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ABSTRACT: The properties of faults and fractures under dynamic reservoir conditions are critical in determining flow and performance of geothermal doublets in fractured reservoirs. The complexity of faults obtained from geological and seismic characterization is usually not honored in flow models for fractured reservoirs. Fault zone architectures may affect flow as well as the geomechanical response of the fractured reservoir to doublet operation. In this study, an analytical model is developed that allows incorporation of fault zone architecture and multiple fault sets in the description of fractured reservoir permeability. The model is used with data from seismic fault characterization to describe bulk permeability for a potential geothermal target in fractured Dinantian carbonates in the Netherlands. The evolution of pressure and temperature during doublet operation is simulated using a 2D numerical flow simulator. The geomechanical response of the fractured carbonates to doublet operation was analyzed using a Mohr- Coulomb failure analysis. Large variations in the spatio-temporal evolution of pressure and temperature are found for different permeability constraints and doublet designs. Increased fracture permeability, reactivation of fractures, shear failure of the reservoir rock, and initiation of tensile fractures can affect flow and need to be taken into account to optimize doublet performance. 1. INTRODUCTION The success of geothermal projects in naturally fractured reservoirs critically depends on the properties of faults and fractures (Hickman et al., 1997; Moeck, 2014). Although the relationship between fluid flow and fault properties has been studied extensively (Frank 1965; Caine et al., 1995; Faulkner et al., 2010), proper incorporation of realistic fault populations and the dynamic response of faults and fractures to flow in fractured reservoirs are largely unresolved issues. In particular, fractured reservoir flow models usually do not account for the interplay between (1) fault zone architecture and variation of permeability in different structural fault units such as damage zones (Caine et al., 1995), (2) site-specific characteristics of fault and fracture populations determined using seismic surveys, outcrop analogues, core material or laboratory experiments (Odling et al., 1999), and (3) changes in permeability due to opening or reactivation of fractures caused by pressure or temperature variations (Safari and Ghassemi, 2016; Fang et al., 2018).
- North America > United States (0.68)
- Europe > Netherlands (0.49)
- Phanerozoic > Paleozoic > Carboniferous > Mississippian > Middle Mississippian > Visean (0.35)
- Phanerozoic > Paleozoic > Carboniferous > Mississippian > Lower Mississippian > Tournaisian (0.35)
- Geology > Structural Geology > Fault (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Petroleum Play Type > Unconventional Play > Fractured Carbonate Reservoir Play (0.41)
Abstract Conventional hydrothermal ("low enthalpy") geothermal reservoirs are already being developed in the Netherlands as a source of heat for greenhouses, district heating. Many of these wells are left with a positive skin after being drilled, and could therefore benefit from stimulation. In order to evaluate the potential benefit of propped fracture stimulation for this type of reservoir, a history match of a real geothermal injection and production doublet was made using an industry standard 3D reservoir simulation model. This model was then used to perform production and injection forecasts for different completion scenarios, including propped fracturing. All available data (logs, PLT, welltests) was used in creating and history matching the reservoir simulation model. The model was history matched to the individual welltests for both the production and injection well and to a 3 year data set of production and injection data. In order to match the long-term pressure trend, an increase in the size of the thermal fracture in the injection well was needed. The history matching results were compared to the predicted increase in the size of the thermal fracture using a new 3D FEM fracture model. The same model parameters were also used to design propped hydraulic fracturing treatments. The propped fractures were also placed in the reservoir simulation model for both the production and injection wells, to predict the performance of the wells with fracture stimulation. Although the actual wells used in this study had negative skins to begin with (after drilling), many wells drilled in the same reservoir (Rotliegend in the Netherlands) have positive skins to begin with. Our simulations showed significant improvement in well injectivity and productivity using propped fractures, compared to a well with a typical initial skin of +2 seen in new open-hole wells in this reservoir. Injection pressure was also reduced significantly, to below the regulatory limits on "safe" injection. The new 3D FEM model showed that injection well thermal fracture propagation was strongly affected by small differences in the Young's modulus between the higher and lower porosity layers in the reservoir. This paper shows, based on actual well data from a real geothermal doublet, the expected benefit from hydraulic fracture stimulation is significant, especially when the wells have a positive skin after drilling. The effect of small differences in rock stiffness on thermal fracture propagation in a geothermal injection well is also a new and surprising result.
- Europe > United Kingdom > North Sea > Southern North Sea (0.28)
- Europe > Netherlands > North Sea (0.28)
- Europe > United Kingdom > North Sea > Southern North Sea > Rotliegend Sandstone Formation (0.99)
- Europe > Netherlands > Slootdorp License > Middenmeer Field > Z2 Carbonate Formation (0.93)
- Europe > Netherlands > Slootdorp License > Middenmeer Field > Slochteren Formation (0.93)
ABSTRACT: In order to improve the location of induced seismicity recorded by a triaxial downhole geophone, we present a method locating seismic events with respect to a reference event. This method requires similar seismic events called "doublets", usually assumed to be generated by a same geological discontinuity. RESUME: Pour ameliorer la localisation de la sismicite induite, enregistree par un capteur triaxe en profondeur, nous presentons une methode localisant les evenements en relatif par rapport ร un evenement de reference. Cette methode necessite I'existence d'evenements sismiques d'une grande similitude appeles "doublets", produits ร priori par la rnerne discontinuite gรจologique. ZUSAMMENFASSUNG: Um die Lokalisierung der von einem in Tiefe gelegten Dreiachssensor registrierten induzierten Erdbebenhaufigkeit verbessern, haben wir eine Verfahrensweise entwickelt, die Relativerfassungen in gezug auf einen Referenzerfassung lokalisiert. Diese Methode erfordert sehr ahnliche seismische "dublette" geheiรte Erfassungen, die von dieselbe geologische Unterbrechung produziert werden. INTRODUCTION Hydraulic fracturing, geothermal energy extraction or salt leaching show an important induced seismicity. This seismicity is very often characterised by the existence of several families of similar events called doublets (figure 1). Taking into account the great simililarity between doublets inside the same family, it is assumed that they have all been generated by the same mechanism. The location of the doublets allows to specify the orientation of the geological discontinuity. Classically, with a triaxial downhole geophone, the seismic hypocentral location is performed using the hodogram method. This method derives the P wave incident angle (azimuth and nutation) from the analysis of the 3D particle motion, and the distance from the time difference between P and S arrivals. This method gives a location for each event independently of the previous locations. Applying this method to a family of doublets, it can be shown that the variable signal to noise ratio introduce a random error on the absolute location of each event, reducing the location accuracy, and blurring the swarm of seismicity. In order to improve the location accuracy, we studied a method using the simililarity between doublets (Moriya H., & al., 1994). This method consists in locating the doublets with respect to a reference event. The reference event is usually located by the classical hodogram method, but all the other doublets are localised with respect to it by computing their relative angle and distance differences. Among a class of doublets, the reference event is the event with the best signal to noise ratio. This ensures a reliable determination of incidence angle, and a good time picking of wave arrivals. THE DISTANCE DIFFERENCE COMPUTATION Hodogram method: the reference event location The reference event is located using the hodogram method (figure 2). By the projection of the 3D panicle motion on the three axis of the downhole transducer, we compute the three eigen vectors (inertia matrix). Using trigonometric functions, the three Eulerian angles (azimuth, nutation, and proper rotation) are determined (Cliet C, & al., 1988) The length of the time window is selected in order to minimize the time delay dispertion computed by the correlation (fig.3c) One can observe a variation of the time delay as a function of the length of the time correlation window. The best accuracy of the time delay corresponds to the minimum standard deviation between the three axis. The evolution of the time correlation window, from one to 10 periods (figure 3c), shows a time delay stability around a same average for the three axis, after 5 or 6 periods (minimum standard deviation). The error of the arrival time picking will also be minimum at this value of the correlation window.
Skeletonization is a syntactic patternrecognition method that is applied to gridded data to produce an automatic line drawing, with an associated event catalog. Previous implementations of skeletonization have been tailored for seismic data. Here, we modify that technique to render it more suitable for other types of gridded data, with particular emphasis on aeromagnetic maps. A modification from previous schemes is the use of a twopass approach, to reduce the effects of an otherwise problematic directional bias that discriminates against events oriented parallel to columns of the grid. The method can be used effectively for filtering aeromagnetic data on the basis of strike direction, event linearity, event amplitude, and polarity. It is based on the delineation of peaktrough pairs (cycles), which are traced throughout the grid to form contiguous events. Cycles and events are characterized by attributes that include amplitude, polarity, and pulse width. Events are further characterized by length, average strike direction, and linearity. The event attributes are stored in a catalog, thus enabling one to perform attributebased analysis and data filtering. We illustrate our algorithm using two regional aeromagnetic examples from different parts of the Canadian Shield. The first, from the Great Slave Lake shear zone, is dominated by linear anomaly trends produced by faults and mafic dikes. The second, from the Manicouagan region of northeastern Quebec, contains abundant subcircular and arcuate anomaly patterns caused by large intrusive complexes and a meteorite impact structure.
- North America > Canada > Quebec (0.34)
- North America > Canada > Alberta (0.28)
- North America > Canada > Northwest Territories (0.25)
- North America > United States > New Mexico > San Juan Basin (0.99)
- North America > United States > Colorado > San Juan Basin (0.99)
- North America > United States > Arizona > San Juan Basin (0.99)
ABSTRACT: Thermal short-circuiting in Enhanced Geothermal Systems (EGS) is a reservoir-scale phenomenon, which arises from coupled thermal-hydraulic-mechanical feedback in response to heat drainage. Coupled feedback on the hydraulic aperture may increase fracture transmissivity drastically and decrease the production fluid temperature lowering the economically recoverable heat energy. This paper identifies the geometrical controls on short circuiting for multi-fracture enhanced geothermal systems to optimize the recoverable heat energy including: fracture spacing, number of parallel factures, wellbore diameter, injector-producer spacing, initial hydraulic aperture, hydraulic aperture distribution and thermo-hydro-mechanical fracture response. We use a combination of simple 1D hydraulic network simulations and fully coupled thermo-hydro-mechanical 3D models solved through FEM. Results show that thermal short-circuiting is more severe in formations (1) with initial or late high hydraulic apertures, in the order of tens of millimeters; (2) when fluid pressure exceeds the minimum total stress, causing large fracture re-opening; and (3) with a large number of fractures in a small rock volume, causing severe thermal and mechanical fracture interaction. Our modeling work suggests that thermal short-circuiting can be mitigated with an appropriate fracture design and operation. 1. INTRODUCTION Heat recovery in Enhanced Geothermal Systems (EGS) requires transmissive fractures connecting injection and production wellbores to increase hydraulic diffusivity of the surrounding rock (Brown et al., 1972; MIT, 2006; US Department of Energy, 2020). High surface area is desirable for large heat recovery because heat conduction in EGS reservoirs occurs at small rates (Fu et al., 2015; Gee et al., 2021). Yet, localized thermo-elastic contraction of the surrounding rock can increase fracture permeability and localize fluid flow, causing thermal short-circuiting (Armstead and Tester, 1986; Tsang and Neretnieks, 1998; Guo et al., 2016). Furthermore, fluid flow localization in EGS reservoirs can be caused by two processes: (1) fluid localization to preferred paths within a single fracture (flow-channeling) and (2) fluid localization to a few fractures within a hydraulic network of several fractures (plane-channeling). The latter is caused by distinct initial fracture geometries and viscous pressure loss between fractures (Shiozawa and McClure, 2014; Asai et al, 2018). These processes can form a thermal-hydraulic-mechanical (THM) coupled feedback loop because permeability increase within a single fracture tends to perturb the hydraulic network such that it accepts more working fluid. Hence, a coupled solution of fluid flow through wellbores and fractures with fluid flow and deformation of the surrounding rock is required to model the full transient behavior of multi-fracture EGS.
- Energy > Renewable > Geothermal > Geothermal Resource for Power Generation > Enhanced Geothermal System (1.00)
- Energy > Renewable > Geothermal > Geothermal Resource (1.00)