Abstract
Heterogeneous reservoirs often have poor sweep efficiency during flooding. Although polymer flooding can be used to improve the recovery, in-depth diversion might provide a more economical alternative. Most of the in-depth diversion techniques are based on the propagation of a system that forms a gel in the reservoir. Premature cross-linking of the system prevents the fluid from penetrating deeply into the reservoir and as such reduces the efficiency of the treatment. We studied the effect of using a polyelectrolyte complex (PEC) to (temporarily) hide the cross-linker from the polymer molecules. In addition to studying the cross-linking process in bulk, we demonstrated its behaviour at the core scale (1 m length) as well as on the pore scale. The gelation time in bulk suggested that the PEC could effectively delay the time of the cross-linking even at high brine salinity. However the delay experienced in the core flood experiment was much shorter. Tracer tests demonstrated that the XL polymer, which is a mixture of PEC and partially hydrolyzed polyacrylamide, reduced the core pore volume by roughly 6.2% (in absolute terms). The micro-CT images showed that most of the XL polymer was retained in the smaller pores of the core. The large increase in dispersion coefficient suggests that this must have resulted in the creation of a few dominant flow paths isolated from each other by closure of the smaller pores.
Highlights
In-depth profile modification is a promising technology for improving the sweep efficiency of water flooded heterogeneous oil reservoirs (Seright et al 2011; Sydansk and Southwell 2000; Bailey et al 2000; Glasbergen et al 2014)
Despite the effective delay of the cross-linking in bulk, the results of the core flood experiment show that the increase in the pressure drop is observed within hours from the beginning of the XL polymer injection (Figs. 14, 15)
The gelation time in bulk suggested that the polyelectrolyte complex (PEC) could effectively delay the time of the cross-linking even at high brine mineralization
Summary
In-depth profile modification is a promising technology for improving the sweep efficiency of water flooded heterogeneous oil reservoirs (Seright et al 2011; Sydansk and Southwell 2000; Bailey et al 2000; Glasbergen et al 2014) It can be implemented by injecting a crosslinking polymer (XL polymer) that propagates deep into the reservoir. There are different parameters which influence the kinetics of cross-linking of the polymer: temperature, different concentration of divalent ions, and cross-linker concentration These parameters were varied during the bulk tests to meet the following requirements: (1) the initial viscosity of a mixture (polymer + cross-linker) has to be low and the gelation time not less than 5 days; (2) the XL polymer has to be stable at 45 °C, which is the target temperature of the experiment; (3) no dramatic precipitation occurs during the gelation time. That allowed for obtaining the dispersion coefficient after the gel was formed in the core
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