Hydraulic fracturing in a sedimentary geothermal reservoir: Results and implications

Field experiments in a geothermal research well were conducted to enhance the inflow performance of a clastic sedimentary reservoir section. Due to depths exceeding 4050 m, bottom hole temperatures exceeding 140 °C, and open hole section (dual zone), technically demanding and somewhat unprecedented conditions had to be managed. The fracturing operations were successful. Fractures were created in two isolated borehole intervals and the inflow behaviour of the reservoir was decisively enhanced. The effective pressures applied for fracture initiation and propagation were only slightly above in situ pore pressures. Nevertheless, the stimulation ratio predicted by fracture performance modelling could not be achieved. Multiple reasons could be identified that account for the mismatch. An insufficient fracture tie-back, as well as chemical and mechanical processes during closure, led to reduced fracture conductivities and therefore diminished productivity. The insights gained are the basis for further fracture design concepts at the given and geologic comparable sites.