Pressure analysis of the hydromechanical fracture behaviour in stimulated tight sedimentary geothermal reservoirs

Hydromechanical phenomena in fractured sediments are complex. They control the flow in stimulated tight sediments and are crucial for the exploitation of geothermal energy from such rocks. We present the analysis of a cyclic water injection/production (huff-puff) process, a promising method to extract geothermal energy from tight sedimentary reservoirs. It uses a single borehole, which considerably reduces investment costs. A huff-puff test was performed in a 3800-m deep sedimentary formation (borehole Horstberg Z1, Lower Saxony, Germany). The analysis presented herein explains the downhole pressure measurements by a simplified reservoir model containing a single vertical fracture. The model addresses the flow behaviour between the fracture and the rock matrix in a layered formation, and the coupling between fluid flow and the mechanical deformation of the fracture. The latter aspect is relevant to predict the efficiency of the geothermal reservoir because cooled regions resulting from a particular injection/production scheme can be identified. The analysis methods include: (1) the curve-fitting code ODA used for a determination of different flow regimes (radial or linear), (2) an analytical solution for the calculation of the injection pressure, assuming a time-dependent fracture area, and (3) the simulator ROCMAS, which numerically solves the coupling between fluid flow and fracture deformation. Whereas each single approach is insufficient to explain the complete test data, a combination of the results yields an understanding of the flow regimes taking place during the test.