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Some Aspects and Problems of Mature Oilfield Geological Reinterpretation

The largest part of the Northern Croatia belongs to southern part of the Pannonian basin. Sava and Drava depression are the main Croatian oil provinces. A number of smalls to medium size oilfields have been discovered there between 1952 and 1970. Oil production scheme in the majority of the oilfields included implementation of water injection early in the life of reservoir. This resulted in favourable secondary oil recovery (recovery factor from 0.25 to 0.40). Revitalization scenaria are being examined for those larger mature oilfields, presently exhibiting declining oil production with increasing watercut. The process includes reexamination of old, as well as consideration of additional petrophysical data, acquired later in the process of reservoir development. The geological reinterpretation is aimed at reevaluation of OOIP in order to obtain more dependable assessment of target oil available for EOR method implementation. There are 15 gas and oilfields in Drava depression, where inhomogeneous clastic rocks (sandstones) are deposited as reservoirs. Initial hydrocarbons-in-place, as estimated at the time of discovery, especially oil reserves, turned out to be low-estimated, since present production data indicate higher reserve values. With time much more geological knowledge about fields’ margins and petrophysical data have been collected. At the onset of production, almost all wells, especially production wells, were cored scarsely or not at all. This is why petrophysical parameters ( , k, Sw) were determined mostly from electric log curves. In that period, with existing log equipment only specific resistivity and spontaneous potential of formation could be measured, and it was not possible to apply method of quantitative analysis of geophysical well measurements, which is standard nowadays. In more advanced phase of oilfield production, new interpolation wells were systematically drilled and main reservoirs were extensively cored. This provided lab derived petrophysical data pool, that was necessary to compare difference between laboratory (core) values and values from log curves analysis. The data from new wells were subsequently used in conjuction with modern and complex quantitative dual-mineral analysis (Complex Reservoir Analysis), that was performed on the old reservoirs. Result was extensive documentation with statistical analyses as well as with graphical reviews and maps. In this paper, case study of Šandrovac oilfield, as an example of these procedures, is presented in this paper.

mature oilfield; reinterpretation; geologic; revitalization; petrophysical properties

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