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

Abstract

The Schönkirchen Tief reservoir is located in the Vienna basin. The reservoir contained 19 std m 3 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 m 3 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 To further increase the amount of gas that can be used for UGS, fluids have to be withdrawn from the reservoir.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 m 3 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.

The Role of Diffusion for Nonequilibrium Gas Injection Into a Fractured Reservoir

“…From outcrop studies and analysis of the production behavior, a vertical fracture spacing of approximately 3 m was determined [for more details, see de Kok and Clemens (2009)]. …”

“…An integrated study showed that sufficient working volume can be achieved and some incremental oil might be recovered (de Kok and Clemens 2009). The Schönkirchen Tief reservoir is characterized by a high-permeability-fracture and a lower-permeability-matrix system.…”

“…The Schönkirchen Tief reservoir is characterized by a high-permeability-fracture and a lower-permeability-matrix system. During the UGS operations, the gas/oil contact (GOC) in the fractures is expected to change by more than 100 m while the matrix is still liquid-filled (de Kok and Clemens 2009). Hence, the matrix is surrounded by gas, and components from the injected gas can diffuse from the fractures into the matrix and vice versa.…”