An Experimental Study of the In Situ Gelation Behavior of a Polyacrylamide/Aluminum Citrate "Colloidal Dispersion" Gel in a Porous Medium and its Aggregate Growth During Gelation Reaction
Authors: Ranganathan R. , Lewis R. , McCool C.S. , Green D.W. , Willhite G.P. 
Published: December 2007

Abstract
A laboratory investigation of a polyacrylamide / aluminum citrate "colloidal dispersion" gel system was conducted to determine whether the system develops in-depth permeability modification in unconsolidated sandpacks. The study includes flow of the polymer and in situ gelation behavior of the gelant in porous media and aggregate growth during the gelation reaction in beakers.
Flow experiments were conducted in long unconsolidated sandpacks where the gel solution was mixed in-line prior to injection. Injection rates were designed to provide adequate residence time for the gel solution to develop in situ flow resistance during displacements based on bulk gel characterization tests. Residual resistance factors were determined following a static rest period where the sandpack was left saturated with the injected gel solution. For comparison, apparent viscosity and residual resistance factors were also determined for the polymer flowing through unconsolidated sandpacks. Membrane dialysis was used to study the aggregate size distribution of the gel system at selected times after mixing.
In-depth in situ flow resistance did not develop when the gelling solution was injected into the sandpacks at frontal advance rates of 2 ft/day. Propagation through the sandpack was similar to a polymer solution. Flow resistance was characterized by the formation of a gelatinous "filter cake" which formed when the injected solution encountered a change in flow medium, such as a screen placed at the ends to retain sand in place, the interface between the 50-mesh coarse sand layer at the inlet and the rest of the sandpack, and void spaces. Delaying injection by two hours after mixing in-line (to simulate field conditions) resulted in severe front-end stripping of gel aggregates. In all cases, residual resistance factors for the gel solution were similar to those obtained with a polymer displacement alone. Effluent fractions from gel solution displacements never developed a gel structure, and their viscosities were significantly lower than the injected solution. In the study of gel size distribution, aggregates were not detected at reaction times up to twelve hours.

Introduction
Application of gelled polymer treatments can significantly improve the amount of oil recoverable by displacement in waterflooding and other enhanced recovery operations in heterogeneous reservoirs containing zones of high permeability. A major factor influencing the success of such treatments is the extent to which the gel solution can be propagated into the reservoir before onset of gelation and formation of an immobile gel structure. Simulation of waterfloods following gelled polymer treatments has shown that in-depth placement of gels has a significant role in the incremental oil recoveries. The use of a partially hydrolyzed polyacrylamide-aluminum citrate "colloidal dispersion" gel has been claimed to produce long-term in-depth permeability modification in certain reservoirs resulting in significant incremental oil recoveries, and has been the subject of an extensive investigation in our laboratory.
The polyacrylamide-aluminum citrate colloidal dispersion gel system developed by TIORCO Inc., Englewood, CO consists of low concentrations of HiVis 350, a partially hydrolyzed polyacrylamide with a viscosity average molecular weight of 27 million, and TIORCO 677, a chelated aluminum citrate solution. Typical concentrations used in this system are 300 ppm polymer and 15 ppm Al3+. This system is reported to be slow forming, thus allowing for in-depth permeability treatment of oil reservoirs.
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