Summary The Bati Raman field is the largest oil field in Turkey and contains approximately 1.85 billion bbl of oil initially in place. The oil is heavy (12°API), with high viscosity and low solution-gas content. Primary recovery was less than 2% of oil originally in place (OOIP). Over the period of primary recovery (1961-86), the reservoir underwent extensive pressure depletion from 1,800 psig to as low as 400 psig in some regions, resulting in a production decline from 9,000 to 1,600 STB/D. In March 1986, a carbon-dioxide (CO2) -injection pilot in a 1,200-acre area containing 33 wells was initiated in the western portion of the field. The gas-injection was initially cyclic. In 1988, the gas injection scheme was converted to a CO2-flood process. Later, the process was extended to cover the whole field. A peak daily production rate of 13,000 STB/D was achieved, whereas rate would have been less than 1,600 STB/D without CO2 application. However, the field has undergone a progressive production decline since 1995to recent levels of approximately 5,500 STB/D. Polymer-gel treatments were carried out to increase the CO2 sweep efficiency. Multilateral- and horizontal-well technology also was applied on a pilot scale to reach the bypassed oil. A water-alternating-gas (WAG) application has been applied extensively in the western part of the field. Current production is 7,000 STB/D. This paper documents more than 25 years of experience of the Turkish Petroleum Corporation (TPAO) on the design and operation of this full-field immiscible CO2-injection project conducted in the Bati Raman oil field in Turkey. The objective is to update the current status report, update the reservoir/field problems that TPAO has encountered (unpredictable problems and results), and provide a critical evaluation of the success of the project. Introduction The Bati Raman field is the biggest oil accumulation in Turkey and is operated by TPAO. It contains very viscous and low-API-gravity oil in a very challenging geological environment. Because of the fact that the recovery factor by primary recovery was limited, several enhanced-oil-recovery (EOR) techniques had been proposed and tested at the pilot level in the 1970s and 1980s. On the basis of the success of the laboratory tests and the vast amount of CO2 available in a neighboring field, which is only 55 miles away from the Bati Raman field, huff ‘n’ puff injection was started in the early 1980s. Because of the early breakthrough of CO2 in offset wells in a short period of time, the project was converted to field-scale random-pattern continuous injection. During more than 20 years of injection, the recovery peaked at approximately 13,000 STB/D and began to decline, reaching today's value of approximately 7,000 STB/D. In the case of Bati Raman, in its mature, the injected agent is bypassing the remaining oil and production is curtailed by excessively high gas/oil ratios (GORs). The naturally fractured character of the reservoir rock has been a challenge for establishing successful 3D conformance from the beginning, and its impact is even more pronounced in the later stages of the process. Therefore, the field requires modifications in the reservoir-management scheme to improve the recovery factor and to improve productivity of the current wells.
Summary 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 in which 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 bbl for each well. Volumes actually pumped ranged from approximately 6,500 to 11,000 bbl. 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 (Lane 2002). A mechanistic semianalytical model based on previously published laboratory work (Lane and Seright 2000) obtained a good match with the field data. The rate/pressure data were fed into the model, and effective fracture widths were backcalculated. Comparisons of results with the Formation MicroImager (FMI) log findings are explained. Gel-monitor well responses were scaled based on field data using a Fetkovich type decline-curve analysis. 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 as shown in Table 1. Sweep efficiency was increased as defined by produced oil/injected gas ratio. The 1-year average post-gel oil rate from 19 offset producers is 720 STB/D, as compared with apre-gel oil rate of 645 STB/D. The rate of increase from the treatments is thus 75 B/D, or 12%, which indicates a payout time of 12 months. Keeping this enlightened approach and seizing on the key concepts, four more CO2 injector wells were treated in 2004 to follow up on the encouraging results.
With 1.85 MMM bbl OOIP, the Bati Raman field is the largest oil field in Turkey. After its discovery in 1961, the field was put on stream for primary production until 1986. The recovery factor was only 2% after twenty five year production mainly due to low oil gravity. The well-known immiscible CO2 flooding project commenced in 1986, and the recovery factor reached 5% at the end of 2007. The recent steady decline in production entails the implementation of new development plans and this paper summarizes these efforts. After reviewing the performance of the current CO2 injection, short and long term development strategies were discussed. Short term plans include the continuation of the CO2 project in the areas where it is still viable. Some parts of the field are under WAG process. To improve the recovery in the short run by a better sweep (or displacement), a chemically augmented water injection process was proposed in those areas. Potential chemicals (surfactants and alkalis) were tested for wettability alteration and IFT reduction applying static (spontaneous) imbibition experiments. The best performing chemicals were determined for the field pilot after an economic analysis. In addition, the possibility of steam injection into the field was evaluated for the long run. Due to extreme heterogeneity and fractured structure, crestal steam injection that uses steam as heating rather than a displacement agent was proposed. An analytical study for the optimization of steam injection was provided. To determine the locations for the above listed processes, an extensive reservoir characterization study was performed using dynamic and very limited static -well- data. Using well recorded primary (1961–1986) and CO2 production data (1986–2007), fracture swarms were mapped. In this process, the changes in the initial production rate and GOR over different time periods were considered. The quickest decline in the initial rate and the lowest GOR areas correspond to highly fractured regions. Highly -vertically- fractured areas (typically the crest) were determined for potential steam injection. This analysis also helped detect high quality matrix areas as candidates for chemically augmented WAG.
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