Application of In-Depth Gel Placement for Water and Carbon Dioxide Conformance Control in Carbonate Porous Media

Abstract This paper presents the results of an investigation of the application of gel placement in an attempt to reduce the effective permeability of a carbonate porous medium to water and supercritical carbon dioxide, as encountered in the CO2 flooding of carbonate reservoirs. Three gel systems based on a high and a low molecular weight polyacrylamide polymer with chromium(III), as crosslinker, were used for this study. Since sodium lactate is commonly used for increasing gelation time at elevated temperatures, experiments were conducted by adding sodium lactate to the gel solution with a ratio of polymer to sodium lactate equal to one. The higher molecular weight polyacrylamide gel system was composed of 7,200 ppm Alcoflood 935 and 300 ppm Cr(III), while the other gel system tested was composed of 5% low molecular weight polyacrylamide (Alcoflood 254S) with a ratio of 1:12 chromium(III)-acetate to polymer as crosslinker. Experiments were conducted at 8,274 KPa and 40 °C with and without the presence of residual oil. Performance and stability of the 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 alternately injecting several pore volumes of water and carbon dioxide into the porous media in several cycles. The porous medium used was crushed carbonate with an initial permeability of over 9.86 µm2. For all experiments, the presence of Sor led to lower residual resistance factors (RRF). Nevertheless, RRFs were between 100 and a few thousands for all experiments conducted. The results obtained are a clear indication of the effectiveness of these gel systems for conformance control purposes during carbon dioxide flooding projects in carbonate reservoirs. Introduction From the pore-scale point of view, dense carbon dioxide is an ideal displacement fluid for many crude oils. Carbon dioxide gains miscibility with oil and is able to achieve very high recovery efficiencies at reservoir pressures near or above minimum miscibility pressure. However, even when pressure conditions for miscibility are met, this high microscopic sweep efficiency is not often observed in field tests. The combined effect of heterogeneity of the reservoir and the low viscosity of the injected carbon dioxide leads to the viscous fingering of carbon dioxide through fractures, channels, or high-permeable streaks in the reservoir. This causes early breakthrough of injected carbon dioxide into production wells, which reduces oil recovery efficiency. Different techniques have been investigated for reducing channelling through fractures or high-permeability zones. One such technique is in-depth gel placement. This process has been applied successfully in many reservoirs, resulting in fracture sealing, water and gas shut-off, and permeability modification. The objective of gel placement is to reduce fingering of the injected fluid through fractures or high-permeability zones of formation while minimizing gel penetration in hydrocarbon bearing regions of the reservoir. The main advantages of using gels over the other methods, such as cements or mechanical plugs, is their flexibility for pumping without a workover rig, high control of setting time, a deeper penetration into the formation, ease of cleaning, and an easy removal from the wellbore by water recirculation(1, 2).

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