Alternatives of Gas Injection as an Enhanced Recovery Method in Advanced Gas Market Environment

Abstract

In Hungary, the oil prices in the range of $20 makes EOR projects commercially viable. Because of their structures and fluid properties, the oil reservoirs in Hungary are suitable for gas injection EOR methods. This EOR potential is well supported by favourable infrastructure in the local oil industry/surroundings. The paper presents the EOR application for a large reservoir in the Pannonian basin. The Szeged-Móraváros is undersaturated fractured reservoir with Triassic dolomite, Miocene sandstone and conglomerate at the depth of 2630–2450 meters below the sea level. The OOIP derived from material balance is 11.56 million cubic meter and oil gravity is 817 kg/m3. The initial pressure was pr=331 bar, initial temperature was tr=140 °C. In the period of the natural depletion between 1975–1980, the cumulative oil production was 1,27 million cubic meter. In this period, because of the limited water influx, the reservoir pressure dropped below the bubble point and secondary gas cap started to increase. The pressure dropped to pr=244 bar. In 1980, water injection was started to maintain the reservoir pressure. The aim was to increase the reservoir pressure above the bubble point pressure. With this water injection from 1980 to the present time, the oil recovery is 36.4 % with a cumulative oil production of 4.21 million cubic meter. The current reservoir pressure is 250 bar. For studying the further recovery enhancement, laboratory and reservoir simulation studies were carried out to evaluate different gases. Natural gas, nitrogen, and CO2 were selected for the gas injection near the structure top. In Hungary, large CO2 reserves are available 50–250 km distance from the oil field. The natural gas resources are easily available because of the developed gas distribution network and the production of nitrogen by stripping from the air can be also easily realised. At the optimal pr=250 bar pressure, the displacement process is not miscible in the reservoir, and the recovery increment is not sensitive to the type of injected gas or its quality. The reservoir simulation shows the recovery increment is about 12 %. We have selected nitrogen gas as an injection gas based upon economic and environment evaluations. The paper describes how we arrived at this conclusion in a country where there is a well-developed natural gas market. The paper also includes discussion about the methods and criteria used to select the process. Introduction The amount of the produced oil and condensate coming from the EOR processes is 170,4 thousand tons recently. More than 70 % of the total amount is connected to gas injection technologies mainly to CO2 injection but methane and ethane rich gases are also used as the material of EOR injection mass. CO2 is available from natural resources in the Western part of the country, here is the largest CO2 injection project is taking part. In the central area of the country, only limited natural CO2 resources are available. Here a gas enriching plant is working where the CO2 is forming as a sideproduct. CO2 reservoirs are also in the eastern part of the country but their development has not yet begun because of the lack of utilisation. However the most important oil resources of the country are in the Southern, Southern-East areas relatively far from the above mentioned CO2 areas. Therefore, in this region, studying the possibility of methane and nitrogen injection is necessary because of the limited or cost requiring availability of CO2. A systematic screening was made between 1997–2000 for the Southern-Hungarian oilfields. The most important conclusion of the study was that the temperature and pressure conditions, the geological features and the PVT characteristics of the reservoir fluids are favourable mainly for the EOR with gas injection.