| Title | Multiple Fracture Stimulation Treatments to Develop an Enhanced Geothermal System (EGS) - Conceptual Design and Experimental Results |
|---|---|
| Authors | G. Zimmermann, A. Reinicke, G. Blöcher, I. Moeck, G. Kwiatek, W. Brandt, S. Regenspurg, T. Schulte, A. Saadat, E. Huenges |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | Enhanced Geothermal Systems (EGS), hydraulic fracturing, acid stimulation |
| Abstract | Enhanced geothermal systems (EGS) are engineered reservoirs developed to extract economic amounts of heat from low permeability and/or porosity geothermal resources. To enhance the productivity of reservoirs, a holistic concept is necessary to actively make reservoir conditions from profitable using specially adjusted stimulation treatments, such as multi fracture concepts and site specific well path design. The results of previously performed stimulation treatments in the geothermal research well GtGrSk4/05 at Groß Schönebeck, Germany are presented. The reservoir is located at a 4100-4300 m depth within the Lower Permian of the NE German Basin. The reservoir rock is classified by two lithological units from bottom to top: volcanic rocks (andesitic rocks) and siliciclastics ranging from conglomerates to fine grained sandstones (fluvial sediments). The stimulation treatments included multiple hydraulic stimulations and an acid treatment. In order to initiate a crossflow from the sandstone layer, the hydraulic stimulations were performed in different depth sections (two in the sandstone section and one in the underlying volcanic section). In low permeability volcanic rocks, a cyclic hydraulic fracturing treatment was performed over 6 days in conjunction with adding quartz in low concentrations to maintain a sustainable fracture performance. Flow rates of up to 150 l/s were realized, and a total of 13170 m³ of water was injected. A hydraulic connection to the sandstone layer was successfully created in this way. However, monitoring of the water level in the offsetting well EGrSk3/90, which is 475 m apart at the final depth, showed a very rapid water level increase due to the stimulation treatment. This can be explained by a connected fault zone within the volcanic rocks. Two gel proppant treatments were performed in the slightly higher permeability sandstones to obtain long-term access to the reservoir rocks. A total of 100 tons of high strength proppants were injected with 500 m³ of cross-linked gel. The subsequent production test in conjunction with flowmeter logging showed an improvement of productivity by a factor of more than 4. Due to assumed residual drilling mud (constituents: calcite, dolomite, aragonite) in the near wellbore vicinity, an acid stimulation was performed using a coil tubing unit. The following nitrogen lift test demonstrated another increase of productivity by 30 – 50%. |