Improving Gel Performance in Fractures: Chromium Pre-Flush and Overload
Authors: Wilton R. , Asghari K. 
Published: March 2010

Abstract
High water cuts during waterflood operations are a major problem encountered in mature reservoirs. Areas of the reservoir that are fractured, either naturally or hydraulically, are excellent pathways for floodwater to bypass oil-bearing pore spaces. Gel placement within fractured zones of the reservoir is a technique that has been employed to decrease water production. In order to utilize this technique more effectively, the improvement of gel placement and its performance within fractures must be investigated.
For the purposes of this study, two experimental setups are developed. Initially an acrylic fracture model is developed in order to obtain qualitative information about flood fluid penetration into the placed gel. The rupture pressure of the HPAM-Cr (III) [hydrolyzed polyacrylamide-chromium (III) acetate] gel system is observed for 1x, 2x?and 3x?gel systems (multiplier refers to chromium concentration) within the fractures. The rupture pressures observed are generally higher for gel systems with greater chromium concentration. The acrylic setup also allows for visual observation of the gel's performance and behaviour during water injection. Water penetration is dominated by one major channel. Smaller channels are often observed to either branch off from the dominant channel or smaller side channels would connect and join the flow path of the major channel.
Secondly, Berea sandstone slabs are cut and an experimental setup is built in order to study two main mechanisms for improved gel placement. The application of Cr (III) acetate pre-flush and overload are investigated in order to determine their effect on gel performance within fractures. Both techniques compensate for the amount of chromium lost to the matrix via molecular diffusion and the integrity of the gel is maintained. This allows for significant fracture blockage without having to place performed gel or placing the gel ant with leak-off in order to achieve a stable gel.

Introduction
Many reservoirs currently under production suffer from excessive water production. Water could be supplied either by a natural water source (e.g. aquifer) and/or because of waterflooding. Waterflooding is normally used in order to displace any remaining oil in the reservoir matrix after the primary stages of oil production. Presence of high permeability zones in the reservoir provides pathways for water to bypass oil-bearing regions and break through into the production wells. Areas of the reservoir that are fractured, either naturally or hydraulically, are excellent pathways for floodwater to penetrate and consequently bypass oil-bearing pore spaces.
Blocking the high permeable thief zones and diverting water towards the unswept regions of the reservoir has been proposed and used by oil producers as a viable remedy for this problem. In-depth gel placement is the most widely used technique for blocking high permeable zones of reservoirs. This technique has been implemented through many field trials around the world and researchers have successfully determined the mechanisms governing this process in porous media.
Although gel placement in fractures is a common practice in the field, the mechanisms controlling the performance of this technique in fractures are not well understood. This has created a challenging opportunity for researchers to study the detailed mechanisms of gel placement and performance in fractures.