- Open Access
Fluid pressure drops during stimulation of segmented faults in deep geothermal reservoirs
© The Author(s) 2018
- Received: 16 March 2018
- Accepted: 8 November 2018
- Published: 16 November 2018
Hydraulic stimulation treatments required to produce deep geothermal reservoirs present the risk of generating induced seismicity. Understanding the processes that operate during the stimulation phase is critical for minimising and preventing the uncertainties associated with the exploitation of these reservoirs. It is especially important to understand how the phenomenon of induced seismicity is related to the pressurisation of networks of discrete fractures. In this study, we use the numerical simulator CFRAC to analyse pressure drops commonly observed during stimulation of deep geothermal wells. We develop a conceptual model of a fractured geothermal reservoir to analyse the conditions required to produce pressure drops and their consequences on the evolution of seismicity, fluid pressure, and fracture permeability throughout the system. For this, we combine two fracture sets, one able to be stimulated by shear-mode fracturing and another one able to be stimulated by opening-mode fracturing. With this combination, the pressure drop can be triggered by a seismic event in the shear-stimulated fracture that is hydraulically connected with an opening-mode fracture. Our results indicate that pressure drops are not produced by the new volume created by shear dilatancy, but by the opening of the conjugated tensile fractures. Finally, our results reveal that natural fracture/splay fracture interaction can potentially explain the observed pressure drops at the Rittershoffen geothermal site.
- Enhanced geothermal reservoirs
- Pressure drops
- Reservoir simulation
- Induced seismicity
- Fracture networks
Geothermal energy development, either for electricity generation or for direct applications of geothermal heat, can be carried out in a wide range of geological settings. In the case of active tectonic zones with abnormally high geothermal gradients, such as Iceland, Italy, New Zealand or Turkey (Moeck 2014), geothermal energy can be widely exploited at shallow depths. However, geothermal projects usually need to exploit deeper reservoirs in regions where radiogenic igneous rocks are not present or where the tectonic activity is minor. In recent years, the exploration and exploitation of deep geothermal reservoirs have significantly increased worldwide (e.g. Tester et al. 2006; Breede et al. 2013; Király et al. 2015). In this context, geothermal projects focusing on heat distribution (low and medium enthalpy) have mainly targeted crystalline basement rocks or large and deep sedimentary basins, such as intracratonic basins and foredeep orogenic belts, as well as continental rifts. Projects focusing on power generation in high-temperature, low-permeability settings generally need to be developed as Enhanced Geothermal Systems (EGS), either in fractured crystalline basement rocks, or in sedimentary and volcanic rocks (Zimmermann and Reinicke 2010; Elders et al. 2014).
Although deep, naturally fractured tight reservoirs are normally characterised by the presence of pre-existing fracture networks that provide some permeability. In most cases, the presence of rocks with low permeability prevents the economic feasibility of the project. Exploitation of low-permeability geothermal reservoirs requires the use of hydraulic stimulation techniques to enhance the permeability of the reservoir, increasing fluid flow and heat transfer between injection and extraction wells (Zimmermann and Reinicke 2010; Schill et al. 2017). However, stimulation processes sometimes produce induced seismicity, a hazard that needs to be mitigated to ensure the social acceptance and viability of the project. Normally, when this type of seismicity is low in magnitude, it is rarely felt and is referred to as microseismicity. Nevertheless, in some cases, the events may have high enough magnitude to be noticed at the Earth’s surface, putting in risk the viability of the project (e.g. Majer et al. 2007; Häring et al. 2008; Dempsey and Suckale 2015). In such contexts, it is essential that we understand the fundamental processes involved in the hydraulic stimulation phase to reduce seismic risks and characterise the uncertainty of seismic hazard estimates.
Meyer et al. (2017) proposed three main hypotheses to explain the mechanisms associated with pressure drops and the triggering of induced seismicity. The first hypothesis considers that pressure drops are caused by fracture reactivation and slip with associated microseismicity. In the second hypothesis, they suggest that the phenomenon can be due to pressure equilibration during the connection between the stimulated/hydraulic fractures and the pre-existing ones. When a pressurised hydrofracture (i.e. a new fracture in the reservoir formed during hydraulic stimulation) gets connected with an unpressurised pre-existing fracture, additional fluid storage space is suddenly generated and, as a consequence, a pressure drop can occur. When fluid pressure recovers, stimulation of the newly connected fracture causes instability and slip takes place creating a swarm of seismic events. Finally, the third hypothesis considers that the pressure drop is produced by the propagation and opening of new cracks (i.e. tensile fractures) as wing cracks growing from pre-existing fractures (e.g. Norbeck et al. 2018). During the sliding stimulation phase of a pre-existing fracture, there is a relative displacement between the two fracture walls. This can induce the growth of tensile cracks at their tips, if the tensile strength of the rock is overcome. Such new cracks would also cause a sudden increase of permeability and an associated pressure drop. After running a series of numerical models, Meyer et al. (2017) concluded that the most plausible hypothesis to explain pressure drops is the propagation of new tensile fractures, although their study was not conclusive and suggested that further work was required.
Both hypotheses 2 and 3 proposed by Meyer et al. (2017) share the condition that pressure drops occur in systems that contain or develop at least two sets of fractures at different orientations with respect to the stress field. As previously demonstrated by several studies (e.g. Garagash and Germanovich 2012; Gischig 2015; Piris et al. 2017), rupture propagation and sliding/tensional behaviour on fluid pressurised fractures depend on their relative orientation with respect to the principal stress axes. Fractures with strikes oriented at moderate angles with the maximum compressive stress (σ1) are characterised by reactivation by sliding and/or opening, and are considered seismically active. In contrast, fractures at low angles with σ1 have the capacity of being stimulated by opening mode and present either an aseismic behaviour or very low-magnitude seismicity (Piris et al. 2017). The new formation or reactivation of the latter set of fractures (i.e., at low angles with σ1) can potentially result in a sudden permeability increase due to their ability to dilate at relatively low fluid pressure. Accordingly, if both fracture sets are connected when a fracture at a moderate angle is reactivated, the slip would induce opening of the aseismic fracture and will, thus, cause a pressure drop. The understanding of this process can potentially be used to identify patterns of the mixed-mechanism stimulation during hydraulic stimulation treatments.
In this study, we present a conceptual model where natural fractures are hydraulically connected by tensile splay fractures. Our overarching aim is to understand the influence of different properties of fracture sets on the system pressurisation and their consequences for seismicity propagation and fault pressurisation in a generic deep geothermal reservoir. We present numerical simulations based on simple fracture geometries, avoiding complex fracture networks, to investigate how pressure drops are related to stimulation and induced seismicity. Our results reveal a direct link between pressure drops and seismicity in systems involving two different fracture sets that are hydraulically connected. Seismicity is produced by sliding of a shear-mode fracture that induces a sudden opening of connected hydraulically tensile fractures, thus triggering the pressure drop.
Friction and fracture parameters used in the simulations
Reference normal traction (mechanical aperture)
Residual mechanical aperture
Dilatation angle (mechanical aperture)
Reference normal traction (hydraulic aperture)
Residual hydraulic aperture
Dilatation angle (hydraulic aperture)
Arbitrary (good coupling)
Water values at 20 °C
0.001 Pa s
To obtain representative magnitudes
Radiation damping coefficient
Arbitrary (common value) (McClure 2012)
Arbitrary (common value)
Arbitrary (common value)
Nominal friction coefficient
Characteristic displacement scale
Velocity effect coefficient
State effect coefficient
Arbitrary (common value)
Matrix tension strength
Arbitrary (common value)(McClure 2014)
Stress intensity factor
1.5 MPa m1/2
For all the models, the geothermal reservoir was assumed to be at a depth of 4500 m, with an initial fluid pressure defined by the hydrostatic gradient. We assumed a strike-slip regime in which the principal stresses σ1 and σ3 are horizontal (parallel to the y- and x-axis of our model, respectively; Fig. 2) while σ2 is vertical (i.e. oriented out-of-plane in the models). A minimum in situ stress of 76 MPa was imposed in the x-direction, while a maximum horizontal stress of 185 MPa was applied in the y-direction. A constant injection pressure of 70 MPa was imposed, with a maximum injection rate of 10 kg/s. The duration of simulation was set to be high enough to pressurise almost the entire domain (t = 50,000 s).
A series of additional models were run to evaluate the influence of key parameters on the pressure drop phenomenon. These include the dilatancy effect, the scale effect and the model setup similar to that of the Rittershoffen geothermal reservoir. To evaluate the sensitivity of the models to the mechanical dilation angle (i.e. the dilatancy effect, defined as the increase of fracture volume by shear displacement), additional models were run with dilation angles (φEdil) of 2.5° and 5°. The potential sensitivity of the length scale was evaluated in the model “88–60” using fracture segments with lengths of 50 m, 40 m, 30 m, 20 m, 15 m, and 6 m. Finally, with the aim of comparing numerical predictions with field observations of seismicity coupled with pressure drops, several models with different length scales (i.e. 80 m, 60 m, 50 m, 40 m, 30 m, 20 m, 15 m and 6 m) were run using a configuration similar to that utilised during the stimulation of the Rittershoffen geothermal reservoir (Cornet et al. 2007; Baujard et al. 2017; Meyer et al. 2017). For these models, the parameters that were varied with respect to the previous ones were the stress state (σ1= 50 MPa, σ3= 29 MPa), the initial fluid pressure of 23.7 MPa and a constant injection pressure of 28 MPa.
The main parameters analysed in our simulations were the fluid pressure evolution, the fracture apertures, the earthquake hypocentre locations, and the earthquake magnitudes. These parameters were used to highlight differences between models.
Finally, the influence of the length scale of the fracture segments on the pressure drops is summarised in Fig. 12b. Using as a reference the model “88–60” and φEdil= 0°, different runs were carried out with different segment sizes (60 m, 50 m, 30 m, 20 m, 15 m, and 6 m). Systematic pressure drops in the well and in the entire domain were identified and the mean values were calculated. There was a systematic decrease of the pressure drop values throughout the system with decreasing segment length (from 6 MPa for the 60-m-length model to 1.5 MPa for the 6-m model). Pressure drop values in the well were very similar, although there was an increase of ranging between 30 and 15 m (Fig. 12b).
Pressure drop mechanism
In situations where the fluid is injected in a fracture segment at a low angle with σ1 (e.g. model “88–60”), the fracture high fluid storage capacity or transmissibility allows it to be initially pressurised without seismicity. Once the pressure front reaches the intersection between fracture segments, and a seismic segment is stimulated, microseismicity occurs. When the tensile strength is overcome in the seismic segment (i.e., α = 60°), the fracture slides and the relative displacement between walls induces stress concentration at fracture tips. In our models, this stress was high enough to open the tensional segments, producing a slight decrease of fluid pressure next to the intersection zones (for example see Fig. 3 around t = 0.5 × 104 s). After that, a time lapse is required to re-pressurise the region prior to the onset of a new pressure drop. This pressurisation is followed by new seismic events that assist the opening of additional tensional segments. These processes are repeated until all the seismic segments are completely stimulated. While the injected fluid progressively flows from the well throughout the fracture network, seismic events migrate from intersections located next to the injection well to more distant ones. Tensional segments are stimulated as aseismic segments or result in seismic events with very low magnitude. Larger events are located along seismic fractures and tend to occur near the intersections. With ongoing stimulation, seismic events progressively occur at longer distances from the injection point and the induced pressure drops are, thus, hardly observable by looking at the fluid pressure measured at the well. Nevertheless, they are continually happening, as illustrated in Figs. 3, 7 and 10 or in Fig. 12b, in which the difference between pressure drops at the well and in the simulation domain increases with increasing of fracture length.
In cases where fluid injection is carried out in a low-transmissivity fracture segment (model “60–88”; Fig. 7), pressure drops are difficult to be detected at the injection point. The fracture acts as a barrier for the pressure drop propagation due to its low storage capacity and low hydraulic aperture. The process producing pressure drops operates in a similar way as in the model previously described. When the tensile strength is overcome in a seismic segment, a sudden aperture change of the intersection is induced, causing the aseismic/tensional segments (i.e., high-capacity fractures) to get open, generating a new volume and producing the pressure drop (for example, see those at t ~ 3.25 × 104 s in Figs. 7, 8 and 9 or between t = 4 and 4.5 × 104 s in Fig. 7).
Another process associated with void aperture can be detected when pressure drops are analysed in detail (Figs. 5, 6, 8 and 9). The opening of aseismic fractures was not homogenous in our models, and regions along the same fracture segment experienced closing and opening during stimulation of the fracture intersections. Some regions are opened suddenly, while others are closed suddenly (e.g. points 1 and 6 in Fig. 6). Since a sudden fracture opening should imply a pressure drop, its sudden close should be associated with a local fluid pressure rise. Such local pressure rises, which get quickly dissipated, are likely to be felt more intensively in low-permeability fractures, i.e. in fractures that are shear stimulated (this can be detected for example in the curve t + 10 s in Fig. 8 for injection in the 60° segment).
Models “60–88” and “88–60” were carried out to explore the influence of the orientation of the fracture in which the fluid was injected. Despite the initial differences between the two models, their dynamic behaviour is very similar, and both show similar pressure drop phenomena. Similarly, the variation of the dilation angle or the length scale does not modify the described processes, but only determines the absolute values of pressure drops (Fig. 12) and the magnitude of microseismicity. Increasing the dilatational angle produces a permeability increase in the shear-stimulated fractures, allowing the propagation of pressure drops up to the well (Fig. 12a). However, the pressure drop process is similar to that in models “60–88” and “88–60”, and is related to the reactivation by sliding of a shear-stimulated fracture and the opening of the tensile conjugated fractures. Figure 13 shows a synthesis of the processes related to pressure drops. The influence of the injection rate was tested (from 2 kg/s up to 100 kg/s), producing a reduction of pressure drop values. However, the main pressure drop values in the system are independent of this parameter.
The same pattern was observed in the model with wing cracks (model “60–hydro”). When the seismic segment is stimulated, the wing crack is forced to open, producing a pressure drop and enhancing its propagation. In our simulations, pressure drops were not related to wing crack propagation, which was associated with the stress concentration at the edges of the pre-existing fracture. Sliding of the seismic segment allowed wing crack propagation, given that injection pressure in our models was lower than the minimum principal stress (σ3). This resulted in hydrofracture propagation with injection fluid pressures below σ3 and in accordance with the model proposed by McClure and Horne (2014), as an explanation of the mixed-mechanism stimulation for EGS projects (i.e., shear stimulation operates jointly with new tensile fracture generation).
As previously mentioned, Meyer et al. (2017) concluded that pressure drops could be produced by the propagation of tensile fractures as a wing crack. This process could be interpreted in a similar way, as observed in breakdown tests and used to identify the minimum principal stress (Prabhakaran et al. 2017). In these tests, the generation of a new hydrofracture produces a pressure drop because the fluid quickly migrates into the newly formed fracture, oriented normal to the minimum stress. However, the process of hydraulic fracture propagation as a wing crack due to the stress concentration at fracture tips was achieved under conditions of fluid pressure below σ3. According to the modelling parameters used in our simulations (specifically the injection fluid pressure and the tensile strength of the material), sudden changes as those observed in breakdown tests (in which the injection pressure reaches σ3) are not observed. Moreover, as discussed above, pressure drops in our models are linked with the tensile fracture opening rather than its propagation, regardless of whether this fracture is a pre-existing or a newly formed one.
Seismicity and pressure drops
In terms of the seismicity associated with pressure drops, we can distinguish two types of events. The first type of seismic event is produced in the seismic segments by fluid pressurisation, acting as a trigger for the pressure drop phenomenon and usually producing high magnitudes (M > 1.5). The second type of seismic event is produced at the aseismic fracture segments next to the regions that are opening. Normally, the latter events appear as low-magnitude seismic swarms (events with magnitude below one or zero), produced to accommodate the displacement generated by the sliding of seismic segments and the opening of the aseismic ones. A similar behaviour can be observed in the model containing a pre-existing fracture combined with wing cracks. This duality of the system’s seismicity was proposed and analysed by Fischer and Guest (2011). In their model, the higher magnitude events are located at the critically stressed natural fractures, while lower magnitudes occur at pre-existing tensile fractures or new hydrofractures. Such behaviour would be expected in a mixed-stimulation mechanism, where these different stimulation mechanisms operate jointly (McClure and Horne 2014; Norbeck et al. 2018).
A key aspect in our simulations is the tendency of microseismicity to cluster next to the intersections between fractures. The influence of intersections between fractures on the seismicity population and location was already proposed by Rutledge et al. (2004). Their interpretation of microseismicity generated during fluid stimulation in the Cartage Cotton Gas field (Texas) showed anomalous dense clusters of seismic events following intersections between fractures. Clusters showed location patterns diverging in time, progressively migrating from the injection zone to far away regions. Additionally, clustering of events was related to fewer and larger precursor events along critically stressed fractures, while other segments oriented at low angles to σ1 experienced an aseismic behaviour. After injection shut-in, new large-magnitude and clustered seismic events were observed. This phenomenon was interpreted by Rutledge et al. (2004) as a result of fluid flow forced by slip-induced loading along critical seismic fractures. During injection, the increase of fluid pressure critically stimulated pre-existing fractures and fracture intersections, allowing fluid migration along the fracture network.
Rittershoffen sensitivity analysis
Furthermore, our models show that the time lapse between the main earthquake event and the pressure drop at the well occurs after a few seconds (less than 2–4 s). This very short time interval probably implies that both phenomena will be almost simultaneously detected in real cases, requiring a highly precise time synchronisation between injection and seismicity data.
Our models use simplified geometries and are intended to help in investigating and understanding physical processes, rather than providing a perfect representation of reality. We chose not to use a model with complex multifracture networks, such as that utilised by Meyer et al. (2017), to isolate the main processes controlling pressure drops and seismicity. With a more complex network, the superposition of effects could attenuate the phenomena. Simulations by Meyer et al. (2017) with multifracture networks also produced pressure drops next to the intersections between fractures. However, their signal in the fluid pressure evolution at the well was attenuated. Additionally, there is a higher chance that more fractures can act as barriers to the propagation of transient variations of fluid pressure in multifracture systems. Our results confirm the interpretation by Meyer et al. (2017) that the conditions required to observe pressure drops in wells are very specific and unlikely to be observed in all reservoir formations. For injection wells located at a fracture with high transmissibility (i.e. model “88–60”), pressure drops at the well are potentially observable. However, pressure drops are hardly detectable in situations where the wells are located in low-transmissibility fractures (i.e. model “60–88”). However, as demonstrated by the numerical simulations presented here, pressure drops may occur in the reservoir even if they are not detected at the injection well.
Our simulations were carried out in isothermal conditions and, therefore, thermal drawdown effects are not modelled. In terms of stress reduction and seismicity, Gan and Elsworth (2014) observed that a second seismic cycle is developed related to the thermal drawdown that could potentially produce a second pressure drop cycle. It would be useful to repeat our analysis with a fully 3D model, since 2D models may enhance the magnitude of early events. Furthermore, the height used in our models (Table 1) is only an assumption required to take into account the third dimension, assuming plain strain for height values much larger than the fracture size (Shou and Crouch 1995).
Using simple fracture geometry configurations, we investigated different hypotheses for the occurrence of fluid pressure drops associated with hydraulic stimulation in Engineered Geothermal Systems (EGS). The results suggest that two fracture sets can influence pressure drops: one system able to be stimulated by shear (that will produce seismic events) and another one able to be stimulated by opening-mode fracturing (that will be aseismic). The tendency of stimulation by shear- or opening-mode fracturing is determined by the operational parameters (i.e. injection pressure, flow rate, etc.) and the stress state.
In the simulations, a pressure drop can be triggered by a seismic event in a shear-stimulated fracture that is hydraulically connected with a tensile or opening-mode fracture. The pressure drop is not produced by the new volume created by dilatancy, but by the opening of the conjugated tensile fracture instead.
This tensile fracture set may be part of the pre-existing fracture network, or alternatively, be developed as a hydrofracture during the stimulation phase. However, in our simulations, no pressure drops are observed during hydraulic fracture propagation at the tips of a pre-existing fracture. Nevertheless, once wing cracks are created, it is possible that slip along the natural fracture causes a significant aperture change on the splay fracture that can result in a pressure drop. In addition, we show how seismicity propagates through fracture arrays while poorly oriented segments slip aseismically. Seismicity is concentrated in critically oriented fractures near fracture intersections. The pressurisation front propagates non-smoothly and can be affected by the interaction of a conjugate fracture with other fractures with tensional or sliding properties. Our simulation results show that natural fracture/splay fracture interaction is a plausible explanation for the observed pressure drops at the Rittershoffen geothermal site.
This work is part of the Ph.D. project of GP. (corresponding author). GP designed and performed the numerical simulations, analysed the data and wrote the first draft of the manuscript; AG designed and supervised the study; EG-R and IH supervised and analysed the data. MM and JN developed the numerical code (CFRAC) used in this study. All authors read and approved the final manuscript.
The Institut Cartogràfic i Geològic de Catalunya is acknowledged for their support in our investigation of Geothermal resources. G. Piris was supported by an AGAUR grant of the Industrial Doctorate programme 2016-DI-031. EGR acknowledges the support of the Beatriu de Pinós programme of the Government of Catalonia’s Secretariat for Universities and Research of the Department of Economy and Knowledge (2016 BP 00208). The authors would like to thank three anonymous reviewers and the editors Dr. Carola Meller and Prof. Olaf Kolditz for their helpful comments that improved this manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
All datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.
G. Piris was supported by AGAUR grant for Industry Doctorate Research 2016-DI-031.
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