Abstract: This paper discusses the complex evaluation of a basement reservoir in south-eastern Hungary. The host rock is gneiss with negligible porosity; hence the recoverable petroleum rests mainly in fractures. Commercial exploitation of the field started in the early 1980s. After producing more than 1.3 MMm oil, currently has reached the phase of significant production decline.
Since the oil accumulated in and is produced through major fracture zones, conventional G&G studies in general and petrophysical studies based on cores in particular do not provide insights to the reservoir's flow system. Consequently, first a large scale geological subdivision was done on the basis of compartments defined from the results of history matching and reservoir simulation. This primordial model has been refined using 3D seismic data to identify major conducting paths (=fractured zones) and to quantify heterogeneity by combining coherence with the acoustic impedance. On the basis of this simplified model a lot of future production scenario has been predicted.
To reduce the risk profile of the forecasts a Discreet Fracture Network (DFN) Model has been constructed. To establish it the reservoir structure and bulk and pay volume of the field had to be verified, the porosity, fracture intensity had to be re-assessed. The input data and their sources have been as follows:Transmissivity (from PLT's, PI's, single hole tests),
Diffusivity data (from lag times in interference tests)
Fracture intensity (spatial frequency) (from petrophysical logs, well test statistics, and aperturetotransmissivityrelationships)
Orientation of fractures (from borehole image log (CBIL))
Distribution/ spatial frequency of fractures and transmissivities (from flow and no flow zones)
The fracture network model was further matched to the seismic response of the field parameters and validated against reservoir pressure history as well as production forecast. Although the model uses only the fracture network (=single phase) it is able to reproduce the major elements of the production history.
(Figure in full paper)
The Dorozsma field is located in SE Hungary. With two adjacent major fields (Moravaros and Algyo fields) it constitutes the most significant petroleum province of the last 50 years. (OOIP ≈ 115 MMm, OGIP ≈ 105 Tm). Discovered in 1973, Dorozsma field's development started only ten years after. Varying initial static pressure surveyed in and different bubble point pressures measured for different wells indicated well in advance, that the reservoir had minimum 3 (two major and a small), compartments with identical geology, They have been duly named as the "Central Block" and the "SE Block" Block of well Do-16.
(Figure in full paper)
The reservoir volume developed in an originally tight metamorphic rock. Tectonic activity produced significant fracture porosity and "casual" fracture network. The only homogeneous property in this field is the heterogeneity. The pressure history with widely scattered data points in space and time suggests that direct connection between the compartments in general locally between drainage areas of wells in particular is poor to moderate or non-existing;