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Enhanced geothermal system (EGS) has the potential to offer a large amount of clean energy by extracting stored thermal energy from the subsurface. The effectiveness of heat extraction is dependent not only on the permeability of fractured rocks but also on the stability of preexisting and induced fractures. A better understanding of fracture slip in granite during fluid injection is critical to optimize the strategies of hydraulic stimulation. We experimentally investigated the shear behaviors of a sawcut fracture, a gouge-filled fracture, and a natural fracture in Bukit Timah granite in response to fluid injection under a constant normal stress and a constant critical shear stress, respectively. In the most cases, the pore pressure at the injection-induced failure exceeds that predicted by the Mohr-Coulomb failure criterion. This is attributed to the nonuniform distribution of fluid over the fracture plane, which is associated with lower permeability of fracture and host rocks and higher injection rate. The shear behaviors of sawcut and natural fractures show a complex combination of creep and stick before injection failure, which is presumably dependent on the state of asperity contacts. The gouge-filled fracture always creeps preceding the injection-induced failure, because the stiffness of testing system overweighs the critical rheologic stiffness of fracture. For these three fractures, the slip rate at injection failure increases with higher injection rate, releasing more strain energy. The slip rate induced by fluid injection in this study falls within the slip rate range of slow slip events observed in natural faults.
Harvesting heat trapped in igneous rocks offers us an affordable and sustainable solution to reduce our dependence on fossil fuels. Because of the extremely low permeability of igneous rocks, fluid injection has been used to create and/or activate fractures, enhancing the permeability of the host rocks. Besides the permeability evolution of rock fractures, the effectiveness of heat extraction is also dependent on the frictional stability of preexisting and induced fractures, because frictional instabilities of fractures result in seismic events. For example, fluid injection-induced seismicity in an enhanced geothermal system (EGS) in Basel, Switzerland led to the closure of the project (Majer et al. 2007), so did the California project in the United States also aiming at extracting underground geothermal energy (Elsworth et al. 2016). Therefore, a better understanding of the frictional stability evolution of fractures in igneous rocks is of critical importance to optimize the hydraulic stimulation strategy for EGS.
The fracture is induced to slip when the shear stress acting on the fracture exceeds the shear strength of it according to the Mohr-Coulomb failure criterion (Zoback 2010) and the effective stress law (Terzaghi 1923). When a fracture starts to slip, the slip can either be seismic or aseismic, depending on the friction rate parameter and relative magnitude of critical rheologic stiffness of the fracture and the stiffness of elastic surroundings (Marone 1998). Specifically, when the friction rate parameter is positive, the fracture is intrinsically rate strengthening and can always slip stably (aseismic), while a fracture with a negative friction rate parameter is conditionally rate weakening, tending to slip unstably (seismic) if the critical rheologic stiffness of the fracture is larger than the stiffness of elastic surroundings.
I started with Shell in 1982 after graduating in business administration from the University of Applied Sciences, Zürich. I was keen to work globally and appreciated the international opportunities that Shell offered. My first impressions of Shell were that of worldwide possibilities and the ability to work nearly anywhere there was a need for energy. Gaining practical, hands-on experience and contributing to a team that is focused on performance is still what drives me today. I was keen to work in a variety of countries and experience other cultures, which I saw as a good way to develop my leadership skills.
Dutler, N. O. (University of Neuchâtel) | Valley, B. (University of Neuchâtel) | Gischig, V. (CSD Engineers) | Jalali, M. R. (SCCER-SoE) | Doetsch, J. (SCCER-SoE) | Krietsch, H. (SCCER-SoE) | Villiger, L. (SCCER-SoE) | Amann, F. (Chair of Engineering Geology and Environmental Management)
ABSTRACT: Various in-situ hydraulic fracturing experiments were carried out in the naturally fractured, crystalline rock mass of the Grimsel Test Site (GTS) in Switzerland. The purpose was to study the geometry of the newly created fractures and their interaction with the preexisting fracture network using transient pressure and rock mass deformation observations. Under controlled conditions, six hydraulic fractures with similar injection protocols were executed in two sub-vertical injection boreholes. The rock mass is intersected by two E-W striking shear zones (S3), and two biotite-rich meta-basic dykes with a densely fractured zone in between. The S3 shear-zone intersecting the rock volume of interest acts as a high-permeability connection to the tunnel for the experiments executed south of it. Strong variation in injectivity enhancement, jacking pressure, break down pressure, instantaneous shut-in pressure and fluid flow recovery among the different injection intervals indicate different stress conditions north and south of S3.
The main requirement for extracting energy from the subsurface is sufficient fluid flow through permeable pathways to transport heat or oil/gas from the underground to a production well. The crustal permeability decreases with depth (Manning & Ingebritsen, 1999; Rutqvist & Stephansson, 2003), which influences the productivity in a negative manner. Thus, the permeability in deep target reservoirs has to be enhanced through hydraulic stimulation. Only by enhancing the permeability of the underground, sufficient conductivity and connectivity are achieved. Geothermal projects utilizing permeability enhancement techniques are referred as enhanced geothermal systems (EGS) (Cummings & Morris, 1979).
Two main processes are typically invoked during permeability enhancement: 1) hydraulic fracturing and 2) hydraulic shearing. Hydraulic fracturing is the initiation and propagation of tensile (mode I) fractures. It occurs when tensile stress exceeds tensile strength and the energy, which is required to create new surfaces in the rock exceeds fracture toughness (Detournay, 2016). When the stimulation operation is completed, the newly formed hydraulic fractures keep a residual aperture resulting in permeability enhancement (Jalali et al., 2018).
GENEVA, Switzerland (28 June 2006) -The Society of Petroleum Engineers (SPE) and the United Nations Economic Commission for Europe (UNECE) have entered into a Memorandum of Understanding (MOU) to further efforts to develop one globally applicable harmonized standard for reporting fossil energy reserves and resources. Such a standard will ensure greater consistency and transparency in financial reporting and enhance energy resources management, energy studies and business processes. The SPE, working together with the World Petroleum Council (WPC) and the American Association of Petroleum Geologists (AAPG), has developed definitions for reserves and resources, and encourages their universal adoption by the oil, gas, and related industries; international financial organizations; governments; regulatory agencies; and reporting bodies.
In offshore steel structures engineers often face the problem of assessing the criticality of existing hot spots to predict the remaining lifetime and thus to develop sound reliability-based inspection programs. One problem with such an approach is that the past fatigue conditions cannot be appropriately modeled, and the degree to which damage has accumulated in hot spot areas cannot be consistently modeled. This paper shows a practical methodology for predicting the remaining fatigue life of hot spots by using a probabilistic fracture mechanics approach and shows how in general the results can be used in reliability-based inspection programs.
At present, the best practice for modeling remaining fatigue life and identifying hot spots in existing offshore steel structures is the traditional S–N approach (Miner’s rule). The uncertainties in the S–N approach for existing structures are significant, and it is often impossible to take into account the stress cycles to which the structure has been subjected in the past, especially in cases where the structure has been strengthened or modified. This results often in very low fatigue life, sometimes even lower than the actual age of the structure. The S–N approach has thus only limited use for identifying measures and for establishing risk- and reliability-based inspection plans for such hot spots in existing structures.
The present paper shows how a probabilistic fracture mechanics approach can be used to analyze the existing hot spots in an offshore steel structure and how reliability-based inspection planning can be established based on these results. The general approach is to postulate cracks of specific length and depth that match the threshold of known inspection techniques, to have cracks located in specific directions at the hot spot locations, and then to predict the crack growth and failure probability of the postulated cracks.