大象传媒

础诲惫颈蝉辞谤蝉:听Inga Berre, Eirik Keilegavlen and Jan Martin Nordbotten

Short description聽of project:

Shear-dilation-based hydraulic stimulations are conducted to create聽enhanced geothermal systems (EGS) from low permeable geothermal聽reservoirs, which are initially not amenable to energy production.聽Reservoir stimulations are done by injecting low-pressurized fluid into聽the naturally fractured formations. The injection aims to activate聽critically stressed fractures by decreasing frictional strength and聽ultimately cause a shear failure. The shear failure leads to a permanent聽permeability enhancement of the fractures, which contributes to the聽overall reservoir permeability, owing to the damage in fracture surface聽characteristics during the shear failure. Shear stimulation is聽considered a key for geothermal energy development; however, seismicity聽is a critical by-product, which has to be controlled. Numerical modeling聽can provide a deeper understanding on governing mechanisms, which is聽essential for reservoir assessments and the control of seismicity. The聽primary goal of this thesis is to aid further development of EGS by聽contributing to the current state-of-the-art for numerical modeling of聽shear-dilation-based hydraulic stimulations.

Numerical modeling of shear-dilation-based hydraulic stimulations聽requires mathematical modeling of flow and mechanical deformation in聽fractured formations. The initial focus of the thesis is the modeling of聽the mechanical deformation of naturally fractured rock. The deformation聽and stress state of the rock are controlled by the deformation of聽pre-existing fractures, which is governed by different equations than聽the deformation of the surrounding formation. A cell-centered聽finite-volume approach is developed where the fractures are represented聽as co-dimension one inclusions in the domain. The method is capable of聽modeling deformation considering open and closed fractures with complex聽and nonlinear relationships governing the displacements and tractions at聽the fracture surfaces. The method aims to provide benefits for studies聽including flow and deformation couplings in a discontinuous rock.

Hydraulic stimulations are essentially coupled hydro-mechanical聽processes, where the deformation of fractures has an impact on the聽permeability as well as on the stress state of the rock. We develop a聽computational model, which has the capability to capture these聽interrelations in two- or three-dimensional domains. Considering the聽significance of the pre-existing fractures, we model the reservoir as a聽network of explicitly represented large-scale fractures immersed in a聽permeable rock matrix. The model can forecast the permeability evolution聽of geothermal reservoirs with complex fracture networks.

To be able to mitigate the seismic hazards, the contributing processes聽and the interaction between them should be examined. The computational聽model developed here also has the capability to investigate the induced聽seismicity. By using the developed model, a novel hypothesis regarding聽the induced seismicity generated after the termination of injections has聽been tested. During the fluid injections, the pressure builds up inside聽the fractures, which causes normal deformation and increases the void聽place between the fracture surfaces. The termination of the injections聽reverses the void increase; i.e., the fracture starts to close owing to聽the pressure decrease. We identify that the fracture closure is one of聽the mechanisms that are responsible for the induced seismicity generated聽after the termination of injections.

Link to thesis at BORA-UiB:聽