Other minerals e. Introduction Properties Common rock-forming minerals Economic minerals. Definitions Cleavage - The tendency of a mineral to break along flat planar surfaces as determined by the structure of its crystal lattice.
Fracture - The way a mineral breaks other than along a cleavage plane. Phone: ACS AuthorChoice. Article Views Altmetric -.
Citations Abstract High Resolution Image. Quantification of flow and transport in fractured rocks, which is needed for reservoir-scale simulations and for interpreting field observations, may require explicit representation of discrete fractures, 10 making fundamental understanding of the dynamic evolution of individual fractures critical.
Previous studies have shown that flow rate plays an important role in controlling fracture alteration that is driven by mineral dissolution. At a flow rate that is slow compared to the reaction rate, i. In this transport-controlled regime, diffusion may become important, which tends to reduce the local concentration gradient and suppress reactive instability.
This is also known as compact dissolution or face dissolution. At a flow rate that is fast in comparison to the reaction rate, the residence time of the fluid is short, and local reactions become surface reaction controlled. In this case, highly reactive fluids persist throughout the length of the fracture, and dissolution is uniform in the fracture.
At flow rates that are comparable to the reaction rate, the interplay between flow and reactions leads to reactive infiltration instability a form of geochemical self-organization phenomena , i. Hydraulic properties of the fractures evolve according to the dissolution patterns. In the case of compact dissolution, fracture permeability does not increase significantly because of the lack of fracture opening at the downstream end.
In the case of uniform dissolution, fracture permeability will increase substantially when no confining stress is present and can decrease under confining stresses because of the removal of asperities, i. These analyses, however, focused on simple geochemical systems, where typically only a single mineral is present, as the choice of a single reaction rate in multimineral systems is less clear.
As revealed by several experimental studies, however, heterogeneity in mineral composition can cause complex alteration of fracture morphology and hydraulic properties. This altered layer serves as a barrier that limits the contact between the fresh reactive fluid and the fast-reacting mineral, resulting in increasingly reduced dissolution of the fast-reacting mineral. If there is fracture wall erosion, the mobilized particles are transported downstream by the fluid, where they can accumulate at narrow points and reduce fracture hydraulic permeability.
These experimental studies have provided important insights regarding fracture alteration in multimineral systems, which are more common in the natural environment. For the same multimineral sample, the spatial patterns of fracture aperture change were observed to shift from compact dissolution to wormholing to uniform dissolution as the flow rate increases.
It does not, however, include any information on the reaction rate and therefore limits the ability to compare across mineral compositions. It has been shown that the flow regime corresponding to a given spatial pattern can be different for a mineralogically heterogeneous sample from what would be expected if no mineral heterogeneity was present. For example, channelization was not observed in an argillaceous limestone even though flow conditions that favor channelization in a pure limestone fracture were used.
The objective of the current study is to develop an analytical framework for the investigation of fracture evolution, and fracture channelization in particular, in multimineral rock matrices such that comparison across mineral compositions is possible.
To this end, we first conduct a series of numerical experiments to examine fracture evolution in multimineral systems under different flow conditions and with different levels of geometric and mineralogical heterogeneity. This investigation is enabled by the recent development of a reduced dimension reactive transport model. In contrast, the reduced dimension model used in our study is computationally efficient. Further, the model can be viewed as conceptually similar to a dual-porosity model where the exchange between the fracture and the matrix is calculated with the rate model eqs 4 — 6.
The model was developed and tested based on experimental studies and captures the complex morphological and geochemical changes that arise from the coexistence of minerals of differing reactivity. The key features of the model are summarized in the Methods section including flow conditions, initial fracture geometries, and geochemical conditions like mineral composition used for the numerical experiments.
We conclude the manuscript with a discussion of the implications for caprock integrity in geological carbon storage systems. To simulate dissolution-driven fracture alteration, we adopt a reduced dimension model that was developed and tested based on experimental observations. Details of the model can be found in previous publications, 34, 35 and only the key features of the model are summarized here. The reduced dimension model is implemented in CrunchFlow, a multicomponent reactive transport code.
The core module provides the fundamental framework in which the fracture is discretized into a 2D mesh within the fracture plane.
Each grid cell is a continuum, i. The reaction term R i accounts for the contribution from dissolution negative and precipitation positive reactions of minerals.
Porosity of each grid cell is calculated based on local fracture aperture and updated over time. Although the configuration of the core module allows the consideration of fracture aperture variations and therefore fracture geometry heterogeneity, the dimension across the fracture aperture is not treated explicitly, i.
Two additional modules were developed to account for the processes across the fracture aperture, specifically the development of an altered layer on the fracture surface and its erosion due to the detachment of the altered rock matrix.
It is assumed that the two fracture halves are symmetric. In the altered layer module, the reaction front for each individual mineral is tracked based on its respective reaction rate and volume fraction. As a result of differential dissolution, mineral fronts retreat into the rock matrix at different rates.
The aperture of a given mineral b i,j,m n is the distance between the dissolution fronts of this mineral m n in both sides of the fracture, i.
The mineral-specific aperture is calculated using eq 2 from the volume fraction of that mineral in the intact rock matrix f r,i,j,m n and the volume fraction of the mineral in the grid V i,j,m n , which is controlled by the reaction rate of the mineral. The mineral front that retreats the slowest defines the fracture surface, and the distance between the two fracture surfaces is the fracture aperture, also referred to as the flow aperture or geometric aperture in this study b i,j eq 3.
The distance between the fracture surface and the mineral front that retreats the fastest is the thickness of the altered layer L. Following previous observations, 23, 39 only diffusion and no flow is considered in the altered layer. Given that the dissolution products of the fast-reacting mineral have to diffuse through the altered layer, the dissolution of the fast-reacting mineral is increasingly subject to a diffusive transport limitation as the altered layer develops.
This diffusion limitation, as quantified by the diffusion controlled reaction rate R diff , is proportional to the ratio between the effective diffusion coefficient of the porous altered layer D eff and its thickness eq 4. The effective reaction rate R eff eq 5 is determined by the lower value of R diff and the surface reaction rate R surf , which is calculated using transition state theory type rate laws in this study.
In eq 6 , k rxn is the kinetic coefficient, A rxn denotes the reactive surface area, IAP represents the ion activity product, and K eq is the equilibrium constant. The exponents n 1 and n 2 here are assumed to be one. These formulations enable the consideration of the development of the altered layer and the diffusion limitation on subsequent geochemical reactions in the fracture.
The dissolution front for each individual mineral and the altered layer thickness are calculated internally and updated dynamically in the model. The only additional input parameter is D eff , which can be calculated from the porosity of the altered layer, i.
The erosion module accounts for the observation that dissolution of a fast-reacting cementing mineral can result in disaggregation and detachment, i. The erosion of the altered layer is triggered after a critical thickness L c is exceeded. This is consistent with the physical process that a mineral grain can only detach from the rock matrix after the bulk of the cementing mineral is removed.
Initial fracture geometries used in the numerical simulations are random fields generated based on a log-normal distribution and geometric statistics reported for fractures used in previous experimental studies Table 1.
Table 1. The statistics of the generated random fields are summarized in Table 1. Compared to random fields 3 and 4 RF3 and RF4 , random fields 1 and 2 RF1 and RF2 have smaller roughness as measured by the ratio between the standard deviation of fracture apertures and the average aperture. RF1 and RF3 are generated with a shorter spatial correlation length.
High Resolution Image. The fracture aperture values capture the range documented for most types of rocks in the geosphere, except for large fissures that may be present in geothermal or karst environments. Three types of mineral composition are considered. The first type is composed of a single carbonate. One example is the Indiana Limestone. As there is no heterogeneity in mineral composition, it allows us to establish a baseline for fracture evolution with impacts from only geometric heterogeneity and flow rates.
The third type of mineral composition is a carbonate-rich shale, i. Examples of this type of mineral composition include the Eagle Ford shale and the Niobrara shale. For type I mineral composition, two cases, pure calcite and pure dolomite, are simulated for comparison with the other two types of mineral composition. This is because the underlying processes involved in fracture alteration in the two rocks and the corresponding parameters were well understood and documented through previous fracture flow experiments.
Table 2. The specific choice of the Duperow dolomite and the Niobrara shale does not mean that the model cannot be applied to other mineral compositions. Previous studies provide valuable insights as to what processes—mineral dissolution, diffusion-limited reaction due to an altered layer, and erosion of the altered layer—are in play for a given mineral composition.
For example, the diffusion limitation from the altered layer can only be present for multimineral systems where the altered layer is able to develop, and the erosion of the altered layer is triggered only when the fast-reacting mineral is also the cementing material and exceeds a certain volume fraction of the rock. The structural strength of the altered layer will be strong enough to maintain a connected framework, and erosion will not occur if the volume fraction of the cementing mineral is too low.
In fact, several generalized mineral compositions are used in this study to investigate how variations in calcite content could affect fracture evolution for a given type of mineral composition section 3.
The effective diffusion coefficient for the shale compositions is calculated using the respective altered layer porosity and parameters that are reported for the Niobrara shale.
Different coefficients are also explored to examine their impacts on fracture alteration. We note that higher temperatures may be encountered in the subsurface, but we do not expect that using different temperature values will affect the validity of our analyses. Table 3. For each mineral composition, all four initial fracture geometries are simulated.
Even though Table 1 indicates that fractures in carbonate rocks tend to have larger apertures in general, probably because of coarser grain size, there are no definitive correlation between mineral composition and fracture aperture size reported.
A constant volumetric flow rate was imposed as the boundary condition. These values are within the range that may be encountered in a natural and engineered subsurface environment. The hydrostatic pressure gradient is on the order of 0. In the simulations, fracture apertures and thus the pressure gradient across the fracture evolve as a result of geochemical reactions.
The updated pressure gradient was then used in a rearranged version of eq 8 to calculate the hydraulic aperture to quantify the change of fracture hydraulic properties over time. Fracture hydraulic aperture is related to fracture permeability by a power law relationship and is used in this study as a measure of fracture hydraulic properties because it can be readily compared with geometric fracture aperture.
Hydraulic aperture is equal to the average geometric aperture, also referred to as the mechanical aperture, in parallel plate fractures and deviates from the mechanical aperture in the presence of geometric heterogeneity. Simulations were run for h except for some low flow rate simulations, which were run for h. The longer simulation time ensures similar pore volumes to the high flow rate simulations and more observable changes in the fractures.
For all simulations, the influent is a CO 2 -saturated fluid with a total dissolved carbon concentration of 1. In this section, we will first show the evolution of fracture geometry, geochemical reactions, and fracture hydraulic properties from the simulations that use a single carbonate mineral and discuss the impacts of flow rates and geometric heterogeneity. This discussion is followed by the simulation results of the low calcite carbonate and the calcite-rich shale and discussions of the role of heterogeneity in mineral composition.
At the end of this section, time scale analyses in multimineral systems are introduced to provide a general framework for analyzing fracture evolution in such systems.
In this subsection, we first focus on the different behaviors in fracture evolution caused by varying flow rate and discuss the observations that are consistent across initial fracture geometries. Then, we discuss the variations in fracture evolution that are due to differences in fracture geometry for a given flow regime.
Figure 2 plots the changes in fracture apertures after h of reactive flow at the three flow rates for the four initial fracture geometries for the pure calcite case.
Instabilities observed at the inlet propagate only a small distance into the fractures. This is because, at the high flow rate, mineral dissolution is surface reaction controlled, and fluid reactivity is maintained throughout the fracture.
This is consistent with the observation that effective surface area, and thus overall reaction rate, is reduced due to flow channeling. However, given the large volumetric flow rate, the total amount of dissolution after the same period of flow is highest in this case. This means that, over time, a smaller fraction of the fluid that flows through the fracture is in contact with the reactive mineral, leading to a decrease in the effluent Ca concentration Figure 3 a.
The evolution of the hydraulic aperture does not necessarily follow that of the mechanical aperture i. In cases where the dissolution is localized at the inlet, the ratio decreases.
This is because the downstream apertures are not enlarged and continue to constrain the flow, and the increase in apertures at the inlet does not translate into an increase in the hydraulic aperture. In cases of fracture channelization, an increase in the aperture ratio is observed after the breakthrough of the channel.
The ratio can grow larger than unity, following the continuous development of the channel. As pointed out in previous studies, this disproportionally fast increase in the hydraulic aperture in comparison to that of the mechanical aperture is a characteristic feature of flow channeling.
This proportional increase in hydraulic aperture with respect to mechanical aperture in uniformly dissolving fractures, however, may be suppressed or even inverted in the presence of confining stress.
Dissolution patterns are uniform at the two higher flow rates. Overall, for each dissolution regime, the evolution trajectory of the effluent chemistry and fracture hydraulic aperture are similar to those of the pure calcite case. The effluent Ca and Mg concentrations track each other Figure 3 b and e because the dissolution is congruent.
The ratio between hydraulic and mechanical aperture decreases initially before the channel breakthrough, and increases after the channel is established Figure 4 b.
At the two higher flow rates, the aperture ratio is maintained at about one through the entire course of the simulations. Unlike the pure calcite simulations, no temporal change is observed at the high flow rates because fracture volume increase and hence residence time change is limited. The effluent concentrations at a higher flow rate are lower than those at the slower flow rate due to shortened residence time and the transport limitation in the fracture plane on local reactions Figure 3 b and e.
The impacts of initial fracture geometry are secondary to those of the flow rates. Across the fracture geometries, the fracture evolution with respect to flow rate shows consistency. However, for a given dissolution regime, there are variations among different geometries. The variations are most evident when there is fracture channelization.
Because local heterogeneity in fracture geometry is a major source of the perturbations that lead to fracture channelization, variations in the initial fracture aperture fields largely affect the initialization and development of channels. In general, in random fields with less roughness and a shorter spatial correlation length e.
Accordingly, the decrease in the effluent cation concentration and thus overall dissolution rate Figure 3 a, b, and e and the increase in the hydraulic—mechanical aperture ratio Figure 4 a and b are observed at a later time step.
Figure 6 plots the calcite aperture, which measures the distance between the calcite fronts in the two fracture halves in comparison with the flow aperture for the low calcite carbonate mineral composition, i.
The former is of interest because the fast-reacting calcite plays an important role in controlling the progress of geochemical reactions affecting fluid in the fracture. For this mineral composition, the flow aperture is controlled by dolomite dissolution.
The spatial patterns of calcite apertures do not show similarity to those from the calcite-only simulations at the corresponding flow rate Figure 2 and instead follow the spatial patterns of dolomite apertures, which control the fluid flow in the fractures. The spatial coverage of the altered layer also shows distinct patterns at different flow rates. The presence of the calcite, however, causes some deviations from the dolomite-only case.
The magnitude of aperture increase is smaller, and the channels are narrower. In this two-mineral simulation, the effluent concentrations of Mg and Ca show different temporal evolution trajectories Figure 3 c and f. The effluent Mg concentration is solely attributable to dolomite dissolution, whereas the effluent Ca concentration is a result of the dissolution of both calcite and dolomite but is dominated by calcite dissolution because of its more rapid dissolution rate.
The decrease in Ca concentration is caused by the development of the altered layer and the resulting diffusion limitation. The altered layer diffusion limitation is the dominant factor at the higher flow rates for which the altered layer is more developed Figure 3 c.
In addition, the focusing of the fluid flow into the channel also contributes to the decrease in overall calcite dissolution. This inference is supported by the observation that Ca concentration decreases more substantially in the fracture with stronger channelization.
As a result of the reduced calcite dissolution over time, a lower pH is sustained in the fracture to promote the dissolution rate of dolomite. The increase in the aperture ratio is more notable than the dolomite-only case because of the more focused channel development in this case. Figure 7 plots the calcite and flow apertures from the simulations that use the Niobrara mineral composition, i.
Results from simulations that use initial fracture aperture fields other than RF4 are not plotted because they show similar patterns.
Given the slow reaction kinetics of minerals other than calcite, dissolution of minerals remaining in the altered layer do not cause an observable change in fracture apertures. Rather, the fracture apertures increase as a result of the detachment or erosion of the altered layer. This erosion process is triggered by the dissolution of the cementing material, i.
Because sufficient calcite has to be removed for disaggregation and detachment of the altered layer to happen, calcite dissolution does not lead to an immediate change in the fracture aperture and flow field. The delay in the feedback between the flow and the reaction causes the shift in the dissolution regime.
Calcite dissolution, as indicated by the effluent Ca concentrations, decreases as a result of the diffusion limitation of the altered layer at the higher flow rates Figure 3 d. The decrease is instead primarily associated with flow channeling. The channelization also corresponds to an increase in the hydraulic—mechanical aperture ratio Figure 4 d.
The results of the numerical experiments demonstrate distinct behavior of fracture evolution for different mineral compositions. The interplay between flow and reactions that ultimately controls fracture alteration can be summarized by the conceptual models Figure 8 for each category of mineral composition. With a single mineral, the feedback between reaction and flow constitutes a complete and simple loop. In the presence of multiple minerals, the feedback loops become more complicated.
In multimineral systems where the intact rock matrix has a low percentage of a fast reacting mineral, i. Whether there is fracture channelization and what the breakthrough time is are controlled by the slow-reacting mineral. However, an additional indirect feedback may exist, because the reaction rate of the slow-reacting mineral may also be affected by the fast-reacting mineral, which is in turn influenced by the flow field in the fracture.
In multimineral systems where the fast-reacting mineral is the cementing material and accounts for a large fraction such that it supports the structure of the intact rock matrix, the dominant feedback loop is the one between flow and the fast-reacting mineral.
Regardless of the presence of effectively nonreactive minerals in the remaining altered layer, fracture aperture change and thus the continuous feedback between reaction and flow is enabled by the erosion of the altered layer.
There is, however, a delay in the feedback, resulting in shifts in the flow regime that eventually leads to fracture channelization. As shown in Table 3 , the kinetic coefficient may depend on different reaction pathways eq 12 , 12 where k i is the kinetic coefficient of the respective pathway, and a i is the activity of the corresponding catalytic or inhibitory species i.
Assuming that the reactive surface area is simply the geometric surface area of the two fracture halves, eq 11 can be simplified to In multimineral systems, the rate of aperture change can be calculated for each mineral k b,m based on its respective kinetic coefficient, molar volume, and volume fraction.
Without erosion, such as in the case of the low calcite carbonate, the actual fracture aperture is controlled by the mineral front that retreats the slowest. Then, the rate of fracture aperture change is given by the minimum of the rates of aperture change for all minerals present.
We can then use the reaction rate of the fast-reacting mineral as a surrogate for the rate of fracture aperture change. However, because the erosion occurs when the altered layer exceeds a critical thickness, the diffusion limitation from the altered layer is substantial, and the effective reaction rate should be used. The diffusion-controlled rate is given by the effective diffusion coefficient and the critical thickness of the altered layer.
This is consistent with the observation that channelization is observed at the lower flow rate for the Niobrara mineral composition and illustrates that the proposed framework successfully captures the underlying processes of fracture alteration in multimineral systems. For example, it does not account for mineral spatial heterogeneity. However, it provides a useful framework for the analysis of fracture evolution in well-mixed multimineral systems. As the calcite content increases, there is a shift from channelization to compact dissolution.
Additionally, as calcite content increases, the decrease in Ca concentration and the increase in Mg concentration are less significant after the same amount of fluid flow because it takes longer for the channel to break through. This observation is consistent across different fracture geometries. With higher calcite content, i. Similarly, the proposed framework allows analysis of calcite content variations and the impact on the evolution of calcite-rich shale fractures. Different from the conventional dual-porosity and double-permeability concept models [ 46 ], the large-scale fracture or fractured space embedded in the rock matrix is conceptualized as main global flow channel.
While low-porosity and low-permeability matrix continua, still mainly providing storage space as sinks, are indirectly or directly interacting with widely connecting fractures. The conceptual discrete model utilized in this study assumes that approximate thermodynamic equilibrium exists locally in each of the two independent units at all times [ 47 ]. On basis of the local equilibrium assumption, the thermodynamic variables, such as fluid compositions, temperature, pressures and saturation, for each continuum can be defined successfully.
According to the outcrop observation, the degree of filling of the fractures changed greatly within a few meters, as a result, we set the conceptual model at scale of meter. The extensively developed intra-fracture fillings e. The ascending hydrothermal system generally consists of high-temperature water H 2 O.
Carbon dioxide, hydrogen sulfide and possible methane released from hydrocarbon reservoirs mainly considered as a gas reservoir , as well as of rock fragments, are dissolved into the water phase.
To ensure the reliability of subsequent simulation results, in this study we applied a simplified fluid thermodynamic model for carbonate reservoirs, i.
It could be considered that the fluid type in this simulation belonged to single-phase flow. The boundary condition is given on the boundary of a seepage zone to express the physical condition at the boundary of the seepage zone, that is, the condition that the water head or seepage flow should meet on the boundary of the seepage zone. According to the current geological features, for the horizontal flow model Figure 3 , its upper and bottom surfaces i.
Neumam condition. For the second type, the corresponding governing equation was: ,. Here, K was permeability coefficient of the study area; H was water head on the top and bottom, and here referred to the underground heat flow; q x,y,z,t was flow into or out of the boundary per unit area and unit time, as a given quantity; Since this water-blocking boundary belonged to an isotropic medium rock, the expression could be simplified to: , indicated that the fluid flow was zero at this boundary.
For this condition, the corresponding governing equation was: ,. Here, was a function of boundary flow as a function of time. Although the water point at each point on this boundary was constant at each moment, the boundary was in communication with the outer formation, and the fluid could still flow as long as the pressure was large enough.
In the model, we further set the deep hydrothermal fluid to flow from the left west side to the right east side, and the CO 2 injection rate was 0. However, the top and bottom boundaries were set to the second type or flow boundaries, that was, the fluid moved from deep to shallow at an initial injection rate of 0.
Table 1 shows initial water chemical elements. The geothermal fluid injection was assumed to last for a period of years. In order to observe the chemical reaction during the fluid flow, the simulation of fluid flow and geochemical transportation was also set to be a period of years. Core and thin section observations indicated that the main lithologies of the Ordovician strata in the study area included pelsparite, biogenic limestone, and some dolomitic limestone, mixed with other small amounts of clay and clastic rocks.
A total of 26 porosity and 39 permeability measurements were conducted on carbonate core samples from the outcrops, as listed in Table 2. Porosity measurements of rock matrix ranged from 0. As shown in Table 2 , the horizontal permeability values of these samples fracture samples and matrix samples were all significantly higher than the vertical permeability, but still by an order of magnitude.
Default values of the thermal conductivity of rocks set by the simulator were used, as the focus of this study is fluid movement and water-rock interactions. In addition, default values in the software were used for some hydrogeological and thermodynamic parameters e.
Minerals filter the groundwater and change the content of their ionic components during ion exchange with minerals [ 33 ]. Many researches have been carried out to investigate the geochemical composition and genesis of formation water of the representative Tahe oil field of northern outcrop area in Tarim Basin [ 54 — 56 ].
According to the classical scheme of Surin, the groundwater of Ordovician stratum is mainly the calcium chloride-type CaCl 2 of oil field water, with average pH of 5. Finally, after generalize the aquifer hydraulic characteristic, vertical boundary and lateral boundary, the groundwater mathematical model has been established on the basis of the hydrogeology conceptual model. The chemical reaction kinetic parameters refer to research results of Xu et al. The aquifer porosity and permeability are constantly changing due to the dissolution of the initial minerals and the precipitation of secondary minerals [ 63 ].
The change in porosity is directly calculated from variation in volume fraction of minerals, and the change in permeability due to the variation in porosity can be calculated by the Kozeny-Caeman sphere particle model [ 41 ]. The specific expression is: where K 0 is the initial permeability of rock; K is the permeability after chemical dissolution or filling; is the initial porosity of rock; is the porosity after chemical dissolution or filling.
The model of porosity-permeability selected for the simulation process is relatively simple [ 63 ]. Though it does not consider the influence of various factors such as the size, shape, and connectivity of the pores, its complexity is sufficient for the purpose of this simulation study. Further reviewing Figure 5 b , we could see that flow rates and temperature increased faster within the fractures compare to the reservoir matrix site, due to a larger permeability.
Many previous studies [ 64 — 67 ]; Duan et al. Therefore, at this moment, calcite slightly dissolved adjacent to the injection side because calcite solubility increased with relatively low temperature and gradually increasing flow rate Figure 5 c , which was different from dolomite. After 50 years, as the reservoir temperature rose and pressure declined, the chemical reaction in fractures changed from dissolution to precipitation gradually. As the fracture aperture increases, the degree of mineral filling was significantly reduced, but the filling range was relatively increased Figure 5 d.
Interestingly, after years, the difference of mineral precipitation in this fracture system became more apparent Figure 5 e. However, in the early stage, due to the different reaction kinetics [ 33 , 68 ], along the flow path, the fluid conditions changed continuously and the dolomite was slightly dissolved in the matrix, but a slight precipitation occurred in the large-aperture fracture, causing competing effects on calcite solubility Figure 5 f. In comparison, with the continuous injection of fluid, the dissolution of dolomite in the fracture was more obvious, and the larger the aperture, the stronger the degree of dissolution Figure 5 g , 5 h.
In order to further explore the filling mechanism in fractures, we extracted the solution ion curves in different time and different distance range Figure 6. When the deep CO 2 -rich dissolved fluid i. At this time, the reactions of calcite and dolomite dissolution were expressed as. In the geochemical modeling, the discrete pore-fluid flow was considered mainly taking place within the 3-D geological fracture zone.
Undoubtedly, mineral dissolution and precipitation lead to changes in the porosity and permeability of the reservoir matrix and fractures Xu et al. Permeability increases within fractures indicated that mineral dissolution was dominant at the early stage Figure 6 b , while permeability reduced when precipitation dominated in the later stage.
The dissolution of dolomite and precipitation of dolomite played a certain role than other factors for calcite, giving rise to calcite redistribution in fracture zones. The reason for this phenomenon was that the surrounding rock of the carbonate rock was less impermeable than the fracture zone, and the vicinity of the contact surface became a typical flow transition boundary, such as flow rate, pressure and temperature.
Moreover, the smaller the fracture aperture, the easier the calcite precipitation and the lower the total permeability of the fracture zone. All of these indicated that due to the 3-D convective pore-fluid flow, the chemical reactions between the chemical species were finally promoted and accelerated within the 3-D geological fracture zones.
The curve of the pH value of the formation water was shown in Figure 6 e. After years, the pH of the formation water was basically stable. According to the A-B section, the pH value in the fracturewas significantly lower than that of the surrounding rock. For fractures with different apertures, when the aperture was larger, the lower pH value, the higher calcium ion concentration and the lower permeability could be observed.
According to the simulations of fractures with different dip angles, the difference of this mineral filling was more obvious Figure 7. Calcite abundance had almost no difference in three fractures at the early injection stage. Native calcite also dissolved close to the injection side but precipitated later in farther region Figure 7 a. Zones of calcite precipitation move gradually away from the injection point due to changes in temperature along the flow path.
Notice that amounts of calcite precipitation and their redistribution depended on their precipitation kinetics.
As a result, it could be seen that after a long geological history evolution, low-angle fractures were easily full-filled by minerals, and high-angle oblique fractures were easily half-filled with minerals, however, near-vertical fractures were prone to no filling.
Generally, as the deep hydrothermal fluid migrates to the shallower depth, the corresponding temperature pressure gradually decreases as the depth decreases [ 70 ]. To investigate the effects of deep fluid transport on the calcite solubility, this study first assumed an ideal situation where fluid temperature and pressure were consistent with formation temperature and hydrostatic pressure in the formation. As shown in Figure 4 b , interactive precipitation and dissolution occurred with depth due to the injection of CO 2 and H 2 O.
It was clearly observed that all variations in the abundance of calcite were focused within the fracture zones.
Interestingly, since the calcite solubility is proportional to pressure Duan and Li, and CO 2 aqueous phase concentration and inversely proportional to temperature, the distribution pattern of calcite was similar to that of these factors, which could be seen from Figures 4 c and 4 d.
Mainly due to gravity segregation, the initial CO 2 concentration aqueous phase rapidly rose along the steep fracture zone to the top outlet, resulting in large area dissolution of calcite in the rock. As the CO 2 continued to accumulate in the aqueous phase, i. Therefore calcite began to precipitate. This phenomenon became more distinct even after years, in which the upper part of the formation was always the precipitation zone and the lower part was always the dissolution zone.
As a final consequence, in the fracture region, maximum calcite precipitated at upper part, while maximum calcite dissolution took place within lower part but dissolved later Figure 4 b.
Of course, from the depth-based simulation results, the size of the fracture at this time had slight effect on the dissolution and filling. In Patterns 1, 2, 3, and 4 Figures 9 a , 9 b , 9 c , and 9 d , the concentration distribution of precipitated calcite at the first 20 years were very similar to those obtained at 80 and years, although the locations of maximum values of mineral abundance were slightly different.
As shown in Figure 9 a , in complex fracture networks with constant apertures, calcite first precipitated in the facture parallel to the direction of flow path near the inlet site and then gradually migrated to the similar fracture in the farther zone. Moreover, as time went on, the high-value zones of mineral precipitation gradually migrated to or localized in the intersections of the fractures, which just showed that the degree of mineral filling was proportional to the fracture intensity measurement for characterizing degree of development.
In contrast, for different fracture combination, the high precipitation was focused in a few low permeability channels in variable-aperture fractures. In summary, the mineral filling degree could be calibrated to a full-filled type in fractures parallel to flow, the oblique small-aperture fractures and the intersections.
Accordingly, the degree of filling at other locations might be defined as a half-filled or unfilled type. This indicated that small-aperture fractures that intersected at a low angle to the flow path not only transmitted sufficient ion concentration i.
Conversely, connecting large-aperture fractures transmitted water at sufficiently large rates, and it was difficult to provide a stable sedimentation environment, thereby mainly causing dissolution. In the same way, the secondary fracture, that was, the fracture in same direction as the main fracture, was more likely to precipitate minerals than the fracture diagonally inclined to the main fracture Figure 9 b.
The reason was mainly that the secondary fracture along the direction of flow was easier to pass through more fluids to maintain higher temperatures and carried more precipitated ions than the reversed fracture. For a fixed-angle combination, it might thus be concluded that those fractures with a small angle to the fluid transport direction e. Generally, as time went on, more calcite was firstly precipitated in the region of up-temperature flow or major fractures i.
We standardized the data and plotted them into a uniform cross-section map to generalize mineral filling pattern for different fracture configurations. Certainly, this inherent relationship between precipitation and structural complexity was not constant. For the second type of curve Figure 10 b , the trend was very similar to the first type.
In detail, another effect that had an influence on filling degree in the conceptual model was the variable dip angles of fractures under complex stress state. For the third curve model, with different fluid injection directions Figure 10 c from the other three Figures 10 a , 10 b , 10 d , there was a clear chemical reaction interface, roughly in the upper half of the reservoir.
Furthermore, vertically, in the matrix and fractures above the interface, calcite dissolution or a small amount of precipitation was dominant, while a large amount of precipitation was dominant under the interface Figure 10 c.
In addition, it could be seen that as the linear density of connected fracture network increased, the precipitation volume fraction or the degree of mineral filling decreased. All of this was mostly due to the structural complexity of fracture networks, which controlled on flow rate, local temperature and pressure changes, and variations of ion concentration, but those were not the only factors that contributed to this effect.
Based on the outcrop observation in the northern area of Tarim Basin, we summarized the principal components to be ideally considered when modelling hydrothermal fluid flow in deep low-permeability carbonate reservoirs: the transported pores and their properties, the geometry of the fissure system, and the host rocks.
The redistribution of minerals e. In early stages, the boundary was roughly one-fifth of the total height from the top surface, and in the later stage, the boundary gradually migrated to one-third 5 For fracture network, the angle between fracture surface and direction of fluid flow and the degree of connectivity had the most significant effects on the degree of filling. And dissolution phenomenon strengthened within large-aperture conjugated fractures gradually along the flow direction.
In addition, in complex system, fractures with higher linear density had a relatively lower filling degree, which could provide more space for fluid to migrate without easily precipitating the carried ions.
The data used to support the findings of this study are available from the corresponding author upon request. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors.
Read the winning articles. Journal overview. Special Issues. Guest Editor: Victor Vilarrasa. Received 19 Feb Revised 10 Jun Accepted 06 Sep Published 15 Nov Introduction In recent years, a new round of oil and gas resources evaluation in China shows that the middle-depth and deep carbonate rocks have become one of the important areas to increase reserves.
Geochemical Modeling 2. Figure 1. Generalized map showing the location and the distribution of tectonic units in the Tabei Outcrop Area. Figure 2. Schematic of conceptualizing fracture filling formation as a fracture-cavity-matrix system with well different-level mineral veins.
Figure 3. Schematic diagram of three-dimensional multiple geological model. Figure 4.
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