Improved Sweep in Bati Raman Heavy-Oil CO2 Flood: Bullhead Flowing Gel Treatments Plug Natural Fractures

Abstract

ABSTRACT This paper covers the successful pilot field application of polymer gels for reservoir conformance improvement in the ongoing CO2 injection project at Bati Raman heavy-oil field in Southeastern Turkey. Bati Raman is a naturally fractured carbonate reservoir where the heterogeneities and the unfavorable mobility ratios between CO2 and the heavy oil cause inefficient sweep of the reservoir. These conditions prompted the pilot application of a conformance improvement fracture-plugging (flowing) gel system in three wells in July 2002. Based on injection tests performed in the field, approximate treatment volumes were estimated to be on the order of 10,000 bbls for each well. Volumes actually pumped ranged from ~6,500 - ~11,000 bbls. All three of the wells showed a gradual increase in injection pressure during treatment, indicating a decrease in injectivity index as treatment progressed. During one treatment an offset producer experienced changes in fluid level consistent with rapid pressure transmission via the connecting fracture early in the treatment, with later loss of such communication. This behavior provides direct evidence of fracture plugging during treatment. A mechanistic semi-analytical model based on previously published laboratory work (Seright, et.al) obtained a good match with the field data. The rate-pressure data were fed into the model and effective fracture widths were back calculated. Comparison of results with the FMI log findings isexplained. Gel monitor wells responses were scaled based on field data using Fetkovich type DCA. These studies enabled the incorporation of the effect of reservoir heterogeneities on the gel propagation radius so that future gel treatment design parameters could be optimized. Pre and post-treatment CO2 injection pressures and the rates are shown in the following table: WELLPRE-GEL (7/2002)POST-GEL (7/2003)Choke Size (x/64")Inj. Pressure (psig)Inj. Rate (MMscf/d)Choke Size (x/64")Inj. Pressure (psig)Inj. Rate (MMscf/d)109123401.12105200.68116102401.14104000.70124102401.44105400.67 Sweep efficiency was increased as defined by produced oil/injected gas ratio. One year average post-gel oil rate from 19 offset producers is 720 stb/d as compared with pre-gel oil rate of 645 stb/d. Rate of increase from the treatments is thus 75 bbl/d or 12 %, which indicates a payout time of 12 months. Keeping this enlightened approach and seizing on the key concepts, 4 more CO2 injector wells are slotted for treatment in 2004 to follow up on the encouraging results.