In-Situ Saturation Monitoring During Polymer Injection for Mobility Control in High-Salinity Fractured Carbonates
Authors: Ahmed Shehzad , Hanamertani Alvinda , Alameri Waleed , Al-Shalabi Emad , Hashmet Muhammad 
Published: September 2022

Abstract
Polymer injection has been widely established for mobility and conformance control during enhance oil recovery (EOR). However, its role in resolving the water-channeling problem in carbonate reservoirs becomes challenging due to the existence of heterogeneous and complex fracture-matrix networks, in addition to harsh in-situ conditions. In practice, the properties and in-situ performance of polymers at reservoir conditions determine the success of in-depth profile modification operation. The requirement to deploy a suitable polymer with high salinity and temperature tolerance has motivated a thorough investigation on the consistency of polymer performance as a mobility control agent. This study presents in-situ saturation monitoring of polymer injection to identify the flow diversion effect established through the mobility control process in fractured carbonate rock. A synthetic polymer, acrylamido tertiobutyl sulfonate (ATBS), was used as the profile modification agent and prepared in 200,000 ppm salinity brine. The rheological behavior of the polymer was studied at 70°C at various concentrations. Also, a set of single-phase flooding experiments was performed using coreflooding system coupled with a CT scanner for real time saturation monitoring in unfractured and fractured (longitudinal) Indiana limestone core samples under reservoir conditions. The recorded pressure drops readings across the core samples were utilized to calculate polymer resistance factor (RF) and residual resistance factor (RRF). The flow diversion across fracture-matrix system during polymer injection was interpreted based on the saturation data. The established rheological behaviors in unfractured and fractured cores were also compared.
The results showed that a mild shear thinning behavior was observed in unfractured core while a pronounced shear thickening flow behavior was established in fractured core. The resistance factor established in the fractured and unfractured core samples were 12.5 and 5.6, respectively, obtained at 10 ft/day with 2000 ppm of polymer. Both cases of fractured and unfractured cores showed that the polymer propagation under in-situ conditions was not subjected to any noticeable degradation. The polymer injection was able to divert the flow from the fracture to the matrix zone; hence, offering a significant mobility control effect. Polymer solutions initially entered the high permeable zone, governing a considerably high-pressure drop and providing effective flow resistance. These occurrences led to flow diversion of the subsequent injected polymer solution into the matrix region. Inaccessible pore volume (IPV) was found relatively high (53%) in fractured core at 1 PV and then, it was reduced to 30% when polymer flooding was extended for 5 injected PVs. The RRF values were moderately low in both core samples (fractured and unfractured); however, a slight increase in RRF appeared in the fractured core sample due to polymer retention in matrix region. This investigation is one of the very few studies on evaluating polymer flooding performance in fractured carbonate reservoirs.