gravitational forces, validating the fines migration inferred experimentally from the pressure drop increase for a set of coreflooding experiments. We show that particle detachment occurs also in carbonates when the salinity drops below a certain value.

Then, we model the fines migration based on the concept of the so-called critical retention function. Our approach is different from the previous reported models, since we calculate this function from the balance of forces, and not taking it as a constant deduced from pressure drop measurements. Using a constant critical retention function would imply that the electrostatic forces do not change along the core/reservoir, which is definitely not the case for chalk reservoirs, where the calcite minerals are highly reactive.

To account for the changes in the electrostatic repulsive forces and therefore in the critical retention function, we couple a CD-MUSIC surface complexation model to our model for fines release. Therefore, at each core position we are able to calculate the critical retention function by considering variables like the ionic strength, pH, pCO2, all of them affecting the electrostatic forces.

The main novelty of this work is the coupling of our optimized surface complexation model with the fluid flow, which allows us to better estimate the electrostatic forces, and consequently the critical retention function that will eventually govern the amount of particle released or reattached. With this model, we are able to predict the critical salinity at which fines migration occurs, the transport and capture of the particles, their impact on the effective permeability of water, and the pressure drop profile during low salinity waterflooding.