ALBERT

Description: The influence of elevated temperature on injection-induced fault slip is poorly constrained. In this study, at steady-state elevated temperatures, triaxial shear-flow experiments on a sawcut fault in granite were conducted to simulate injection-induced slip of a critically stressed fault. Our results suggest that an elevated temperature favors a more uniform fluid pressure distribution over the fault surface mainly by reducing water viscosity. At temperatures above ambient, a larger perturbation force from the injected fluid is required to reactivate the fault primarily because of the enhanced thermally activated fault healing processes, resulting in a faster fault slip rate upon failure. This study may partially explain the causal link between higher reservoir temperature and higher maximum magnitude of injection-induced earthquakes in geothermal systems, and the observation that larger magnitude seismic events concentrate near the deeper part of the reservoir, where temperature is higher.

Description: Previous studies have revealed that hydraulic fracturing behavior in crystalline rock (i.e., granite) is highly dependent on mineral content, grain size, and heterogeneity and is therefore quite different from the behavior observed in sedimentary rock (i.e., sandstone). We investigated hydraulic fracture paths generated via continuous and cyclic injections in Gonghe granite through uniaxial testing. Hydraulic fracturing breakdown pressure was reduced by 20% in cyclic injection. In particularly, we analyzed the role of biotite grains and pre-existing defects – such as natural fractures – in controlling hydraulic fracture propagation. We found that hydraulic fracture crossed natural fracture either without or with an offset in continuous injection, while hydraulic fracture was arrested by natural fracture in cyclic injection, and NF is more likely to be activated during high-cycle injection. This can be beneficial if hydro-shearing of pre-existing flaws is anticipated.

Description: Previous studies have revealed that hydraulic fracturing behavior in crystalline rock (i.e., granite) is highly dependent on mineral content, grain size, and heterogeneity and is therefore quite different from the behavior observed in sedimentary rock (i.e., sandstone). We investigated hydraulic fracture paths generated via continuous and cyclic injections in Gonghe granite through uniaxial testing. Hydraulic fracturing breakdown pressure was reduced by 20% in cyclic injection. In particularly, we analyzed the role of biotite grains and pre-existing defects – such as natural fractures – in controlling hydraulic fracture propagation. We found that hydraulic fracture crossed natural fracture either without or with an offset in continuous injection, while hydraulic fracture was arrested by natural fracture in cyclic injection, and NF is more likely to be activated during high-cycle injection. This can be beneficial if hydro-shearing of pre-existing flaws is anticipated.

Description: This study investigates numerically several hydraulic fracturing experiments that were performed on intact cubic Pocheon granite samples applying different injection protocols. The goal of the laboratory experiments is to test the concept of cyclic soft stimulation which aims to increase permeability sustainably among others. The Irazu 2D numerical code is used to simulate explicitly coupled hydraulic diffusion and fracturing processes under bi-axial stress conditions. Using the hybrid finite-discrete element modelling approach, we test two injection schemes, constant-rate continuous injection and cyclic progressive injection on homogeneous and heterogeneous samples. Our study focuses on the connection between the geometry of hydraulic fractures, fracturing mechanisms and the permeability increase after injection. The models capture several characteristics of the hydraulic fracturing tests using a time-scaling approach. The numerical simulation results show good agreement with the laboratory experiments in terms of pressure evolution characteristics and fracture pattern. Based on the simulation results, the constant-rate continuous and cyclic progressive injection schemes applied to heterogeneous rock sample with pre-existing fractures show the highest hydraulic aperture increase, and thus permeability enhancement.

Description: The effect of normal stress variations on fault frictional strength has been extensively characterized in laboratory experiments and modelling studies based on a rate-and-state-dependent fault friction formalism. However, the role of pore pressure changes during injection-induced fault reactivation and associated frictional phenomena is still not well understood. We apply rate-and-state friction (RSF) theory in finite element models to investigate the effect of fluid pressurization rate on fault (re)activation and on the resulting frictional slip characteristics at the laboratory scale. We consider a stepwise injection scenario where each fluid injection cycle consists of a fluid pressurization phase followed by a constant fluid pressure phase. We first calibrate our model formulation to recently published laboratory results of injection-driven shear slip experiments. In a second stage, we perform a parametric study by varying fluid pressurization rates to cover a higher dimensional parameter space. We demonstrate that, for high permeability laboratory samples, the energy release rate associated with fault reactivation can be effectively controlled by a stepwise fluid injection scheme, i.e. by the applied fluid pressurization rate and the duration of the constant pressure phase between each successive fluid pressurization phase. We observe a gradual transition from fault creep to slow stick–slip as the fluid pressurization rate increases. Furthermore, computed peak velocities for an extended range of fluid pressurization rate scenarios (0.5 MPa/min to 10 MPa/min) indicate a non-linear (power-law) relationship between the imposed fluid pressurization rate and the peak slip velocities, and consequently with the energy release rate, for scenarios with a fluid pressurization rate higher than a critical value of 4 MPa/min. We also observe that higher pressurization rates cause a delay in the stress release by the fault. We therefore argue that by adopting a stepwise fluid injection scheme with lower fluid pressurization rates may provide the operator with a better control over potential induced seismicity. The implications for field-scale applications that we can derive from our study are limited by the high matrix and fault permeability of the selected sample and the direct hydraulic connection between the injection well and the fault, which may not necessarily represent the conditions typical for fracture dominated deep geothermal reservoirs. Nevertheless, our results can serve as a basis for further laboratory experiments and field-scale modelling studies focused on better understanding the impact of stepwise injection protocols on fluid injection-induced seismicity.

Description: Cyclic fluid injection has been demonstrated as a plausibly effective and controllable strategy to mitigate the seismic risks during hydraulic stimulation. The mechanism involved remains largely unconstrained, and our ability to control the activation of critically stressed, locally undrained faults is still limited. Injection-induced activation of these faults can be one of the most threatening scenarios as they likely perturb the stability of nearby faults beyond the stimulation volume. Here, we perform a series of laboratory fluid injection tests on critically stressed, locally undrained faults in low-permeability granite to offer insights into cyclic fluid injection as a possible solution for seismic risk mitigation. Our results show that cyclic fluid injection promotes fluid pressure diffusion on the faults, but a reduction in seismic moment release depends on several cycle-related factors, such as the critical injection pressure and injection frequency. Particularly, cyclic fluid injection could be inefficient for fluid pressure diffusion if the critical injection pressure is very close to the predicted pressure at fault failure, or over-reduced to cause excess fluid injection and long-term frictional healing. A proper design of injection parameters is thus essential to balance the energy budget between the seismic energy and hydraulic energy. Our study reveals that the effectiveness of cyclic fluid injection is also dependent on fault drainage conditions, stimulation requirements, as well as dynamic responses of faulted reservoirs, which could guide the future development of cyclic fluid injection.