Discrete Fracture Network Model Research Articles

A novel approach to investigating 3D fracture connectivity in ultrahigh steep rock slopes

Ensuring the high-quality construction of the ST Railway is a top priority for construction organizers and managers. In this paper, a stepwise approach combining bounding box (BB) technology and the Gilbert–Johnson–Keerthi (GJK) algorithm was proposed to determine the intersections between fractures. With the use of a stepwise approach, most of the nonintersecting fracture pairs were excluded, the computed speed and effectiveness were greatly improved compared with those in previous studies, and the length and direction vectors of the fracture intersections were obtained. A 3D discrete fracture network (DFN) model was generated and validated according to fracture data acquired from the right bank slope of the Nujiang Grand Bridge (NGB) of the ST Railway by unmanned aerial vehicle (UAV) photogrammetry technique. Based on the DFN model, the stepwise approach was implemented for intersection analysis and connectivity analysis. Connectivity parameters, including the number of intersections per unit volume and the intersection length per unit volume, were calculated as 1.279 m-3 and 2.296 m-2, respectively. The intersection analysis suggested that the mean length of the intersections was 1.796 m, and fracture sets 2 and 3 constituted the main flow pathways and played a predominantly combined cutting role for the rock masses in the study area. In addition, the investigation of the direction vector of the intersections indicated that the combined cutting action of fracture sets 2 and 3 easily formed wedge-shaped failure surfaces, and the lateral expansion produced by bedrock fracture groundwater was also unfavourable to the stability of the slope.

On the onset of nonlinear fluid flow transition in rock fracture network: Theoretical and computational fluid dynamic investigation

Fluid flow regimes affect the determination of hydraulic conductivity of fractured rocks, and the critical criteria for the onset of nonlinear fluid flow transitions in discrete fracture networks (DFNs) of rocks have yet to be established. First, the factors causing the fluid flow transition regime of fracture intersections and rough surface fractures are theoretically and numerically analyzed. This reveals that the fluid flow regime is governed by the fracture aperture, density of fracture intersections, surface roughness, and Reynolds number (Re). Then, these identified parameters are redefined in DFN models, and their influence on the onset of nonlinear fluid flow is further investigated by performing computational fluid dynamic analysis. The results show that the fracture intersection and aperture play a more significant role in the linear-to-nonlinear fluid flow transition than the fracture aperture heterogeneity. With the increase in the fracture aperture, unevenness of fracture surfaces, and connectivity of DFNs, the onset of the nonlinear fluid flow appeared at the lower flow velocity. With the Forchheimer equation, it is found that the critical hydraulic gradient Jc, defined as the hydraulic gradient at which inertial effects assume 10% of the total pressure loss, is highly correlated with the fracture aperture, fracture intersection, and roughness of the surface. Finally, the mathematical expression of Jc and the Forchheimer coefficients are formulated based on the regression analysis of fluid dynamic computation results, which provides an approach to determine whether the cubic law should be applied as governing equations for the computation of fluid flow in DFNs.

A FEM-DFN model for the interaction and propagation of multi-cluster fractures during variable fluid-viscosity injection in layered shale oil reservoir

To investigate the height growth of multi-cluster fractures during variable fluid-viscosity fracturing in a layered shale oil reservoir, a two-dimensional finite element method (FEM)-discrete fracture network (DFN) model coupled with flow, stress and damage is proposed. A traction-separation law is used to describe the mixed-mode response of the damaged adhesive fractures, and the cubic law is used to describe the fluid flow within the fractures. The rock deformation is controlled by the in-situ stress, fracture cohesion and fluid pressure on the hydraulic fracture surface. The coupled finite element equations are solved by the explicit time difference method. The effects of the fracturing treatment parameters including fluid viscosity, pumping rate and cluster spacing on the geometries of multi-fractures are investigated. The results show that variable fluid-viscosity injection can improve the complexity of the fracture network and height of the main fractures simultaneously. The pumping rate of 15 m 3 /min, variable fluid-viscosity of 3-9-21-36-45 mPa s with a cluster spacing of 7.5 m is the ideal treatment strategy. The field application shows that the peak daily production of the application well with the optimized injection procedure of variable fluid-viscosity fracturing is 171 tons (about 2.85 times that of the adjacent well), which is the highest daily production record of a single shale oil well in China, marking a strategic breakthrough of commercial shale oil production in the Jiyang Depression, Shengli Oilfield. The variable fluid-viscosity fracturing technique is proved to be very effective for improving shale oil production.

Mechanical behavior and failure modes of jointed rock mass based on rough discrete fracture network models

Mechanical behavior and failure modes of jointed rock mass based on rough discrete fracture network models

On the parameterisation of dual-continuum models for reactive transport modelling in fractured media

In sparsely fractured rocks, the rock matrix is an important geochemical buffer and provides significant retardation to contaminants advected through the flowing fractures. Accounting for geochemical reactions and mass exchange between these two regions is key to properly capture the overall buffering capacity and the related hydrogeochemical evolution of a fractured medium. Reactive transport modelling in these kinds of fractured media is routinely performed using Equivalent Continuous Porous Media (ECPM) models: i.e. continuum models based on permeability and porosity fields that somehow preserve the underlying fracture properties, which are in turn described by companion Discrete Fracture Network (DFN) models. However, the proper parameterisation of these models, in terms of mass exchange between fractures and the bordering matrix, is still a largely unresolved issue. Here, we leverage the Dual Continuum Disconnected Matrix Model (DCDMM) formulation included in the massively parallel code PFLOTRAN to propose a novel parameterisation approach that honours the local volumetric fracture density (P32). The proposed approach is first benchmarked against a semi-analytical solution with a problem that entails flow and transport along two different and consecutive fractures. Two demonstrative large-scale reactive transport problems are also presented and discussed: the first is related to the generation and migration of radiogenic helium and the second assesses the buffering capacity of a realistic fractured medium against the infiltration of acidic water. The latter simulation, which includes more than three hundred million transport degrees of freedom is one of the largest subsurface reactive transport models ever formulated and solved. This simulation was made possible by the highly efficient implementation of the DCDMM in PFLOTRAN, which makes the solution of the secondary continuum equations embarrassingly parallel.

Simulation Method and Application of Three-Dimensional DFN for Rock Mass Based on Monte-Carlo Technique

In this study, the authors simulate a polygonal discrete fracture network (DFN) in rock masses. The probability models of the relevant geological parameters, including the orientation, trace length, volume density, and coordinates of the centroid, are firstly developed as fractures are in the shape of rectangles. In the process, the probability distribution of rectangular fractures with side lengths as random variables is introduced and described in terms of mean trace lengths on the basis of the probability model of disk-shaped fracture with the diameter as the random variable. The relationship between the volume density and the linear density of rectangular fractures is given for a negative exponential distribution. Following this, the coordinates of the vertices of fractures are derived based on spatial algebraic geometry, and the data for the three-dimensional DFN model are generated using the Monte-Carlo technique. The resulting three-dimensional DFN is visualized by calling the Open GL graphics database in the environment of Visual C, and the process of implementation of the DFN simulation is given. Finally, the validity of the simulation is verified by applying it to engineering practice.

Influence of preexisting discontinuities on hydraulic fracture complexity in a naturally fractured reservoir

Preexisting discontinuities, such as joints and beddings, are widely distributed in unconventional reservoirs. During fluid injection, hydraulic fractures strongly interact with preexisting discontinuities, leading to great uncertainty in prediction of fracture geometry. To investigate how preexisting discontinuities affect fracture complexity, numerical simulations are carried out using the distinct element method. A discrete fracture network (DFN) model for a shale reservoir in the southeastern Ordos basin, China is built. Four cubic models, including the (1) rock matrix model, (2) bedding model, (3) joint model and (4) bedding and joint model, are extracted from the DFN model and then are used for numerical simulations. The results indicate that preexisting discontinuities strongly influence fracture complexity: they impose a barrier for fracture growth in most cases, and thus result in a smaller fracture area and a higher fluid pressure for fracture extension. However, the roles that joints and beddings play in fracture complexity differ. Compared with the bedding model, the joint model causes a less complex fracture geometry under the same injection condition, suggesting that the joints play a more significant role in reducing fracture complexity.

A multiscale neural network model for the prediction on the equivalent permeability of discrete fracture network

An equivalent permeability approach can upscale the discrete fracture network (DFN) model to an equivalent DFN model and significantly reduce the gas flow simulations in a large-scale fractured gas reservoir. Current equivalent permeability prediction models are only applicable to the reservoir with a simple fracture network. However, an equivalent permeability prediction model has not been available for a reservoir with a multiscale discrete fracture network. This study proposes a multiscale convolutional neural network model (called MsNet) and introduces three mainstream structures with high performance convolutional neural network (CNN) (ResNet-18, VGG-16 and GoogLeNet) to efficiently predict the equivalent permeability of a complex multiscale fracture network. These CNN models use both the images and features of DFN as their input and the equivalent permeability as their output. This MsNet model is validated with the simulation results simulated by Lattice Boltzmann method and compared with the three mainstream CNN structures and an existing permeability prediction model (CNN-4). It is found that this MsNet model innovatively considers the multiscale characteristics of DFN by a multiscale convolution feature fusion and combines the residual connection for further performance enhancement. Both DFN dataset and MsNet model structure affect the model prediction ability. A deeper network structure of MsNet model can enhance its prediction ability, but significantly increases training time. The MsNet-8-4 (a MsNet structure with 8 multiscale connection modules and 4 sub-networks in each module) has the least convergence time and the lowest mean absolute error on the test set. It performs obviously better than other four models on the DFN dataset with higher fracture density. The MsNet model can well accelerate the simulation on the gas flow in a complex discrete fracture network.

Mixture distribution model for three-dimensional geometric attributes of multiple discontinuity sets based on trace data of rock mass

The geometric attributes of discontinuities are critical to an accurate discrete fracture network (DFN) model of rock mass. It is difficult to estimate three-dimensional (3D) geometric parameters of multiple groups of discontinuities by observation data. The study proposes a mixture distribution model for the 3D geometric attributes of multiple discontinuity sets. The method of estimating the mixture distributions of 3D geometric attributes of all discontinuity sets from the two-dimensional (2D) trace data in a sampling window is established and modified, including discontinuity diameter distribution estimation, volume density estimation, orientation distribution estimation, and distribution mixing. Discontinuities are assumed to be thin disks uniformly distributed in space, and their roughness and aperture are ignored. Discontinuity diameter and orientation follow lognormal distribution and bivariate normal distribution, respectively. Orientation distributions defined on the definitional domain of the geological coordinate system are derived to represent the real frequency distributions of orientation measurements in 3D rock mass space. The mixture distribution model is verified by using Monte Carlo method to generate DFNs with different volume densities, and distribution parameters of orientation and diameter. The maximum error of volume density is <0.5 m−3. The estimated distributions of diameter and orientation are in good agreement with the true distributions, while there are limited symmetry estimation errors of the discontinuity sets with orientation samples covering both sides of the 0° or 180° polar axis. The model is applied to Sejilashan tunnel, and the result indicates that the surrounding rock of two excavation faces have a high probability to be rated as the V classification according to the engineering classification of rock mass in China, which is in good agreement with the reality according to the geological survey and field observation.

Some considerations on the role of joint spacing in rock slope stability analyses using a 3D discrete element approach

Stability assessment of potential wedge failures in rock slopes is usually carried out based on stereographic projection techniques and limit equilibrium analyses. Nevertheless, this methodology presents several limitations (i.e., lack of information on joint mechanical properties, oversimplification of the structure, etc.) that could lead to poorly accurate conclusions. In line with this idea, the main objective of this work is to evaluate the effect of joint spacing in the estimation of the factor of safety of rock slopes affected by potential wedge failures. For this purpose, a 3D numerical distinct element code (3DEC) was selected to carry out a substantial number of simulations in which the factor of safety of two types of slopes, affected by different discontinuity sets, was studied by using the shear strength reduction (SSR) method. Different values of joint spacing, cohesion, and friction angles were considered, combined with two angles of the slope face under study. A discrete fracture network (DFN) model was also implemented, with the aim of benchmarking results with those obtained from the original models. The joint spacing has been found to relevantly affect the values of the factor of safety of the slope, which showed variations of up to 40% in comparison with those obtained from limit equilibrium methods. This work provides an insight to a more realistic interpretation of rock slope analyses against wedge failures, particularly to better estimations of the factor of safety.

Probabilistic analysis of heat extraction performance in enhanced geothermal system based on a DFN-based modeling scheme

With the uncertain fractures involved in the water-rock interaction in the enhanced geothermal system (EGS) taken into account, numerical simulation of heat extraction is challenging to implement, especially in the situation where the EGS is conducted in a complex fracture network. In this study, a discrete fracture network (DFN) modeling procedure based on finite element method (FEM) is proposed to simulate the uncertainty of heat extraction performance in the fractured hot dry rock (HDR), in which the rock matrix and fractures are discretized into triangular elements and zero-thickness elements, respectively. To verify the effectiveness of the established numerical model, the simulation results are compared with the analytical solution and a good agreement is reached. Further, Monte Carlo simulation (MCS) method is adopted to probabilistically study the production performance of EGS considering the randomness of the fracture network. According to the MCS results, heat production performance, i.e., outlet average temperature, EGS lifetime, output thermal power, heat extraction ratio, and levelized cost of energy (LCOE), are statistically analyzed. The study results indicate that the heat extraction performance can be more effectively assessed by the probabilistic analysis, and the uncertainties involved in the heat extraction process cannot be ignored.

Characterization and Modelling of Multiscale Natural Fractures in Shale Reservoirs: A Case Study from a Block in the Southern Sichuan Basin

Natural fractures are vital to the efficiencies of drilling and completion operation. The purpose of this paper is to characterize and model multiscale natural fractures in shale reservoirs. Based on the seismic and log responses, as well as outcrop and core observations, we divide natural fractures into Macro, Meso, and Micro three scales. Macroscale fractures are the faults picked directly in seismic profiles. Also, Mesoscale fractures are the natural fracture corridors analyzed by ant tracking technique in 3D seismic data. Furthermore, Microscale fractures are the fractures observed in imaging logs and cores. The fracture intensity is obtained by the correlation between ant tracking attributes and fracture density in borehole. The fracture aperture, dip, and azimuth are three main parameters, which are recognized by the loggings and cores. Stochastic modelling is applied to factures. We find that faults identified by the ant tracking result are excellent in line with Macroscale faults interpreted directly from seismic data. In addition, Mesoscale fractures are indicated from the ant tracking result, which are in accord with breakpoint in the well and in keeping with tectonic history of the area. Such high consistency indicates the ant tracking result is reliable. Moreover, image logs and cores reveal that it mainly develops high angle natural fractures and the fracture aperture is about 1 mm. The fracture strike includes three sets (NNW-SSE, NE-SW, and NNE-SSW). The distribution of the natural fractures in discrete fracture network (DFN) system is distributed controlled by the ant tracking result. Comparing the histograms of DFN results and fracture characterized by seismic and logging responses, as well as outcrop and core observation, it suggests that the major part of the observed natural fractures is retained into our DFN model.

A Review of Simulation Models of Heat Extraction for a Geothermal Reservoir in an Enhanced Geothermal System

This paper reviews the heat transfer model for geothermal reservoirs, the fracture network in reservoirs, and the numerical model of hydraulic fracturing. The first section reviews the heat transfer models, which contain the single-porosity model, the dual-porosity model, and the multi-porosity model; meanwhile the mathematical equations of the porosity model are summarized. Then, this paper introduces the fracture network model in reservoirs and the numerical method of computational heat transfer. In the second section, on the basis of the conventional fracture theory, the PKN (Perkins–Kern–Nordgren) model and KGD (Khristianovic–Geertsma–De Klerk) model are reviewed. Meanwhile, the DFN (discrete fracture network) model, P3D (pseudo-3D) model, and PL3D (planar 3D) model are reviewed. The results show that the stimulated reservoir volume method has advantages in describing the fracture network. However, stimulated reservoir volume methods need more computational resources than conventional fracture methods. The third section reviews the numerical models of hydraulic fracturing, which contains the finite element method (FEM), the discrete element method (DEM), and the boundary element method (BEM). The comparison of these methods shows that the FEM can reduce the computational resources when calculating the fluid flow, heat transfer and fracture propagations in a reservoir. Thus, a mature model for geothermal reservoirs can be developed by coupling the processes of heat transfer, fluid flow and fracture propagation.

Coal seam gas reservoir characterisation based on high-resolution image logs from vertical and horizontal wells: A case study

Drilling horizontal surface-to-inseam (SIS) wells along low permeability coal seams, e.g. <10 mD, is a crucial technology to increase the drainage area for this type of unconventional reservoir. For the first time in the Bowen Basin, a continuous 1900 m long formation resistivity-based high-resolution image log from a SIS well is used to investigate key coal reservoir characteristics including faults, fractures, coal plies, porosity and permeability. This study presents a workflow from image log interpretation to discrete fracture network (DFN) modelling and then to well performance analysis. The interpreted faults from the image logs were used to analyse the uncertainty of fault interpretation from 3D seismic data. The image log was also used in analysing the variation of fracture distribution vertically and horizontally and compared to similar data obtained from vertical wells. All these data were used to understand the variation in productions from three nearby chevron well groups. Results show that the fracture density decreases with increasing Gamma-ray response from the in-seam section, including the intra-seam partings. Regionally, the orientation of fractures and cleats is parallel, sub-parallel or even perpendicular to the regional maximum stress orientation; however, in the study area, the majority of fractures are generally parallel or near parallel to the maximum stress orientation. This study confirmed the trend that the orientation of fractures/cleats rotated from NE to NW from the study area to the north. Because the maximum stress orientation is NNE, this cleat orientation will yield a decrease in coal permeability from south to north. This can explain at least partially the decrease in well performance from south to north. Based on the modelled fractures and their apertures, the upscaled fracture porosity ranged from 0% to 0.01%, with fracture permeability in the x-direction (easting direction) ranging mainly from 1 mD to 10 mD, and with the ratio of permeability in the x-direction to y-direction (northing direction) ranging from 5 to 20.

Upscaling the Mechanical Properties of a Fractured Rock Mass Using the Lattice-Spring-Based Synthetic Rock Mass (LS-SRM) Modeling Approach—Comparison of Discontinuum, Continuum and Empirical Approaches

A numerical characterization of a fractured rock mass and its mechanical behavior using a discontinuum approach was carried out utilizing lattice-spring-based synthetic rock mass (LS-SRM) models. First, LS-SRM models on a laboratory scale were created to reproduce standard rock mechanical tests on Triassic sandstone samples from a quarry in Germany. Subsequently, the intact rock properties were upscaled to an element volume representative for geotechnical applications, recalibrated and combined with a Discrete Fracture Network (DFN) model. The resulting fractured rock mass properties are compared to predictions from empirical relationships based on rock mass classification schemes and the DFN-Oda-Geomechanics approach. Modeling results reveal a significant reduction in the strength of the fractured rock mass compared to the intact rock, showing a high agreement with empirically calculated values. Results for the deformation modulus reveal a significant reduction induced by the fracture network and a good agreement compared to the results obtained by other approaches. It is shown that the LS-SRM allows analyzing the complex mechanical behavior during failure of rock masses, including crack initiation, propagation and coalescence. The resulting rock mass properties are key parameters for a wide range of geotechnical applications and can be used for large-scale numerical modeling as well.

An optimum solution for coal permeability estimation from mesoscopic scale calibrated stochastic and deterministic discrete fracture network models

The efficiency of a coal bed as a reservoir of methane and CO2 sink primarily depends on its fracture permeability. Conventional techniques of permeability estimation from drilled-core plugs in the laboratory and analytical computations are often different from in-situ measurements by multiple orders. Hence, pre-drill estimation of coal bed permeability remains one of the major challenges for the operators. Researchers usually try to solve this issue by building DFN models based on the direct use of µCT images of millimeters scale coal chips. However, such small-scale DFN models also overestimate permeability by many folds. As a solution, we propose the deployment of calibrated Discrete Fracture Network (DFN) modeling technique based on mesoscale coal images. Deterministic and stochastic DFN models are simulated by replicating thirteen coal specimens collected from the Gondwana coal seams in the Raniganj coalfield of India. Different model outputs such as directional permeabilities, permeability anisotropy, connectivity, etc. are used to distinguish and calibrate these models. The derived average permeabilities from these models closely match with available field measurements. Thus, the proposed technique helps in minimizing the huge gap between the laboratory and well-test measurements. In the process, we derive the threshold sample/model size that would sufficiently address the average cleat permeability at the well-drainage scale. Additionally, sensitivity analysis of key fracture attributes is carried out to quantify their effect on coal permeability.

Estimation of the representative elementary volume of three-dimensional fracture networks based on permeability and trace map analysis: A case study

A three-dimensional (3D) discrete fracture network (DFN) was generated to evaluate the representative elementary volume (REV) of granitic rock masses based on the statistical geometry parameters mapped from outcrop surveys at the Xinchang site, China. Then, extensive numerical simulations were performed to investigate the effect of fracture density on the side length of models of the REV size (LREV). The relationship between LREV of 3D models and the geometric parameters of two-dimensional (2D) cutting planes parallel and perpendicular to the direction of hydraulic gradient variation were analyzed. This relationship was determined by calculating fluid flow through 90 original 3D models, 810 3D submodels, and 270 2D cutting models. Finally, two multivariable regression equations were proposed for predicting the LREV of the 3D fracture network and equivalent permeability of the REV size. The results show that the clustering results obtained from the 3D DFN model are similar to those obtained from field measurements. The values of LREV of the 3D DFN model based on statistical geometry parameters mapped from outcrops are 18.89 m, 19.56 m, and 24.86 m in three flow directions, respectively, which clearly highlights the anisotropic characteristics of fractured rock masses. The LREV decreases with increasing fracture density following an exponential relationship. The multivariable regression function estimates the evolution of the LREV of 3D DFN models with a wide range of fracture densities from 0.53 to 1.33 m/m2. The predicted results are more accurate than those predicted by the fitting function between the fracture density of 2D cutting planes perpendicular to the z-axis and LREV. The proposed model provides a method to approximate the LREV of 3D DFN models and equivalent permeability of the REV size (KREV) using geometric parameters of the 2D cutting planes obtained from trace map analysis of fractured rock masses.

Permeability measurement of the fracture-matrix system with 3D embedded discrete fracture model

In this work, we performed the pulse-decay experiment on a fractured shale core and applied the 3D embedded discrete fracture model (EDFM) to simulate the experiment process. The irregularly rough fracture surface was depicted in detail using a 3D scanner. Then the fracture surface profile was imported in the discrete fracture network model to simulate the flow process in the fracture-matrix system realistically. History matching was efficiently performed to simultaneously obtain porosity and permeability for the matrix and fracture. Gas properties after applying nanopore confinement were obtained by modifying Sutton's viscosity correlation with sensitivity analysis on the pore diameter ranging from 2 to 50 nm. Nanopore confinement results in low matched permeability due to viscosity reduction in the matrix-only porous media. The fracture-matrix system complicates the interpretation of nanopore confinement impact on flow behavior in shale reservoirs. Different scenarios of the highly fractured and matrix-dominated systems were discussed in detail by constructing the digital twin model.

CO2 and heat energy transport by enhanced fracture permeability in the Monterotondo Marittimo-Sasso Pisano transfer fault system (Larderello Geothermal Field, Italy)

Carbon dioxide is one of the most important gasses naturally released from geothermal systems. Establishing the processes and pathways that regulate the CO2 diffuse degassing can provide valuable information for exploration and exploitation purposes of geothermal reservoirs. In this work, a high-resolution CO2 flux (with records up to 2927 g m–2 d–1) and soil temperature (with records up to 98.8 °C) survey was carried out along with detailed fracture parameters measurements in a selected area of the Monterotondo Marittimo-Sasso Pisano transfer fault (Larderello geothermal system, Tuscany, Italy). The main aim was to define the behavior of diffuse CO2 through the fault system and investigate how the soil CO2 flux and steam change with respect to the architecture of the fault damage zone (i.e., volumetric fracture intensity, permeability, and persistence of the fractures). The presence of multiple populations of CO2 flux suggested that three different transport mechanisms control soil degassing: i) purely diffusive, ii) mixed diffusive-advective, and iii) purely advective, characterized by efflux values of <20, between 20 and 300 and >300 g m–2 d–1, respectively. The spatial distribution of these fluxes well agrees with the fracture distribution and features of the Jurassic radiolarite (Diaspri Fm) dissected by NNE-striking faults. Areas with high CO2 emissions are indeed able to reveal major upflow areas from deep reservoirs through deep-reaching permeable fault zones. The interaction between pre-existing fractures and fracture-related fault-zone locally enhances the secondary rock permeability as highlighted by the correlation between Discrete Fracture Network (DFN) modeling and advective flux. Eventually, by normalizing the CO2 output to the fault strip (1350 m2), a release of CO2 equal to ∼155 t d–1 km–2 was estimated.

Hydrothermal Numerical Simulation of Injection Operations at United Downs, Cornwall, UK

The United Downs Deep Geothermal Project (UDDGP) is designed to utilize a presumably permeable steep dipping fault damage zone (constituting the hydrothermal reservoir in a very low permeability granitic host rock) for fluid circulation and heat extraction between an injection well at 2.2 km depth (UD−2) and a production well at 5 km depth (UD−1). Soft hydraulic stimulation was performed to increase the permeability of the reservoir. Numerical simulations are performed to analyze the hydraulic stimulation results and evaluate the increase in permeability of the reservoir. Experimental and field data are used to characterize the initial reservoir static model. The reservoir is highly fractured, and two distinct fracture networks constitute the equivalent porous matrix and fault zone, respectively. Based on experimental and field data, stochastic discrete fracture networks (DFN) are developed to mimic the reservoir permeability behavior. Due to the large number of fractures involved in the stochastic model, equivalent permeability fields are calculated to create a model which is computationally feasible. Hydraulic test and stimulation data from UD−1 are used to modify the equivalent permeability field based on the observed difference between the real fractured reservoir and the stochastic DFN model. Additional hydraulic test and stimulation data from UD−2 are used to validate this modified permeability. Results reveal that the equivalent permeability field model derived from observations made in UD−1 is a good representation of the actual overall reservoir permeability, and it is useful for future studies. The numerical simulation results show the amount of permeability changes due to the soft hydraulic stimulation operation. Based on the validated permeability field, different flow rate scenarios of the petrothermal doublet and their respective pressure evolution are examined. Higher flow rates have a strong impact on the pressure evolution. Simulations are performed in the acidized enhanced permeability region to make a connection between the ongoing laboratory works on the acid injection and field response to the possible acidizing stimulation.