Exploitation of Crude Oil and Natural Gas Reservoirs in Poland

Summary

1. Crude oil

Crude oil reserves occur in Poland in the following three geological regions:

• - the Carpathian Mountains;

• - the Carpathian Poreland;

• - the Polish Lowland.

The crude oil reservoirs of each geological region differ markedly (i.e. in geological structure. sort of reservoir rock, drive mechanism and degree of depletion).

The Carpathian crude oil fields have been exploited for a number of decades; some even for more than a century. For many years the gravity system has been the drive mechanism. In many crude oil reservoirs gas injections are performed. However due to the complicated geological structure of these deposits, their small area and high degree of depletion, a substantial increase in production cannot be expected. Despite this fact, different tests concerning secondary and even tertiary oil-recovery techniques are being developed on an industrial scale. They are mainly gas drive method (foaming agent-pressure method, alternative gas and water-drive method) as well as waterflooding method (water drive with polymers or surface-active agents, gasoline barrier method).

In one of the Carpathian crude oil fields projects are being initiated for an in-situ combustion application.

The decrease of oil production is counterbalanced here by different technologies directly related to the wells (preheating with electric heaters, paraffin solvent flushing).

The crude oil reservoirs located on the area of Poreland and Polish Lowland are rich in crude oil. Mithin the Foreland region reservoir rocks are formed by sandstones and limestone while in the Lowland region dolomite reservoirs can be found.

In comparison with the Carpathian crude oil reservoirs the afore-going reservoirs also differ in their drive mechanism. They are water drive fields and therefore waterflooding is recommended.

For 20 years now hydraulic fracturing and acidification have been performed on all crude oil wells where permeability has to be increased these operations are primarily accomplished on new oil wells to force the oil or gas to flow into the well.

2. Natural gas reservoirs

Similar to the crude oil reservoirs, natural gas reservoirs occur in the following three geological ragions, namely;

• - the Carpathians

• - the Carpathian Foreland

• - the Polish Lowland.

The Carpathian natural gas reservoirs are old and exploited to a considerable degree (depletion reaches the level up to 90 per cent). They are frequently still exploited owing to the implementation of local low-pressure gas piping systems.

The principal natural gas producing formations present in the Carpathian Foreland region are of Miocene age. Gas resources accumulated there represent 80 per cent of total proven reserves in this region. Gas reservoirs are multihorizontal.

The Polish Lowland reservoirs occur in the geological region named the Fore—sudetic Monoclinal Fold. The principal gas producing reservoirs are red beds covered with limestone and main dolomite. The trap properties of reservoir rocks are highly diversified and range from low to high permeable rocks.

The chemical constitution of the Polish Lowland gas also varies. The principal component of the natural gas here is nitrogen; its content ranges from 20 to 80 per cent (in most cases: 40 to 60 per cent). Furthermore, the natural gas sometimes contains hydrogen sulphide, helium and mercury. The natural gas produced in the dolomite reservoirs contains many heavy hydrocarbons which appear in the form of condensate or light crude oil. Hence, proper utilization of the natural gas is related with many complex problems.

In the case of the Lowland small gas reservoirs the above-mentioned problems must be also considered from the economical point of view. The construction of cleaning installation for a field with reserves of the order of hundreds of million cubic meters is not always justified economically; in particular for the Lowland natural gas reservoirs which are also surrounded by more of less chemically active edge waters. As regards small reservoirs wells completion is followed by water encroachment. Therefore the degree of recovery of small gas deposits tunas to be low.

In order to solve the difficulties related to the exploitation of small sulphurous gas reservoirs it is important to develop and test several simple and cheap gascleaning technologies. It is obvious that such technologies should provide for a complete environmental protection against contamination.