Abstract The ability to predict the magnitude of an earthquake caused by deep fluid injections is an important factor for assessing the safety of the reservoir storage and the seismic hazard. Here, we propose a new approach to evaluate the seismic energy released during fluid injection by integrating injection parameters, induced aseismic deformation, and the distance of earthquake sources from injection. We use data from ten injection experiments performed at a decameter scale into fault zones in limestone and shale formations. We observe that the seismic energy and the hydraulic energy similarly depend on the injected fluid volume ( V ), as they both scale as V 3/2 . They show, however, a large discrepancy, partly related to a large aseismic deformation. Therefore, to accurately predict the released seismic energy, aseismic deformation should be considered in the budget through the residual deformation measured at the injection. Alternatively, the minimal hypocentral distance from injection points and the critical fluid pressure for fault reactivation can be used for a better prediction of the seismic moment in the total compilation of earthquakes observed during these experiments. Complementary to the prediction based only on the injected fluid volume, our approach opens the possibility of using alternative monitoring parameters to improve traffic-light protocols for induced earthquakes and the regulation of operational injection activities.
Un dispositif amovible d'auscultation in situ du comportement hydromecanique des fractures permettant la realisation de mesures simultanees de pression et de deplacement a ete mis au point. Les mesures sont realisees a l'aide de capteurs a fibre optique qui se revelent etre d'un ordre de grandeur plus precis que les mesures par capteurs a cordes vibrantes. La frequence des mesures est egalement bien superieure (120 Hz), ce qui permet d'enregistrer avec beaucoup plus de finesse les variations temporelles des parametres mesures. Ce dispositif a ete teste sur le site experimental de Coaraze, petit massif calcaire fracture situe au Sud-Est de la France. Les experimentations ont consiste a injecter ou pomper un certain volume d'eau (en controlant la pression ou le debit) au niveau de l'intersection d'un forage et de la faille que l'on souhaite caracteriser. Le dispositif instrumental s'est revele pertinent pour caracteriser in situ le comportement hydromecanique des fractures. Les simulations hydromecaniques ont permis de reproduire correctement les experimentations et de determiner par calage les caracteristiques hydromecaniques des fractures (raideur normale, ouverture hydraulique).
Author(s): Oldenburg, C; Dobson, Patrick; Wu, Yuxin; Cook, Paul; Kneafsey, Timothy; Nakagawa, Seiji; Ulrich, Craig; Siler, Drew; Guglielmi, Yves; Ajo-Franklin, Jonathan; Rutqvist, Jonny; Daley, Thomas; Birkholzer, Jens; Wang, HF; Lord, NE; Haimson, BC; Sone, H; Vigiliante, P; Roggenthen, WM; Doe, TW; Lee, MY; Ingraham, M; Huang, H; Mattson, ED; Zhou, J; Johnson, TJ; Zoback, MD; Morris, JP; White, JA; Johnson, PA; Coblentz, DD; Heise, J | Abstract: In support of the U.S. DOE SubTER Crosscut initiative, we established a field test facility in a deep mine and designed and carried out in situ hydraulic fracturing experiments relevant to enhanced geothermal systems (EGS) in crystalline rock to characterize the stress field, understand the effects of rock fabric on fracturing, and gain experience in monitoring using geophysical methods. The project also included pre- and post-fracturing simulation and analysis, and laboratory measurements and experiments. The kISMET (permeability (k) and Induced Seismicity Management for Energy Technologies) site was established in the West Access Drift of the Sanford Underground Research Facility (SURF) 4757 ft (1450 m) below ground (on the 4850 ft level (4850L)) in phyllite of the Precambrian Poorman Formation. We drilled and continuously cored five near-vertical boreholes in a line on 3 m (10 ft) spacing, deviating the two outermost boreholes slightly to create a five-spot pattern around the test borehole centered in the test volume 40 m below the drift invert (floor) at a total depth of ~1490 m (4890 ft). Laboratory measurements of core from the center test borehole showed P-wave velocity heterogeneity along each core indicating strong, fine-scale (~1 cm or smaller) changes in the mechanical properties of the rock. Field measurements of the stress field by hydraulic fracturing showed that the minimum horizontal stress at the kISMET site averages 21.7 MPa (3146 psi) trending approximately N-S (356 degrees azimuth) and plunging slightly NNW at 12°. The vertical and horizontal maximum stresses are similar in magnitude at 42-44 MPa (6090-6380 psi) for the depths of testing, which averaged approximately 1530 m (5030 ft). Hydraulic fractures were remarkably uniform suggesting core-scale and larger rock fabric did not play a role in controlling fracture orientation. Analytical solutions suggest that the fracture radius of the large fracture (stimulation test) was more than 6 m (20 ft), depending on the unknown amount of leak-off.
Abstract Fluid injections into the deep subsurface can, at times, generate earthquakes, but often, they only produce aseismic deformations. Here we analyze the influence of fault hydromechanical properties on the growth of injection‐induced aseismic slip. Using hydromechanical modeling, we show how permeability enhancement in addition to the background stress and frictional weakening has an important effect on the pressure diffusion and slip growth during injection. We find that the more pronounced the fault permeability enhancement, the stronger is the growth of the aseismic slip zone. The effect of enhanced permeability is more pronounced when the fault is initially close to failure. Our results show that aseismic slip grows beyond the pressurized zone when the fault permeability increases, while slip remains behind the pressurized zone when permeability does not vary from its initial preslip value. Thus, fault permeability increases should be considered as complementary mechanism to current models of fluid‐induced aseismic slip.
Abstract The presence of fluid within a fault zone can cause overpressure and trigger earthquakes. In this work, we study the influence of fault‐zone architecture on pore pressure distribution and on the resulting fault reactivation caused by CO 2 injection. In particular, we investigate the effect of the variation and distribution of lithological and rock physical properties within a fault zone embedded in a multi‐layer sedimentary system. Through numerical analysis, we compare several models where the complexity of the fault‐zone architecture and different layers (such as caprock and injection reservoir) are incrementally included. Results show how the presence of hydraulic and mechanical heterogeneity along the fault influences the pressure diffusion, as well as the effective normal and shear stress evolution. Hydromechanical heterogeneities (i) strengthen the fault zone resulting in earthquakes of small magnitude, and (ii) impede fluid migration upward along the fault. We also study the effects of the caprock and aquifer thickness on the resulting induced seismicity and CO 2 leakage, both in heterogeneous and homogeneous fault zones. Results show that a thin caprock or aquifer allows smaller events, but a much higher percentage of leakage through the caprock and into the upper aquifer. The amount of leakage reduces drastically in the case of a multi‐caprock, multi‐aquifer system.
