Evolution Of Fracture Permeability Research Articles

Calculation model of shale fracture compressibility and evolution of permeability under water-bearing conditions

Water saturation of shale reservoirs significantly influences the permeability and compressibility of propped fractures. This study focused on the Longmaxi Formation shale reservoir in northern Guizhou, China, where the permeability of water–saturated shale under varying gas and confining pressures was measured. A compressibility model for proppant embedment and compaction deformation was developed and validated against the experimental results. This study examined the compressibility of supported fractures considering water–rock interactions and elucidated the intrinsic relationship between compressibility and water saturation. The findings demonstrated a decreased trend in shale fracture permeability with increasing water saturation under identical conditions. Compared to dry shale, the permeability decreased by 1.2%–16.4% and 2.0%–17.8% at water saturation of 15% and 50%, respectively. The results of the model calculations demonstrate that fracture compressibility is contingent on the degree of variation of the fracture width. Prolonged water–rock interactions intensified the variation in the fracture width increasing the compressibility under the same stress conditions. As the water saturation increased from 0% to 50%, the fracture closure rate increased from 0.034 to 0.179 with the increase in effective stress. Increased water saturation also increases the sensitivity of the fracture compressibility to effective stress while decreasing the elastic modulus of the rock, thereby enhancing the proppant embedment depth and significantly increasing the fracture compressibility. This study provides critical insights into the dynamic evolution of fracture permeability during hydraulic fracturing and offers valuable implications for gas production forecasting.

Shear-induced permeability evolution of natural fractures in granite: Implications for stimulation of EGS reservoirs

Permeability evolution of natural fractures during shearing is one of the critical issues in enhanced geothermal system (EGS). In this study, we conducted a series of numerical simulations of shear-flow tests under different normal stresses to investigate the permeability evolution and shear behavior of natural fractures during shearing. All numerical simulations in this study are executed based on a coupled hydro-mechanical pore network model (PNM) for fluid flow within the discrete element method (DEM). The numerical model uses a 33 mm × 25 mm fracture surface profile extracted from X-ray computed tomography scanning data of joints in a rock core retrieved from a 4.2-km-deep well at the Pohang EGS site. The results demonstrate a positive correlation between fracture permeability and shear displacement when the normal stress is relatively high. The central aspect of permeability evolution lies in the variation of local aperture distribution along the pathway: an increase in the variance of fracture aperture distribution leads to an increase in fracture permeability. Permeability evolution of fractures is significantly affected by normal stress, fracture roughness, and relative flow direction. Larger fracture roughness facilitates the formation of high-permeability pathways. Higher local normal stress often corresponds to lower local permeability. Moreover, high normal stress amplifies the effect of fracture roughness on permeability evolution. These findings enhance our comprehensive understanding of hydraulic-mechanical coupling effects during EGS stimulation.

Frictional process, AE characteristics, and permeability evolution of rough fractures on Longmaxi shale with specific roughness under water injection

Studying the frictional process, acoustic emission (AE) characteristics and permeability evolution of fractures under water injection is fundamental for proposing theories and methods to control the stability of geological structures. In this work, the shear-flow test was conducted on Longmaxi shale fractures with different specific roughness to analyze the frictional deformation and mechanical properties of shale fractures during the frictional slip process, and the influence of effective normal stress and surface morphology was investigated. AE monitoring technology was also utilized to reveal the AE evolution during the frictional slip process. Based on the AF/RA values, AE signals were subjected to cluster analysis using the Gaussian Mixture Model (GMM) to distinguish and quantify the crack types and proportions during frictional slip. Combining the development of cracks and considering the ratio of worn area, a theoretical analytical expression was established to reasonably describe the friction-damage evolution, classifying the friction-damage during the frictional slip process into tensile and shear damage, respectively. Meanwhile, the permeability evolution of Longmaxi shale fractures was also studied in detail. A theoretical model capable of describing and predicting the permeability evolution was proposed under the condition that the permeability evolution and damage process of Longmaxi shale fractures correspond to each other. Finally, the interaction between asperities on shale fractures with different surface morphologies during the frictional slip process was discussed, and the influence mechanism of surface morphology on permeability evolution was revealed.

Numerical investigation of heat extraction performance affected by stimulated reservoir volume and permeability evolution in the enhanced geothermal system

The high-permeability stimulated reservoir volume (SRV) containing artificial fractures is the main region for deep geothermal extraction. Evaluating the SRV distribution and the contributions of fluid pressure and thermal stress to permeability evolution is essential to accurately quantify heat extraction. Here, we employ the numerically calculated pore pressure and the field-measured rock failure as evaluation indicators to reasonably identify the SRV and investigate the effect of rock and fracture permeability evolution on heat extraction performance. We find that simply assuming the entire model as the SRV would significantly overestimate the heat extraction, and scientifically defining the SRV based on hydraulic fracturing is crucial. The variable rock permeability model extracts marginally less heat as the working fluid squeezes and cools surrounding rocks causing their lower permeability. In regions around the injection well, both the high fluid pressure and the thermal stress increase the fracture aperture and permeability, whereas the fluid pressure shows the opposite effect around the production well as it is lower than the initial pore pressure. When the main fractures connect injection and production wells, the widened fractures accelerate fluid flow, causing higher heat extraction for a short time (first 4 years); however, after 4 years’ operation, the heat extraction remains at a lower value for a long time as the increased fracture permeability causes more fluids to flow directly from main fractures to the production well without playing participating in heat exchange. When the main fractures do not connect injection and production wells, the increased fracture permeability slightly enhances the heat extraction over the geothermal extraction process with a relative error less than 0.025. This study provides guidance for the appropriate selection of geothermal extraction simulation assumptions.

Permeability Evolution of Bituminous Coal and Its Dynamic Control, a Case Study from the Southeastern Ordos Basin, China

Coalbed methane (CBM) reservoirs’ permeability is the result of dynamic variations influenced by tectonics, hydrology and the CBM production process. Taking samples from the southeastern Ordos Basin, China, the permeability evolution of bituminous coal and its control were analyzed in three steps: (1) the coal fracture permeability evolution was acquired via X-ray CT scanning and permeability evolution experiments; (2) the permeability variation was determined while considering the coupling characteristics effective stress, gas slippage, and matrix shrinkage effect and its influencing factors; and (3) a dynamic permeability model was built while considering those effects. For samples in which neither fractures nor bedding developed, the permeability decreased first and then increased as the gas pressure increased. For samples with fractures that developed parallel to the axial direction, with a gradual increase in gas pressure, the permeability also increased. As the gas pressure decreased, the matrix shrinkage effect became positive, resulting in a permeability increase. The gas slippage effect was positive in the low-pressure stage, which also resulted in a permeability increase.

Study on permeability evolution and damage mechanism along the EGS fracture in heat mining stage under thermal stress/cracking

As main heat exchange channel in enhanced geothermal system, the evolution of hydraulic conductivity in fracture is significance for efficient heat mining. For the thermal stress or thermal cracking spontaneously induced by the temperature difference between low-temperature fluid and hot rock in heat mining stage, it is necessary to explore the damage mechanism along EGS fracture and the corresponding permeability evolution. Firstly, the long-term permeability tests under high temperature (50–200 ℃) were conducted by the self-developed high temperature seepage experimental device. Then, a coupled THM-D model was constructed to describe the damage distribution along fracture. Combined with experimental and simulation results, relationship between the thermal stress/cracking and the evolution of fracture permeability is revealed. The results indicate that during high-temperature (200 ℃) experiments, the fracture permeability first increases rapidly under the low-temperature induced thermal stress/cracking, then decreases due to the blockage effect induced by the debris particles generated in thermal cracking along fracture. The enhancement of injection velocity and heterogeneity are all conducive to the emergence of thermal cracking in matrix along fracture. Simultaneously, high confining pressure has a negative effect on the migration of debris particles of thermal cracking, which contribute to prevent the blockage of debris particles.

Relationship Between Permeability and Resistivity of Sheared Rock Fractures: The Role of Tortuosity and Flow Path Percolation

AbstractThe fluid‐flow properties of fractures have received increasing attention regarding the role of geofluids in the genesis of slow and fast earthquakes and recent advances in geoengineering developments. Geophysical observations are promising tools to remotely estimate crustal permeability changes; however, quantitative interpretations are limited by the rock‐physical models' paucity for fractures. This study investigated changes in permeability, resistivity, and their respective relationships at elevated stress by performing numerical simulations of different fracture models with varying fracture size, roughness, and shear displacement. Numerical results and microscopic flow analysis demonstrate that permeability–resistivity relationships are controlled by percolation and are less dependent on fracture geometric characteristics. Our finding suggests that the permeability evolution of fractures can be formulated with resistivity changes independent of both fracture size and microstructure, the trends of which can be predicted using Archie's exponent. The extension to the electro‐mechanical relationship further derives the potential applications of estimating stress changes.

Comparison of two approaches for modelling fracture opening due to cold water injection in geothermal reservoir

Heat and electricity production from deep hot reservoirs through hydrothermal or petrothermal system requires to inject cold fluid in a naturally or artificially fractured medium. Cold water injection in a hot reservoirs causes thermo-hydro-mechanical (THM) coupled processes that may have several influences on operations and long term production [1]. Cold water has a higher viscosity, which means injection can become difficult due to higher flow impedance, while thermal diffusion in the rock matrix causes shrinkage and thus an increasing fracture aperture over time that can lead to flow channeling and a reduction of operation time. A deep understanding of the contribution of coupled THM processes to injection into fractured media is thus important to predict the long-term performance of a geothermal power plant. In this study, coupled THM processes in a single fracture are numerically investigated with two different approaches to model the discontinuity.
 The first numerical approach proposes modelling an implicit fracture in a fracture zone using solid elements (Fig.1 (a)). The impact of the fracture is introduced by means of a stress dependent fracture aperture, which in turn determines the fracture transmissivity. This function allows recovery of the opening, and thus the model replicates opening and closure of the fracture. Then, an equivalent continuum permeability [m2] is obtained as follows:
 where [m] is the elastic opening, br and bmax [m] are the residual and maximum apertures, respectively, a is the stress dependency coefficient; [Pa] is effective normal stress, and [m]the fracture spacing. The combination of the continuum elements to model an implicit fracture embedded in a fracture zone and the evolving permeability has been used successfully used to model elastic opening in volcanic rock [2] and granodiorite rock [3, 4].
 In the second approach, zero-thickness interface elements are used to explicitly model the opening of an existing fracture (Fig.1 (b)), following the work by Liaudat et al. [5]. A constitutive law that is capable of describing the fracture initiation, propagation, closing and opening is implemented in this approach, considering heat transfer in the discontinuity. In this approach, the cubic law is used to estimate the longitudinal transmissivity [m3] as a function of the normal fracture aperture:
 where [m] is the normal separation and [m3] is a constant value which makes it possible to assign an initial longitudinal transmissivity even if the fracture is mechanically closed.
 In both approaches, the simulation of cold water injection requires to consider the heat and hydraulic flows in the fracture alongside mechanical behaviour, since changes in pore water pressure and temperature influence the fracture aperture, thus modifying the fracture transmissivity.
 In order to compare both approaches, a simple synthetic model is studied. The model with its boundary and initial conditions are shown in Fig.2. Water with a temperature of 20 °C and pressure varying from 4 MPa – 10 MPa is injected at the centering point of the fracture. Figure 3 shows the injection pressure profile. Fracture permeability evolution is compared in Fig.4, which shows the two different methods capture different characteristics of the natural fracture behaviour. In the continuum method, the fracture is modelled as a continuum element, thus any pressure or stress change will result in corresponding changes in the fracture permeability, which is reflected in Fig.4 showing that the permeability changes with the injection pressure changing, In contrast, in the interface element, Fig.4 shows only when the injection pressure reaches 10 MPa, the permeability increases sharply from the initial value to the peak value. This is because penalty method is used to allow the small overlapping of the two faces of the discontinuity, thus keeping negligible permeability variation, before it opens. The result in Fig.4 also illustrates the effect of thermal stress on the fracture opening, considering the total vertical stress is 10 MPa while injection pressure is below or equal to 10 MPa.
 In conclusion, though the peak permeability predicted from two methods are consistent, both around 1.5 x 10 -12 D, the behaviour before fracture opens in these two approaches are quite different. For the interface method, permeability variations keep negligible before fracture opens, while for the continuum method, permeability changes with corresponding pressure or stress changes without dependence on the opening of the fracture.

Permeability evolution due to combined thermal stress and in-situ stress during fractured geothermal reservoir production

Understanding the process of fluid circulation within fractures is crucial for the utilization of geothermal energy. However, accurately predicting the evolution of heat transfer performance remains challenging due to the complex physical mechanism behind the injection-withdraw fluid balance and poor understanding of thermal transient cooling in changing the stress state of fracture and followed aperture evolution. This study used the coupled thermal-hydraulic-mechanical simulator FLAC3D-TOUGHREACT to investigate the evolution of fracture permeability in a fractured geothermal reservoir under a variety of initial stress conditions. Results show that the produced thermal stress increases the fracture permeability first, but when the heat influence disappears, the permeability starts to decrease. Fractures that are oriented sub-parallel to the minimal principal stress have a greater slip potential, dilate, and increase permeability than other fracture types. Lower differential stress allows for significantly more stress alleviation, and the shear stress is greater which promotes the development of fracture shear failure. With a constant stress ratio of 0.65, the higher mean stress case showed a greater extent of stress reduction and shear stress build-up, which allows permeability and porosity to increase. The findings revealed that the combination of thermal stress and in-situ stress state plays a critical role in determining the evolution of fracture permeability during the fracture process.

Fracture Permeability Enhancement During Fluid Injection Modulated by Pressurization Rate and Surface Asperities

AbstractWe present a series of controlled fluid injection experiments in the laboratory on a pre‐stressed natural rough fracture with a high initial permeability (∼10−13 m2) in granite using different fluid pressurization rates. Our results show that fluid injection on a fracture with a slight velocity‐strengthening frictional behavior exhibits dilatant slow slip in association with a permeability increase up to ∼41 times attained at the maximum slip velocity of 0.085 mm/s for the highest‐rate injection case. Under these conditions, the slip velocity‐dependent change in hydraulic aperture is a dominant process to explain the transient evolution of fracture permeability, which is modulated by fluid pressurization rate and fracture surface asperities. This leads to the conclusion that permeability evolution can be engineered for subsurface geoenergy applications by controlling the fluid pressurization rate on slowly slipping fractures.

Numerical simulation on Geothermal extraction by radial well assisted hydraulic fracturing

The mid-deep geothermal energy is extracted from the fracture network created by hydraulic fracturing. Generally, the hydraulic fracture created in the deep thermal reservoir shows the planar shape but not the network shape, and this significantly impacts the geothermal extraction efficiency. In this study, a geothermal extraction technology by radial well assisted hydraulic fracturing, which connects hydraulic fractures to each other, was proposed. An operation technique of radial well-assisted hydraulic fracturing in geothermal reservoirs without well-developed NFs was established. The model considering evolution of fracture permeability was resolved by thermo-hydromechanical (THM) coupling numerical simulation. The results show that the radial wells should be placed vertically within a large range. The thermal production increases by 20%, about 0.25 MW as the radial well length increases from 50 m to 200 m. As many fractures as possible are needed in thermal exploitation, and the fracture number should be at least 7. The simulation demonstrates that the radial well-assisted hydraulic fracturing heat recovery method is feasible in simulation. In this thesis, a more accurate numerical simulation method of geothermal production capacity is proposed, which has engineering guiding significance for the actual development and prediction of geothermal energy.

Frictional stability and permeability evolution of fractures subjected to repeated cycles of heating-and-quenching: granites from the Gonghe Basin, northwest China

Fluid injection into enhanced geothermal system (EGS) reservoirs can reactivate subsurface fractures/faults and trigger earthquakes—requiring that frictional stability and permeability evolution characteristics are adequately evaluated. This behavior potentially becomes more complicated when the impacts of temperature and cycled thermal stresses, and the resulted damage accumulation on both stability and transport characteristics are getting involved. We conducted coupled shear-flow experiments on saw-cut fractures recovered from an analog surface outcrop representative of a reservoir at 2450 m in the Gonghe Basin of northwestern China. The rocks were subjected to variable numbers of repeated heating-quenching (25-180-25 °C) cycles for shear-flow experiments at an effective stress of ~ 3 MPa and with velocity stepped between 10-1-10-1-10 μm/s. The smooth fractures return frictional coefficients in the range ~ 0.69 to 0.72 and are little affected by the thermal cycling. The frictional stability parameter (a–b) decreases and the instantaneous permeability increases with an increase in the number of heating-quenching cycles, during which intergranular and intragranular microcracks were generated in fracture surface. The above results indicate that the heating-quenching cycles during hydraulic fracturing of geothermal reservoir could affect both the fracture frictional instability and permeability evolution.

Permeability evolution during pressure-controlled shear slip in saw-cut and natural granite fractures

Fluid injection into rock masses is involved during various subsurface engineering applications. However, elevated fluid pressure, induced by injection, can trigger shear slip(s) of pre-existing natural fractures, resulting in changes of the rock mass permeability and thus injectivity. However, the mechanism of slip-induced permeability variation, particularly when subjected to multiple slips, is still not fully understood. In this study, we performed laboratory experiments to investigate the fracture permeability evolution induced by shear slip in both saw-cut and natural fractures with rough surfaces. Our experiments show that compared to saw-cut fractures, natural fractures show much small effective stress when the slips induced by triggering fluid pressures, likely due to the much rougher surface of the natural fractures. For natural fractures, we observed that a critical shear displacement value in the relationship between permeability and accumulative shear displacement: the permeability of natural fractures initially increases, followed by a permeability decrease after the accumulative shear displacement reaches a critical shear displacement value. For the saw-cut fractures, there is no consistent change in the measured permeability versus the accumulative shear displacement, but the first slip event often induces the largest shear displacement and associated permeability changes. The produced gouge material suggests that rock surface damage occurs during multiple slips, although, unfortunately, our experiments did not allow quantitatively continuous monitoring of fracture surface property changes. Thus, we attribute the slip-induced permeability evolution to the interplay between permeability reductions, due to damages of fracture asperities, and permeability enhancements, caused by shear dilation, depending on the scale of the shear displacement.

Effect of pore pressure magnitude on the frictional properties and permeability evolution of fractures in schist

Effect of pore pressure magnitude on the frictional properties and permeability evolution of fractures in schist

Evolution of fracture permeability and its compressibility in proppant-supported shale

Hydraulic fracturing is one of the effective means to increase shale gas production, during which, proppant was widely used to support fractures to form stable seepage channels. Following which changes in permeability are restricted by the compaction and embedding of proppants, so studies on the impact of proppant on the fracture conductivity are highly desirable. In this paper, we discussed first the change mechanism relating to fracture compressibility under the combined effects of proppant compaction and embedding, and, further, proposed a new calculation model denoting fracture compressibility. Moreover, the effect of the number of proppant paving layers was also incorporated in that model. Secondly, based on the exponential relationship between stress and permeability, and by taking fracture compressibility as a bridge, a shale fracture permeability model, that considered the combined effects of proppant compaction and embedment, was constructed. The proposed permeability model was, then, verified through evidence from publicly released test data, and the permeability change law relating to shale fractures. In addition, we focused on the influence of factors: such as, stress, proppant types, rock types, and the number of proppant layers on fracture compressibility. The results revealed that fracture compressibility in its original state was a higher than that of the supporting fracture. The increased stress leads to reduced fracture compressibility It was found that the higher the rock hardness, the smaller the corresponding fracture compressibility, the smaller the elastic modulus of the proppant, and an increase in fracture compressibility. The analysis of these results would help to improve the basic understanding of the key processes affecting proppant performance and permeability alterations.

On the role of proppants and geomechanics on flowback behavior in complex fracture networks

Flowback analysis is often used for the post-fracturing characterization of unconventional wells to estimate key reservoir and fracture parameters. However, conventional flowback analysis relies solely on the fluid flow analysis, the potential impact of proppant or geomechanics on flowback behavior is seldom discussed. To fill this gap, an integrated numerical model is proposed here to simulate fracture propagation, well shut-in and finally flow back from fracture network. To simulate the fracture propagation with branches, a cohesive zone model with node displacement is adopted. During fracture closure process, proppant stress is integrated to consider the presence of proppants and non-linear fracture permeability evolution is also considered to represent the transition from fracture flow to Darcy's flow.Simulation results show that the presence of proppants and geomechanics may significantly impact the fracture closure mode and flowback behavior in fracture networks. When proppants are placed in the fracture, an infection point can be identified in the bottomhole pressure (BHP) response. After one day flowback, BHP with proppants is nearly 65% larger than that without proppants. Notably, fracture complexity can also be identified during flowback. As more proppants placed deeply into the fracture network, corresponding BHP would decline at a lower rate with later appearance of the inflection point. Besides, the formation properties near wellbore also affect the flowback behavior. As the permeability in the stimulated zone improves, BHP declines much milder after the inflection point. Finally, larger flowback rate can accelerate the fracture closure and enhance the overall BHP decline rate no matter when the inflection point is. Since most flowback models rely solely on fluid flow component, this study provides a new insight on flowback behavior which incorporate interactions of fluid flow, geomechanics and the presence of proppants.

Numerical simulation of geothermal energy productivity considering the evolution of permeability in various fractures

The optimization of geothermal exploitation model and simulation of productivity is a hot spot in the thermal industry recently. Previously, the fracture permeability was considered as a constant during the thermal exploitation. However, the fracture permeability varies constantly during the thermal exploitation, and this causes an error in the numerical simulation of geothermal productivity. Here, the experiment in evolution of the fracture permeability in granite was performed, and the equations for evolution of the permeability in major fractures, branch fractures and natural fractures were established. On this basis, the thermal-hydro-mechanical coupling geothermal model was established to investigate the effect of well profiles, conditions for evolution of fracture permeability and natural fracture development degree on geothermal exploitation capacity. The results show that when considering the evolution of fracture permeability, the vertical well model is affected after 20 years of exploitation, and the produced thermal energy in the vertical well with developed fractures is about 4% (0.4 MW) lower than in the conventional vertical well model. Under the same conditions, the difference between horizontal wells is within 1%, which can be ignored. More heat is outputted in reservoirs with developed fractures than in those without fractures.

Coupled thermo-hydro-mechanical modelling for geothermal doublet system with 3D fractal fracture

• 3D fractal fracture existed in a geothermal doublet system. • Coupled thermo-hydro-mechanical interaction for bedrock and fracture. • Normal and shear deformations of fracture based on thin elastic layer assumption. • Evaluation of thermal breakthrough due to preferential fracture flow. The existence of preferential flow paths, such as fractures and/or fault play an vital role on the thermal breakthrough of geothermal doublet system. The interaction between the preferential flow path and bedrock is often uneven and may have typical fractal characteristic. This study proposes a thermo-hydro-mechanical coupling model considering the deformation of fractal fractures. The fractal fracture is regarded as a thin elastic layer existed in the bedrock, whoes deformation depends to the bedrock and its own mechanical properties. Subsequently, the geothermal doublet system with a three-dimensional fractal fracture is modelled and the parameters affecting thermal breakthrough are investigated numerically. Research results indicate that the thin elastic layer assumption is remarkably robust for modelling fracture opening and closing under coupling conditions. Owing to complex fracture geometry, the fracture permeability evolution presents certain heterogeneity, which is related to fractal dimension, in-situ stress, and geothermal wells layout. The cool water in fracture with larger fractal dimension can interact with the bedrock more fully and further affect the thermal breakthrough. This further suggests that the site selection of geothermal wells should consider the specific geometry of preferential flow paths to avoid premature thermal breakthrough inducing low system efficiency.

Evolution of fracture permeability and aperture during CO2 storage in varyingly cemented sedimentary rocks

In this study, CO2-enriched brine was injected into two pure limestone and two calcite- and quartz-cemented sandstone specimens, artificially fractured by a circular saw. The CO2-enriched brine was injected under two flow rates differing by a factor of 2, to simulate the reservoir condition closer and farther from the injection well. Also, the specimens were subjected to different stress levels under loading/unloading stages to simulate different reservoir conditions. The ICP-OES (Inductively coupled plasma–optical emission spectrometry) analysis was performed at the end of each experiment to evaluate the extent of potential dissolution/precipitation. Injecting CO2-enriched brine into the pure calcite specimen, under the higher injection rate, resulted in the creation of new flow channels, leading to a significant increase of hydraulic aperture and permeability (∼20.6 times). However, under the lower injection rate, the fracture channelization was limited to the areas close to the inlet, which led to fracture permeability enhancement of ∼6 times. On the other hand, in calcite- and quartz-cemented sandstone specimens, the increase of hydraulic aperture and fracture permeability was more pronounced under the lower injection rates, which was attributed to the increased kinetic rate of geochemical reactions. In this regard, the chemical analysis revealed that increasing the effluent concentration of Ca2＋ and K＋ was higher under the lower injection rates. The results of ultrasonic velocity measurements revealed a velocity slow-down upon injecting CO2-enriched brine, however, the velocity increased upon increasing the CP level. The results of this study shed light on the importance of different mineralogical features of reservoir rocks under various reservoir conditions. Due to the complexity of fracture response during CO2 storage, the results of this study can serve as a benchmark for numerical simulations. Also, the chemical analysis and fracture permeability evolution results can be used as inputs in reservoir scale modeling.

Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)

The long-term sustainability of fractures within rocks determines whether it is reasonable to utilize such formations as potential EGS reservoirs. Representative for reservoirs in Variscan metamorphic rocks, three long-term (one month each) fracture permeability experiments on saw-cut slate core samples from the Hahnenklee well (Harz Mountains, Germany) were performed. The purpose was to investigate fracture permeability evolution at temperatures up to 90 °C using both deionized water (DI) and a 0.5 M NaCl solution as the pore fluid. Flow with DI resulted in a fracture permeability decline that is more pronounced at 90 °C, but permeability slightly increased with the NaCl fluid. Microstructural observations and analyses of the effluent composition suggest that fracture permeability evolution is governed by an interplay of free-face dissolution and pressure solution. It is concluded that newly introduced fractures may be subject to a certain permeability reduction due to pressure solution that is unlikely to be mitigated. However, long-term fracture permeability may be sustainable or even increase by free-face dissolution when the injection fluid possesses a certain (NaCl) salinity.