Farshid Sanandaji, Mobeen Fatemi, Mohammad Hossein Ghazanfari
Production of excess undesirable brine, is one of the severe problems in highly heterogeneous and fractured oil reservoirs which are under water based enhanced oil recovery (EOR) methods. Recently, preformed particle gels (PPGs) are proposed and used as a diverting agent to control the conductivity of the fractures. Although numerous studies in the literature report improved structural strength and rheological properties of PPGs in the presence of silica nanoparticles (SNP), the possible effect of SNP concentration on such results have not been investigated in details. In addition the effect of rheological parameters such as structural strength and elastic/viscous modulus on the flow dynamics of the secondary water injection (after replacement of the PPG in the fractures) are less studied and rarely visualized. Furthermore, the possible effect of the presence of water soluble fractions (WSF) of the crude oil in the brine (due to long term interactions between formation water and crude oil) on swelling of particles and strength of the PPG samples are overlooked in most cases. With this aim, in the first step of the present research, four PPG samples with different concentrations of SNP were synthesized and assessed through different bulk and rheological tests. Bulk tests showed that presence of the SNP increases the swelling capacity of the samples, since the gel particles containing 0.4 wt% SNP had the better swelling factor (e.g., 25 in 100 times diluted sea water, 100xdSW) compared to the sample without nano particles (15 in 100xdSW). However further increase in the amount of nano particles to 0.8% and 1.0% decreases the swelling capacity of the PPG particles (24 and 19 in 100xdSW, respectively). It was shown that the type and concentration of ions in the brine, as well as the oil/brine interactions affect the swelling behavior. Absence of water soluble fractions of crude oil in the brine and increasing the salt concentration reduce the swelling capacity of the gel particles. Moreover, presence of WSF can hinder the effect of high salinity in the swelling kinetics of the PPG particles. While WSF in the brine enhances the swelling capacity of PPG particles, Hele-Shaw dynamic plugging tests revealed that the PPG samples swelled using these brines, have reduced resistance against secondary water injection compared to the samples swelled in counterpart brines without WSF. Rheology tests of the PPGs confirmed the significant effect of SNP concentration on the storage and loss moduli. Dynamic tests in the model fracture show that although generally the SNP improved the dynamic strength and plugging performance of the PPG against secondary water injection, only the PPG sample with 0.8 wt% nano silica showed the desirable plugging performance (although its swelling capacity is slightly less than 0.4% SNP gel). This PPG sample creates an extra low permeability porous media in the fracture which due to the proper viscoelastic properties allows very low mobility of water through the fingers created between some particles, with no significant or visible gel washout from the fracture. However in the case of other PPG samples, the injected secondary water pushed out some amount of the gel particles and created a wide high conductive flow path inside the treated fracture. Based on the combination of swelling capacity, rheological strength, and dynamic plugging performance, among the synthesized gels, the one with the 0.8 wt% SNP is the recommended optimal concentration for effective water shut-off of the designed fracture.