Laboratory Investigation of Indepth Gel Placement for Carbon Dioxide Flooding in Carbonate Porous Media
Authors: Asghari K. , Taabbodi L. 
Published: December 2007

Proposal
This paper presents the results of an investigation on the application of gel placement in an attempt to reduce the permeability of a carbonate porous medium to water and supercritical carbon dioxide, as encountered in the CO2 flooding of carbonate reservoirs. The high and low molecular weight polyacrylamide polymers with chromium (III), as crosslinker, were used for this study. Since sodium lactate is commonly used for increasing gelation time at elevated temperatures, additional experiments were conducted by adding sodium lactate to the gel solution with a ratio of polymer to sodium lactate equal to one.
Experiments were conducted at 1200 psi and 40°C, with and without the presence of residual oil in order to investigate any role the residual oil might play in the performance of gel. Performance and stability of above gel systems for reducing the permeability of the carbonate medium to the injected water and carbon dioxide was tested in a series of flow experiments by alternatively injecting several pore volumes of water and carbon dioxide into the porous media in several cycles.
The porous medium used was crushed carbonate with initial permeability of over 10 Darcies. For all experiments the presence of Sor led to lower residual resistance factors (RRF). Nevertheless, RRFs were between 100 and few thousands for all experiments conducted. The results obtained are clear indication of the effectiveness of these gel systems for conformance control purposes during carbon dioxide flooding projects in carbonate reservoirs.

Introduction
Application of gel treatment is an attempt to reduce or block channeling through the fractures or high permeability layers of oil reservoirs. This technique can divert fluid flow to lower permeability zones and improve the oil recovery. This method has been applied over last few decades as one of the most effective tools for controlling water and gas production. The polymer-gel technology has been applied successfully in many reservoirs, resulting in fracture sealing, water and gas shut-off, and permeability modification.7 The objective of gel placement and similar blocking-agent treatments are to reduce channeling through fractures or high-permeability zones of oil reservoirs without significantly damaging hydrocarbon productivity and improve the overall oil recovery from the flooding process. The goal of gel treatments is to maximize gel penetration and permeability reduction in high permeable zones while minimizing gel penetration and permeability reduction in less permeable zones or hydrocarbon producing zones.1 Although no treatment has been found that reduces water permeability without effecting on oil permeability,2 gel treatments are one of the most aggressive types of conformance control or profile modification techniques. The main advantages of using gels over the other methods such as cements or mechanical plugs, is their flexibility for pumping without a work-over rig, high control of setting time, a deeper penetration into the formation, ease of cleaning, lack of milling time, and an easy removal from the well-bore by water recirculation.3 Gels can be applied to both injection and production wells.4 Injection well treatments are often larger than producing well treatments, then the goal is to fill as large a portion of the conductive channels from injector to producers as possible.5 In order to ensure an effective blocking of high permeable regions, gel systems have to be placed deep enough into the target zones.6
Polyacrylamide polymers cross-linked with chromium (III) have been used primarily to treat fracture systems, casing leaks, near well-bore regions, water shut-off and permeability reduction in reservoirs. Hence, experiments were designed to investigate the stability and performance of several polyacrylamide based gel systems by alternatively injecting water and carbon dioxide through the gelled porous media. The residual resistance factor, RRF (the ratio of the permeability before gelation to the permeability after gelation), was measured for each gel system against the flow of water and carbon dioxide. Gel systems made of high-molecular weight polyacrylamide (Alcoflood 935)-chromium (III) and low- molecular weight polyacrylamide (Alcoflood 254S)-chromium (III) were tested during this research. The goal of this research was to study the stability and resistance of each of the above gel systems for reducing the permeability of a carbonate medium to water and carbon dioxide.