The oilfield Patos-Marinza, located in the southern part of Albania with an OOIP of approximately 2 billion bbls, is the biggest oilfield in the country. One intriguing feature of this heavy oil field is that it dips from outcrop in the hills to the south down to 2000 m over a distance of 17 km. The oil gravity improves accordingly from 6 °API up to 33 °API with the depth of the formation. Due to the broad range of depth and oil quality a multitude of enhanced oil recovery methods have been performed in the past, one being cyclic in situ combustion which was applied in the upper part of the reservoir.
In situ combustion, originally designed as flood patterns, has been practiced in the south of the field for about 20 years starting in 1973. Here the Driza formation is 450 m deep, oil gravity is 11 °API, and viscosity is approximately 9000 cp. Difficulties in production wells (lack of production, sand flow) prompted the state-owned operator, Albpetrol, to take remedial action in the form of cyclic in situ combustion for sand consolidation and production increase. Injection of 1200 - 3000 m?/d/well (cum. ± 200,000 m?/well) air resulted in a burned zone of approximately 5 m radius, and a stabilized reduction of sand inflow from 8 % down to less than 2 % was observed.
In successful applications the improved oil production was as high as 650 tons cumulative, and the subsequent oil production yielded considerably less sand content. Technical difficulties especially with burners resulted in two failures. Cyclic in situ combustion proved nonetheless to be a successful application to consolidate the formation sand around the wellbore and increase the oil production significantly.
The Patos-Marinza Oilfield is an onshore field located east of the city of Fier in South Central Albania (fig.1). It occupies an area of approximately 24,000 hectares. The Patos Oilfield was discovered in 1926 and started commercial oil production in the early 1930's. Development to the north continued with drilling over 2300 wells up to now reaching the boundaries of the field in the late 1980's.
The main sandstone reservoirs dip at 8 - 13 °?from an outcrop in the South containing mineable tar sands to the oil water contacts at 1800 m in the north. There are multiple stacked sand reservoirs of Upper Miocene (Messinian) age, the most important ones being Gorani, Driza, and Marinza (top to bottom). The Driza reservoir contains the majority of OOIP and reserves. It consists of up to 6 sand layers numbered from top to bottom, with the D1 being the main sand continuous throughout the field. The Driza is a suite of unconsolidated sandstones with porosity of 25 - 26 % and net pay varying areally from 7 to 95 m (south to north).
Oil gravity also varies throughout the field depending on the depth of the reservoir and the type of formation. Marinza oil (north) has a gravity up to 860 kg/m? (33 ° API) due to greater depths and an areally closer position to the source rock.
