Improving Gel Performance in Fractures: Chromium Pre-Flush and Overload

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.

Polymers and Polymer-Based Gelants for Improved Oil Recovery and Water Control Applications in Naturally Fractured Chalk Formations

“…HPAM polymers (without crosslinker) injected in the fractured cores containing oil for the experiments shown in Figure 10, clearly ЉinteractЉ with the rock matrix delaying the oil recovery. This interaction in the form of leakoff during the placement of gelants in a fractured system will obviously affect gelation time (and ability to form gel) and gel strength as has been demonstrated by several publications (Sydansk, 1988;Seright, 1995;Seright, 2001;Ganguly et al, 2002;Wilton and Asghari, 2007;Brattekås et al, 2015a).…”

“…An acrylic fracture experimental model was used by Wilton and Asghari (2007) to obtain qualitative information about flood fluid penetration into a placed gel within the fractures, and reported rupture pressures which were generally higher for gel systems with greater chromium concentration. The use of either Cr(III) acetate pre-flush or overload was investigated in Berea sandstone slabs to determine gel performance.…”

New Insight from Visualization of Mobility Control for Enhanced Oil Recovery Using Polymer Gels and Foams

“…The mechanical strength of a gel in a fracture is an indication of its blocking ability and is often determined in the laboratory by measuring the gel rupture pressure-the pressure at which the gel in the fracture breaks apart and permits fluids to pass through it. If the injection pressure exceeds, the rupture pressure of the gel during a chaseflood, the gel-filled fracture will be partially re-opened to flow and fluid channelling through the blocked fracture will start again [4,5,7,16]. Measurements of rupture pressures in open fractures with defined geometries have shown that gel mechanical strength after placement is dependent on the gel state during placement [7].…”

“…It was suggested that gelation did not always occur [7], because gelant may experience compositional changes that interfere with gelation upon contact with the reservoir rock and fluids [5,17], e.g. cross-linker diffusion into the surrounding matrix that may lead to gelation failure [16]. Although initially comparable, the blocking capacity over time during chasefloods was consistently higher for pre-formed gels compared with gels with in situ gelation.…”

The Effect of Cr(III) Acetate-HPAM Gel Maturity on Washout from Open Fractures

“…Polymer gel resistance to washout from fractures was previously studied after placement of immature and mature gel (Ganguly et al (2002), Seright (2003), Wilton and Asghari (2007)). Ganguly et al (2002) placed gelant in fractured cores and slabs, with and without matrix taps to promote leakoff to the matrix.…”