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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