Geological description of Szőreg-1 underground gas storage
1. General summary
Szőreg-1 reservoir is one of the gas cap-type oil reservoirs of Algyő structure, as an Upper Pannonian sandstone reservoir. Production in this reservoir started in 1967 (primary oil production), then with water injection, and from 1994 gas production. The well network and the reservoir engineering technology were based on the former reservoir geological model. In 1997 a new 3D geological interpretation was prepared and it found that the storage is a staggered reservoir with relatively thick structure.
2. Summary of storage information
2.1 Geological information
Szőreg-1 reservoir is located within a dome-type reservoir sequence; where the trap evolution factor was the structural closure emerging from the dom. The aggregate thickness of the sequence is 25-40 m, whereas the effective thickness varies between 6-28 m – and it is higher in the north-western and lower in the south-eastern section of the reservoir. The procession in 1980 showed 11-31 % porosity, and 1-2000 mD permeability. The leff/laggr values vary, and in most parts of the reservoir it is 0,4-0,6, and the average is 0,53.
The highest structural position of Szőreg-1 reservoir can be found in Algyő-608 and –634 wells at -1715.6 and -1719.2 mbsl (meter below sea level) depth; between the said two sections there is a saddle where the storage roof is nearly 15 m lower. The reservoir inclination is between 1-5o, higher in the north-eastern and lower in the south-western wing.
The two geological processions performed at different dates define the same contact values. The gas -oil contact is at -1756 mbsl depth. There is an oil body with extreme asymmetry located below the large gas cap: the oil belt is the largest at the western wing and it varies between 12-17 m. This figure keeps on gradually decreasing towards east. The oil body disappears on the eastern wing of the reservoir, according to the 1980 procession document, and the storage has the gas-water contact here. Wells drilled at this section (Algyő-768, -801, -839) can provide evidence for this. It must be noted however that the geophysical interpretations suggest some kind of oil saturation, moreover Algyő-839 well delivered minimum oil production that was soon water flooded.
2.2 Reservoir fluid parameters, reservoir temperature and initial pressure
The initial (1967) reservoir pressure was at -1756 m (gas-oil contact) 182.5 bar, and the temperature 93 oC.
The oil density at 20 o C was 800 kg/m3.
Value of reservoir space parameters at initial pressure and temperature:
Boi = 1.398 m3/m3
Rsi = 121.0 m3/m3
Bgi = 6.068 m3/th m3
As a consequence, the oil reservoir density is 643 kg/m3, and its viscosity is 0,32 cP.
The gas-condensate ratio calculated with the cap gas initial composition is 5054 m3/m3.
2.3 initial geological reserve with volumetric estimation and the current status
The initial geological reserve of Szőreg-1 reservoir:
Oil, th m3: 12814 (th t: 10251)
Saturated gas, Mm3: 1550.5
Cap gas, Mm3: 6560.6
The production from the reservoir for oil started in 1967, then for gas in 1994. (gas production stopped on December 31, 2006)
During this period oil wells produced:
5 307 600 m3 oil
2 390.9 Mm3 gas
15 978 000 m3 water
At the same time injected from gas wells:
20 960 900 m3 water
323 Mm3 gas
23 085 m3 water
Reservoir pressure changes during production: 182.5 -> 155.2 bar (-1756 m).
Current production ratios: in oil wells with 97 % water content (!)
in gas wells 1 300 m3/m3 GFC (cca. 50% condensate)