GEOTHERMAL PROJECT GENESYS, HANNOVER: 3D RESERVOIR MODELING OF THE WEALDEN (BÜCKEBERG) FORMATION

The GeneSys (Generated Geothermal Energy System) project in Hannover, Germany, aims at demonstrating that deep geothermal energy can be used in sedimentary basins with average heat flow, producing from tight sedimentary rocks. To enable sufficient water production rates, an artificial circulation system has to be generated in the target formation – in the form of fractures. The goal is to heat the offices of Geozentrum Hannover, housing three geoscientific institutions with some 1000 employees. To keep costs down, GeneSys aims at producing heat from a single well. A cyclic “huff-puff” concept is pursued, requiring a large water storage facility. This storage reservoir can be provided by the Wealden sandstones of the Lower Cretaceous Berriasian stage (also called Bückeberg Formation).

The goal of this paper is to present a 3D reservoir model of the Wealden, as input for simulating the pressure evolution caused by cyclic injection and production of 100,000 m³ of water, through well Gross-Buchholz GT1 (GBGT1).

The facies of the North German Wealden is lacustrine with very minor marine influence, deposited in the small, east-west trending Lower Saxony Basin. In the Hannover area, the facies is coastal to deltaic, with the nearest shoreline and sediment source towards the south. To constrain the facies distribution, information from 24 neighbouring hydrocarbon exploration wells has been incorporated in the model. No particular marker horizons could be correlated from well to well across the entire model, but a trend from more claystones at the bottom of the Wealden to more sandstones at the top is visible.

These facies settings have been mimicked using Petrel’s “Truncated Gaussian With Trend” algorithm, using a high influence range. The constraint given by the wells forces lobe shaped sandstone-claystone distributions trending at high angles to the shoreface. Based on the facies distribution, petrophysical properties such as porosity and permeability are then attached to each cell. These are available from well GBGT1, where porosity and permeability have been derived from log, lab and microscope analysis. Porosity is in the order of 15 - 18 % in the sandstones and 5 - 10 % in the claystones, while permeability is highly variable, with 2 - 347 mD in the sandstones and 10^-4 - 5 mD in the claystones.