A Case Study of Foamy Oil Recovery in the Patos-Marinza Reservoir, Driza Sand, Albania

Abstract

Abstract With a current recovery of only 5%, Albania's largest oilfield, Patos-Marinza, (first commercially produced in 1939) still contains potentially more than 100 MMbbl of reserves of 10 to 12 ° API oil at a depth of 1,600 m. A consortium consisting of Albpetrol Albania, Preussag Energie (Germany), Premier Oil (Britain), and the IFC (the private sector arm of the World Bank) are making plans to improve oil recovery in the area. Test wells producing 1 to 2 m3/d yielded large rate improvements to over 30 m3/d using "progressive cavity pumps" up to 30 m3/d with producing sand cuts around 10 to 35%. PVT analysis of the oil, with a relatively high solution GOR of 31 m3/m3, shows a strong tendency to foam (gas in microbubbles without forming a continuous gas phase) below the bubble point, near the initial reservoir pressure. Core tests with different depletion rates resulted in primary oil recoveries of greater than 30%. This paper discusses the detailed suite of laboratory measurements used to examine the Driza crude oil, in the context of present theories of foamy oil and cold heavy oil production with sand (CHOPS). Introduction The Patos-Marinza oilfield is an onshore field located east of the city of Fier in South Central Albania (Figure 1). It occupies an area of approximately 24,000 hectares and was discovered in 1926, with commercial oil production starting in the early 1930s. Development to the north continued with drilling of over 2,300 wells, reaching the boundaries of the field in the late 1980s. The main sandstone reservoirs dip at 8 ° to 13 ° from an outcrop in the South containing mineable tar sands to the original oil water contacts at 1,800 m in the north. There are multiple stacked sand reservoirs of Upper Miocene (Messinian) age, the most important ones being the Gorani, Driza, and Marinza (top to bottom). The Driza reservoir contains the majority of the OOIP and reserves. It consists of up to six sand layers numbered from top to bottom, with D1 being the main sand continuous throughout the field. The Driza is a suite of unconsolidated sandstones with porosity of 25 - 28% 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. The Marinza oil (north) has a gravity up to 825 kg/m3 (40 ° API) due to greater depths and a closer position areally to the source rock. The oil gravity in the Patos area (southern half of the field) is about 998 kg/m3 (11 ° API) with dead oil viscosity over 9,000 mPa.s under reservoir conditions. Due to the unfavourable fluid properties, oil recovery to date in the entire reservoir amounts to less than 5% under depletion drive. The wells with a 2.5 acre spacing generally produce less than 1 M3/d of oil with tubing insert pumps landed halfway to bottom due to equipment limitations.