Combined Underground Gas Storage and Enhanced Oil Recovery in a Fractured Reservoir

Abstract

Summary The Schönkirchen Tief reservoir is located in the Vienna basin. The reservoir contained 19 std m3 oil originally in place. The current recovery factor after 45 years of production is 59%. The field was produced by water injection. The wells, located at the crest of the high relief structure are exhibiting a high water cut. In 2006–07, a comprehensive study was performed to optimize the future development of the field. The field is made up of different types of dolostones. The lower part of the field consists of fractured dolomite, the upper part of weathered dolomite and dolomite debris. A dual-permeability model was used to simulate the flow in the dolomite and the weathered zone, whereas a single-permeability model was sufficient for the debris. The main recovery mechanisms for this reservoir are imbibition and water/oil gravity drainage. Laboratory experiments indicated that the dolomite is water-wet to mixed-wet at reservoir conditions of 100°C. The results of the study show that an economically attractive option for the field development is establishing an underground gas storage (UGS) and using the injected gas to increase the oil recovery. Because of the fact that the current reservoir pressure is below initial pressure, within half a year, approximately 1 billion std m3 can be injected. Only approximately one-third of this gas has to remain as cushion gas during the first cycle. To further increase the amount of gas that can be used for UGS, fluids have to be withdrawn from the reservoir. Drilling horizontal wells close to the original oil/water contact enables injection of an additional 2 billion std m3 gas. The gas injection leads to gas/oil gravity drainage of the oil and water present in the matrix of the fractured reservoir. The oil is collected by the horizontal wells, resulting in incremental oil recovery of up to 5% of oil originally in place.