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.

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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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The Bati Raman field is the largest oil field in Turkey and contains some 1.85 billion barrels of oil initially in place. The oil is heavy (12 oAPI) with high viscosity and low solution gas. Primary recovery has been inefficient, less than 2% of OOIP. Over the period of primary recovery, from 1961 to 1986, the reservoir underwent extensive pressure depletion from 1,800 psig to as low as 400 psig in some regions, with a related production decline from a peak of approximately 9,000 Bbls/day to 1600 Bbls/day. In March 1986, a CO2 injection pilot scheme in a 1200 acre area containing 33 wells was initiated in the west portion of the field. The gas injection was initially cyclic; "huff and puff" method was applied. Later, in 1988, the gas injection scheme was converted to a CO2 flood process. Later, the process was widespread to cover the whole field. A peak daily production rate 13000 STB/d was achieved in 1993 in comparison to what would have been less than 1600 STB/d without CO2 application. However, since 1995, the field has undergone a progressive production decline to recent levels at approximately 5,500 Bbls/day. Polymer gel treatments were carried out to increase the CO2 sweep efficiency and arrest the decline. Multilateral and horizontal well technology was also applied on pilot scale to reach the bypassed oil. WAG is applied widespread now. Current production is 7000 Bbls/day. This paper documents TPAO's 25+ years of experience on the design and operation of full field immiscible CO2 injection recovery project conducted in the B.Raman heavy oil field, in Turkey. The objective is to give an up-to-date status of the performance of the application; reservoir/field problems that TPAO had, unexpected occurrences and results and just a general idea of how successful the project has been. Introduction The Bati Raman field, which is the known highest oil accumulation in Turkey, contains very viscous and low gravity oil in a very challenging geological environment. Due to the fact that the recovery factor by primary recovery was limited, several EOR techniques had been proposed and tested in pilot level in the 70s and 80s. Based on the success in the lab tests and vast amount of CO2 available in a neighboring field which is just 55 miles away from the Bati Raman field, field scale huff-and puff injection was started in the early 80s. Due to 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. Over more than 20 years of injection, the recovery peaked at ∼13,000 bbls and began to decline reaching today's ∼7,000 bbl value. In the case of Bati Raman, at this mature state of the process, the injected agent is increasingly bypassing the remaining oil and production is curtailed by excessive high gas oil ratios (GOR). The naturally fractured characteristics of the reservoir rock has been a challenge for establishing a successful 3D conformance from the beginning and its impact is even more pronounced in the later stages of the process. Because of that reason, the subject field requires modification on the reservoir management scheme to improve recovery factors as well as improving productivity of the current wells. BATI RAMAN FIELD Bati Raman was discovered in 1961 in Southeastern Turkey with the completion of BR-1 (Fig-l). The producing formation is the Garzan Limestone, a very heterogeneous carbonate of Cretaceous age. The reservoir fluid is a very heavy crude oil, having an API gravity ranging from 9.7 to 15.1 and a viscosity ranging from 450 to 1000 centipoises at reservoir conditions. The structural trap is a long; partly asymmetric anticline oriented in the east-west direction which measures about 17 km. long and 2 to 4 km. wide. It is limited by an oil/water contact at 600 meters subsea in the north and west, by a fault system in the southwest and south, and by a permeability barrier in the southern and southeastern part of the field. Formation has a gross thickness of 210 ft (64m). The oil column from the top of structure to the OWC is about 690 ft.