Zhai, Wenbao (China University of Petroleum) | Li, Jun (China University of Petroleum) | Chen, Zhaowei (CNPC Engineering Technology R&D Company Limited) | Zhou, Yingcao (CNPC Engineering Technology R&D Company Limited)
Many weakness planes (such as faults, joints, and micro-fractures, etc.) are usually pre-existed in rock. However, the stress statement changed by hydraulic fracture (HF) propagation may have an important impact on hydraulic fracturing, which is closely related to stress statement of weakness planes. Firstly, the rock samples containing the pre-existing weakness planes were analyzed according to the curves of volumetric strain versus stress difference acquired by laboratory experiment. Secondly, the effective normal stress and shear stress of weakness planes were calculated by the tensor transformation method. And then, weakness planes were divided into four kinds according to the relationship between stress statements of weakness planes and failure lines of Mohr diagram and the kinds of weakness planes were visually described in the Mohr diagram. Finally, it was respectively discussed that pre-existing weakness planes did have an influence on hydraulic fracturing under different stress statements. The research results show that when the effective stress is more than zero, with the effective stress decrease, weakness planes are the more easily inclined to become the dilatation phenomenon where the self-propping effect can improve the reservoirs permeability due to surface asperities of weakness planes. However, there are very complex mechanical phenomena induced by weakness planes under higher effective stress. When hydraulic fractures encounter the pre-existing weakness planes under the approximate stress statement, it may be easy to occur shear slipping of weakness planes or it is difficult to be opened by hydraulic fractures. The latter is extremely beneficial to not become the maximum simulated reservoir volume (SRV) and should be avoided by fracturing operation as early as possible. It is somewhat different that the influence of different mechanical phenomena on hydraulic fracturing, which has a certain guidance for improving hydraulic fracturing stimulation.
Compared to the conventional oil and gas resources, there is usually not natural production in unconventional oil and gas resources, and it needs to rely on hydraulic fracturing to improve development effectiveness (Shrivastava and Sharma, 2018). The complex fracture networks may be created in hydraulic fracturing, which is a combination of shear and tensile failures (Lin et al., 2018). The shear failure of weakness planes (such as faults, joints, and micro-cracks, etc.) resulted in long-term geological tectonic movement is anticipated to dominate in hydraulic fracturing. However, it is a fact that rock dilatation may be caused by rock plastic behavior that the horizontal stress is balanced by the pressure near the fractures tip in hydraulic fracturing (Alko and Economides, 1995). With the development of unconventional oil and gas resources, these weakness planes are a double-edged sword that they can act as a good oil and gas flowing channel, but they can also lead to hydraulic fracturing failure (Ye, 2017). Therefore, it is necessary that considering the role of weakness planes in hydraulic fracturing will be used to optimize the hydraulic fracturing design.
ABSTRACT: Hydraulic stimulation on the geothermal reservoir is the well-known operation for improving the transmissivity and fracture connectivity within the reservoir. In this operation, by increasing pore pressure, shear slip on pre-existing fractures are triggered or new fractures generate from the tip of pre-existing fractures. Due to these mechanisms, mechanical, hydraulic, and seismic properties of fracture/fracture network evolve during the stimulation, but it is unclear how these properties evolve and link in each other. We conduct the laboratory experiments to concurrently monitor the strength, permeability, and acoustic emissions during the hydraulic shearing of rough-walled fracture. Through the experiments, we find the shear slip is limited to between 0.023% and 0.33% of the representative length of pressurized zone, and the fracture permeability increases to from 4 to 12 times of the initial permeability (before slip). Interestingly, more than 50% of the permeability enhancement is achieved during the aseismic motion, which is commonly precedes the seismic/fast slip with acoustic emissions. These findings are well consistent with the mesoscale experiment at URL and will be useful in the actual operation of pressurization for geothermal reservoirs.
Hydraulic stimulation on the geothermal reservoir is the well-known operation for improving or maintaining the transmissivity and fracture connectivity within the reservoir (Evans et al., 2005; Haring et al., 2008). In this operation, by injecting pressurized water into the reservoirs, pre-existing fractures are reactivated in shearing mode with the opportunity for self-propping on asperities (Esaki et al., 1999). Due to these mechanisms, mechanical and hydraulic properties of fracture/fracture network evolve. During the shear slip on fractures, seismicity is possibly increased (Ellsworth, 2013; Majer et al., 2007), and it is necessary to be explored how the mechanical-hydraulic-seismic properties evolve and link in each other during the hydraulic shearing.
One of the most significant advances for understanding the linkages between mechanical, hydraulic, and seismic properties is achieved by Guglielmi et al. (2015a) through the in-situ reactivation experiment of mesoscale fault, which cuts thorough a carbonate formation. They concurrently monitor the mechanical, hydraulic, and seismic properties of the fault during the fluid injection and reveal that slow/aseismic slip primarily and initially occurs and then triggers the micro-earthquakes. Similar characteristics are also observed in in-situ reactivation experiment of shale fault (Guglielmi et al., 2015b). Thus, such a finding is possibly useful in assessing the seismic hazard associated with the pressurized water injection into the deep geothermal reservoir.
The shearing of pre-existing fractures plays an important role in the permeability enhancement of shale reservoirs during hydraulic fracturing or refracturing treatments. The process reactivates pre-existing fractures around a hydraulic fracture causing them to slip and dilate and can also cause fracture propagation in the shear and tensile modes creating secondary cracks resulting in increased permeability. However, laboratory data on fluid flow and fracture slip in reservoir rocks particularly shale rocks are rare, and the mechanisms of permeability evaluation with shear slip and dilation are still not well understood. In this work, we present the results of laboratory scale shear stimulation tests and numerical simulations to illustrate fracture permeability changes with fracture shear slip and complex network formation. Eagle Ford shale samples containing a natural fracture have been used to run triaxial shear tests and injection-induced shear tests. The multistage triaxial shear test has been performed to measure fracture mechanical properties including shear strength, friction angle, normal stiffness, and shear stiffness; the injection-induced shear test has been used to investigate fracture dilatant shear slip and the coupled permeability evolution. The multistage triaxial shear test show that this type of Eagle Ford fracture has a 37° friction angle, and average 1.39*106 psi/in. normal stiffness and 1.11*106 psi/in. shear stiffness. In the injection-induced shear test, we achieved 6 times increase in flow rate even with only a small induced shear sliding (<0.1 mm or <0.004 inch). Furthermore, permeability evolution during injection-driven shearing tends to linearly evolve with the shear slip and dilation. The irreversible behavior of shear slip was found to explain the permeability hysteresis during shear sliding. The relevant laboratory data has been used in numerical simulations to quantify the impact of shear slip along natural fractures during stimulation. This has been achieved using a newly developed complex fracture network model which robustly simulates hydraulic fracture propagation in a naturally fractured reservoir. The numerical results indicate that shear slip induced permeability enhancement in ultra-low permeability reservoirs is a critical component of stimulation particularly when most of the natural fractures are mechanically closed and may not be favorable for proppant placement.
Shear slip caused by increased pore pressure due to injection has been explored as a mechanism for permeability enhancement of unconventional hydrocarbon reservoirs. The process reactivates pre-existing fractures around a hydraulic fracture causing them to slip and dilate and can also cause fracture propagation in the shear and tensile modes creating secondary cracks resulting in increased permeability. Control and optimization of shear stimulation can be achieved by studying how fluid flows through fractures as the stresses (shear and normal) change and how fracture permeability evolves with slip. However, laboratory data on fluid flow and fracture slip in reservoir rocks particularly shale rocks are rare, and the mechanisms of permeability evaluation with shear slip and dilation are still not well understood. In this paper, we present the results of a laboratory scale testing program to address these questions. Salt water (7% kcl) was injected into the two Eagle Ford shale samples under triaxial conditions having a single natural or an induced tensile fracture to induce shear slip, and flow rates during shear slip processes were measured to characterize fracture permeability evolution. In addition, fluid flow through shale fractures under different values of confining stress and injection pressure were examined to investigate the stress-dependent permeability of shale fractures. The hydrostatic flow tests show that flow rate linearly rises with the increase of injection, while an exponential relationship can be observed between flow rate and effective confining pressure. In the injection-driven shear tests, we achieved 6 to 15 times increase in flow rate even with only small shear sliding (<0.1 mm) induced. In addition, permeability tends to linearly evolve with the increase of shear slip and normal dilation. The results quantify the role of shear slip in enhancing permeability of shale fractures, and would help engineer solutions for maintaining these fractures open, reducing costs (proppant/water and additive cost savings).
ABSTRACT: Permeability enhancement through shear slip has been considered as standard treatment of engineered geothermal systems (EGS). The process reactivates pre-existing fractures, making them slip and dilate using fluid pressures below the minimum principal stress resulting in increased permeability. It can also cause fracture propagation in the shear and tensile modes creating secondary cracks. Control and optimization of shear stimulation can be achieved by studying how fracture permeability evolves with shear slip and dilation. However, most experimental studies that have considered fracture slip and permeability evolution have used force-driven shear tests or have manually displaced the specimens to represent fracture slip. A few studies have considered fluid injection-driven slip but only using saw-cut smooth joints. In this work, we have conducted shear slip test by water injection on rough fractures. Water was injected into a granite sample containing a single tensile rough fracture to induce shear slip under triaxial conditions. Flow rate during shear slip was measured to investigate fractures’ permeability evolution. In addition, the effects of confining pressure, differential stress, and injection pressure on stress-dependent permeability of the granite fractures were characterized. We tested three separate samples using different methods. Non-shear flow tests were conducted on a fractured Sierra White granite sample (SW #1) under both hydrostatic and triaxial conditions to characterize stress-dependent fracture permeability. We observed a linear relationship between flow rate and injection pressure, and an exponential relationship between flow rate and confining pressure. In addition, fluid injection-driven shear tests were performed on fractured samples SW #2 and SW #3 using constant stress mode and constant displacement mode, respectively. Shear rates observed during the constant stress test were ˜10−3 m/s and yielded up to 3 orders of magnitude increases in flow rate while the constant displacement mode caused ˜10−5 m/s sliding rate and 20 times increase in flow rate through the fracture. Furthermore, permeability evolution during injection-driven shearing tends to linearly evolve with the shear slip and dilation. The irreversible behavior of shear slip was found to explain the permeability hysteresis during shear sliding.
Enhanced Geothermal Systems (EGS) represent enormous renewable energy reserves (Tester et al., 2006; Brown et al., 2012). Field experiments have been carried out at sites such as Newberry, Desert Peak, Fenton Hill, Soultz-sous-Foretz and Rosemanowes. The primary objective of these field demonstrations has been to create flow paths through basement rock to facilitate heat transfer. Various stimulation mechanisms have been tested at these sites depending on the geology of the host rock, fault structures, natural fractures etc. Shear slip (Pine and Batchelor, 1984; Willis-Richards et al., 1996; Baria et al., 1999; Rahman et al., 2002; Nygren and Ghassemi, 2005; Cheng and Ghassemi, 2016) and the propagation of natural fractures are important mechanisms of permeability enhancement in EGS reservoirs (Min et al., 2010; Ghassemi, 2011; Huang et al., 2013; Jung, 2013; McClure and Horne, 2013; Kamali and Ghassemi, 2016). Often, crystalline basement rocks have pre-existing sealed fracture networks that are activated prior to the creation of new fractures since the sealing material is usually much weaker than the surrounding rock. The objective of shear stimulation is to create a stimulated volume with increased permeability to circulate large volumes of water. This is carried out at pressures below the minimum principle stress to avoid excessive fracture growth and large surface area. Fractures that are sheared tend to self-prop due to asperities.
ABSTRACT: Stimulation mechanisms in unconventional geothermal and petroleum reservoirs are poorly understood. Permeability enhancement via shear slip is commonly accepted as the main stimulation mechanism. On the surface, this appears to exclude the propagation in tensile and shear mode of the natural fractures that experience slip. The misconception, has led to some even claim the discovery of a new stimulation mechanism via the formation of wing cracks. But, wing cracks are an integral part of the shear slip stimulation mechanism because shear slip increases the stress-intensity at the fracture tips, potentially leading to fracture propagation. This is particularly the case when natural fractures are subjected to direct fluid injection. On the other hand, natural fractures subjected to pore pressure through diffusion in the rock matrix could respond differently during stimulation. In an effort to better understand the impact of pore pressure and poroelastic stresses on the stimulation of naturally fractured reservoirs, a poroelastic displacement discontinuity model is developed and used in this study to illustrate various stimulation mechanisms. Mohr-Coulomb contact elements are used to represent pre-existing natural fractures. Our results indicate that natural fracture propagation is less likely to occur when water is injected in the rock matrix outside the natural fractures. Moreover, the orientation of natural fractures with respect to the wellbore is found to significantly impact their response. We also observe local destabilization and non-uniform distribution of shear deformation which is usually neglected or greatly simplified in other models.
Unconventional resources are produced by reservoir stimulation by water injection. The pore pressure change due to injection/production can trigger shearing on critically-stressed natural fractures. Permeability enhancement through shear slip is a commonly accepted stimulation mechanism (Pine and Batchelor, 1984; Murphy et al. 1999). At times, this has been misunderstood to exclude the possibility of slip induced fracture propagation leading to the suggestion that a new and different stimulation mechanism is operating. In fact, some have even claimed the discovery of a new stimulation mechanism via the formation of wing cracks (McClure and Horne, 2013). But, the formation of wing cracks is an integral part of shear slip stimulation mechanism when fracture shear slip increases the stress-intensity at the fracture tips which can cause fracture propagation. It was this understanding that motivated the development of mixed mode fracture propagation models for geothermal reservoir development (Huang et al., 2013; Min et al., 2010). This is particularly the case when natural fractures are directly subjected to fluid injection at pressure below the minimum in-situ stress. Several studies have addressed the possibility of natural fracture propagation and coalescence as a stimulation mechanism (Min et al., 2010; Huang et al., 2013; Jung, 2013; Kamali and Ghassemi, 2016a; Kamali and Ghassemi, 2016b). Jung treated the subject analytically and provided field evidence for wing crack formation but did not consider the possibility of propagation in the “shear” mode. The wing crack is a well-established concept (Hoek and Bieniawksi, 1984; Horri and Nemat-Nasser, 1986; Shen and Stephansson, 1994; Rao et al., 2003), describing the extension in tension or shear mode of mechanically-closed cracks under applied compressive stresses. Kamali and Ghassemi (2016) have explicitly simulated the phenomenon during injection and have shown shear slip occurs at injection pressures below the minimum in-situ stress and triggers the out-ofplane wing cracks (Min et al., 2010; Huang et al., 2013; Kamali and Ghassemi, 2016a). Further propagation and coalescence might be achieved by maintaining the injection at pressures slightly higher than the minimum in-situ stress.
Shear induced permeability evolution is a crucial mechanism in understanding changes in the subsurface hydraulic system driven by natural seismic events and in the stimulation for subsurface energy recovery. We investigate permeability response during aseismic-through-seismic shear deformation on saw-cut fractures of varying low-order roughness. We report shear-instability-permeability direct shear experiments on Westerly granite in a triaxial pressure cell that concurrently and continuously measures friction and permeability during multiple sequential slide and hold events. We observe two clearly different responses in permeability evolution representing end-of-spectrum behaviors. These are (i) shear induced permeability reduction which is dominant in the initial stage of shear slip to a shear offset of several millimeters and (ii) gradual permeability enhancement with further shear offset. When the sample is static (experimental hold period), permeability continuously decreases and follows a power law decay with time. We observe that, with a given (~1mm) shear displacement, the normalized permeability enhancement Δ q/qinitial is greater after a longer hold period. This observation implies that the permeability of natural pre-existing faults that are locked over long durations can be significantly enhanced by shear deformations. The result suggests that permeability can be engineered in reservoirs by reactivated shear deformation and also suggests that the well-documented seismicity-associated permeability increase in natural hydraulic systems (e.g., Elkhoury et. al., 2006) may also be contributed to by systematic shear deformations.
Understanding the mechanism of fracture permeability is an important scientific and engineering challenge. Natural seismic activity is observed to cause transient changes in the natural hydraulic system (Elkhoury et. al., 2006; Manga et. al., 2011). Also, it is observed that shear slip on pre-existing faults may strongly influence the stimulation of shale gas reservoirs (Zoback et. al., 2012). The evolution of permeability in fractures under static loads has been discussed as driven by both mechanical and fluid driven processes. Pressure solution may drive continuous reduction of fracture aperture (Polak et. al., 2003; Yasuhara et. al., 2003) or and free-face etching may increase permeability. Conversely, pore pressure perturbations have been observed to instantaneously enhance fracture permeability (Elkhoury et. al., 2011; Candela et. al., 2014). In the following, we investigate the direct effect of dynamic shear deformation on fracture permeability in multiple slide and hold motions.
Shear slip on the natural fractures is proposed as a viable stimulation mechanism in unconventional geothermal and petroleum reservoirs. Fracture mechanics studies show that the propagation of mechanically closed fractures often involves both mode I and II propagation. Wing cracks are triggered by shear deformation at the tip of the natural fractures. These wing cracks tend to reorient and extend in the direction of maximum compressive stress. In addition to mode I propagation (i.e., wing cracks), natural fractures may also propagate in mode II in a plane approximately parallel to the pre-existing natural fracture. A displacement discontinuity method with Mohr-Coulomb elements is used in this paper to study the response of natural fractures to water injection. Modeling results indicate that the onset of fracture slip occurs when the initial shear stress exceeds the shear strength of the Mohr-Coulomb contact elements. The results show that injection into a single natural fracture may lead to the coalescence of multiple natural fracture which can be regarded as an important advantage of this stimulation technique. However, it was found that network connectivity and fracture coalescence is less likely achievable through simultaneous injection into multiple natural fractures mainly due to the compressive stress shadow in the vicinity of the neighboring fractures caused by the shear slip.
EGS design concept in a number of field projects (Soultz, Desert Peak, Newberry) relied on the conceptual model of permeability increase by slip on natural fractures due to water injection. The conceptual model envisions injection pressures below the minimum in-situ stress to cause slip on the critically-stressed fractures and or induce shear failure of the rock mass. However, Jung (2013) presented a review of the results and observations from a number of EGS experiments and suggested that the hitherto adherence to stimulation by shear or “hydro-shearing” is the main reason for the poor progress in the EGS success. Based on interesting interpretations of a number of phenomena, Jung argued that tensile fracturing and not shear slip or propagation is the main mechanism of stimulation, recommending a return to the conventional stimulation concept. In this paper, we review the concept of wing-crack propagation and show this mechanism is in fact, an integral part of the shear slip stimulation mechanism and that a shear propagation mode is also plausible and can contribute to permeability and MEQ. The process is similar to the shear failure in laboratory triaxial compression tests on rock whereby tensile and shear cracks coalesce to form a macroscopic shear crack or fault across the sample. And although individual tensile cracks do form in the process, the failure is referred to as shear failure.
Induced seismicity associated with hydraulic stimulation for the development of underground resources has been recognized as a risk factor in causing seismic hazards and is of public concern. To understand the simple physics behind induced seismicity, we analyzed the stress state of a fault plane where large seismicity was induced during hydraulic stimulation in the Cooper Basin, Australia and Basel, Switzerland. Using information regarding the stress magnitude and orientation, and the geometry of the fault plane where large events occurred, the stress state of these events was evaluated and the pore pressure necessary to cause shear slip was estimated. The fault plane of the large event in the Cooper Basin was close to being well oriented and only needed small increase in pore pressure (~10MPa) to induce shear slip. It was also discovered that the fault plane of the largest event at Basel required a moderate increase in pore pressure of around 20 MPa to induce a seismic event. Other large events occurring at different depths needed much lower pore pressures to induce shear slip. On the fault planes at Basel where these large events occurred, large shear stress was present, suggesting causality between shear stress and event magnitude.
Hydraulic stimulation is a necessary technology for the enhancement of formation permeability or improvement in system productivity in Enhanced/Engineered Geothermal System (EGS) projects. With the increasing demand for renewable energy, this technology has been used in many EGS projects to create economically feasible geothermal reservoirs. This method, also known as “fracking”, is used in the extraction of unconventional resources such as shale gas/oil. Fluid injection can induce shear slip on existing fractures or can initiate fractures leading to increased permeability. Acoustic energy released simultaneously with rock failure is often observed as induced seismicity, which is considered as the evidence relating these phenomena.
The growing number of large-magnitude, induced seismic events has recently been recognized as a serious problem associated with hydraulic stimulation in geothermal development (Majer et al., 2007). Although the magnitude of induced seismicity is typically less than 1.0, these large magnitude events have had moderate magnitudes (Mw 2~) and it is possible for them to be felt by local residents and also cause seismic hazards. Therefore, a quick development of methods to control seismic activity is necessary, and regulations or protocols based on scientific knowledge for sustainable and reliable geothermal development are required. However, many aspects of the physical mechanism related to large events remain unknown.
In modeling the propagation of injected fluid, as fluid flows through the flow paths in an existing fracture, pore pressure in the reservoir increases with pumping pressure. In an arbitrary existing fracture under tri-axial stress, this increase in pore pressure weakens the effective normal stress. When the shear stress working on a given fracture overcomes shear friction, the result is shear slip. This is known as the Coulomb failure criterion and is the principal behind induced seismicity. It can be described mathematically using equation (1),.
The interaction of hydraulic fractures with the pre-existing natural fractures may play a major role in increasing productivity from unconventional formations. When a hydraulic fracture meets a natural fracture, the hydraulic fracture can cross the natural fracture or be arrested. If the natural fracture is permeable, fracturing fluid can leak from the hydraulic fracture into the natural fracture causing elevation of pore pressure in the natural fracture and reducing the effective normal stress acting on the natural fracture, which could then lead to shear failure or slippage along the natural fracture plane. Shear-slip causes dilation, potentially increasing fracture conductivity and enhancing fluid flow deeper into the natural fracture. The conductivity of unpropped shear-induced fractures can play an important role in enhancing the productivity from ultralow-permeability formations like shale. In this paper, we first evaluate analytically the shear-slip condition and its propagation along a natural fracture under remote normal and shear stresses, when it is exposed to the fluid pressure in a hydraulic fracture. Analytical approximations under some limiting conditions are considered. A rigorous 2D numerical model based on coupling between fluid flow and rock deformation using displacement discontinuity method and fluid flow in the fracture is then described. The results of numerical simulations are presented to illustrate the effect of rock stress anisotropy, initial natural fracture conductivity, and fluid properties on the evolution of the fluid and slip fronts along the natural fracture and the associated permeability enhancement.
In the last decade, following the success of horizontal drilling and multistage fracturing in the Barnett Shale, exploration and drilling activities in shale gas and shale oil reservoirs have skyrocketed in the US and abroad. Economic production from these reservoirs depends greatly on the effectiveness of hydraulic fracturing stimulation treatment. Microseismic measurements and other evidence suggest that creation of complex fracture networks during fracturing treatments may be a common occurrence in many unconventional reservoirs [1-3]. The created complexity is strongly influenced by the preexisting natural fractures and in-situ stresses in the formation. To optimize the fracture and completion design to maximize the production from these reservoirs, engineers must have a good understanding of the fracturing process and be able to simulate it to obtain information such as the induced overall fracture length and height, propped versus unpropped fracture surface areas, proppant distribution and its conductivity, and potential enhanced permeability through stimulation of the natural fractures.