Porous Rock Mass Research Articles

A Fully Coupled Discontinuous Deformation Analysis Model for Simulating Hydromechanical Processes in Fractured Porous Media

Numerical simulations play a key role in the optimization of fracturing operation designs for unconventional reservoirs. Because of the presence of numerous natural discontinuities and pores, the rock masses of reservoirs can be regarded as fractured porous media. In this paper, a fully coupled discontinuous deformation analysis model is newly developed to simulate the hydromechanical processes in fractured and porous media. The coupling of fracture seepage, pore seepage, and fracture network propagation is realized under the framework of DDA. The developed model is verified with several examples. Then, the developed DDA model is applied to simulate the hydraulic fracturing processes in fractured porous rock masses, and the effects of rock mass permeability on fracturing are investigated. Our findings suggest that high rock permeability may inhibit the stimulation of fracture networks, while increasing the viscosity of fracturing fluids can enhance the fracturing efficiency. This study provides a valuable numerical tool for simulating hydromechanical processes in fractured and porous media and can be used to analyze various geo-mechanical problems related to fluid interactions.

The 15 January 2022 Hunga (Tonga) eruption: A gas-driven climactic explosion

An extraordinarily powerful, explosive eruption occurred from Hunga volcano in the Tonga island arc on 15 January 2022 and generated an eruption column 58 km high. The explosive eruption also generated atmospheric gravity waves, extreme runup tsunamis and quite unusual and destructive meteotsunamis. Together these place this VEI 6 eruption as, globally, one of the largest of the past 300 years.Based on the oceanic context of Hunga volcano, it has previously been assumed that the eruption was phreatomagmatic through a fuel-coolant Surtseyan-type interaction, but this is not supported by satellite imagery. Similarly, it has been suggested that a caldera-collapse was the eruption trigger, but this is not supported by bathymetric data or the seismicity recorded during the eruption. Here we develop a new model based on the observed energetics and time sequence of the eruption integrated with understanding of the internal structure of active volcanoes and their characteristic high flux discharges of volcanic gas.It has been shown elsewhere that magma-derived reactive gases (H2O, CO2, SO2, HCl, etc) aggressively alter the volcanic rocks in the core of a volcano leading to self-sealing of gas flow to the surface and consequent changes to deviatoric stress in the structure. Common minerals developed by these reactions include anhydrite (CaSO4), sulphides and silica (quartz), all of which have been recorded in volcanic ejecta including at Hunga.We here develop a first order numerical model that quantifies how the free discharge of such gas to the surface may progressively become choked by these sealing reactions leading to increased internal gas pressure. Hydraulic fracture of the seal occurs when the transmitted pressure of the compressed magmatic gas beneath the seal increases to a value greater than the lithostatic pressure plus the tensile strength of the sealed rock. This initiates the explosive release of compressed gas whose high-power discharge progressively develops and enlarges a crater. At the same time, the explosion feeds upon itself by generating larger pressure gradients in the pressurized gas within the fractured porous rock mass of the core of the volcano. Excavation of the crater may intersect high level intrusions and produce the pumice rafts that were observed after the eruption. The eruption itself diminished in intensity as the gas pressure in the reservoir declined.At Hunga, the eruption excavated an 850 m deep, 2-3 km diameter steep-walled crater. This volume may be assumed to approximate the volume of fractured porous rock (the control volume of the eruption) whose trapped gas was mined by the eruption until surrounding gas pressure was depleted. Our numerical model shows that the calculated potential energy of the trapped compressed gas matches the independent observations of the scale of the eruption. Sensor data have since shown that gas bubble flares continued for at least 6 months after the eruption indicating continued depletion of the gas reservoir of rocks surrounding the new crater. The systems-based, gas-driven model for the Hunga climactic eruption developed here also applies to Plinean-type eruptions on subaerial arc volcanoes such as at Pinatubo (Philippines) 1991.

An Extended Hydro-Mechanical Coupling Model Based on Smoothed Particle Hydrodynamics for Simulating Crack Propagation in Rocks under Hydraulic and Compressive Loads

A seepage model based on smoothed particle hydrodynamics (SPH) was developed for the seepage simulation of pore water in porous rock mass media. Then, the effectiveness of the seepage model was proved by a two-dimensional seepage benchmark example. Under the framework of SPH based on the total Lagrangian formula, an extended hydro-mechanical coupling model (EHM-TLF-SPH) was proposed to simulate the crack propagation and coalescence process of rock samples with prefabricated flaws under hydraulic and compressive loads. In the SPH program, the Lagrangian kernel was used to approximate the equations of motion of particles. Then, the influence of flaw water pressure on crack propagation and coalescence models of rock samples with single or two parallel prefabricated flaws was studied by two numerical examples. The simulation results agreed well with the test results, verifying the validity and accuracy of the EHM-TLF-SPH model. The results showed that with the increase in flaw water pressure, the crack initiation angle and stress of the wing crack decreased gradually. The crack initiation location of the wing crack moved to the prefabricated flaw tip, while the crack initiation location of the shear crack was far away from the prefabricated flaw tip. In addition, the influence of the permeability coefficient and flaw water pressure on the osmotic pressure was also investigated, which revealed the fracturing mechanism of hydraulic cracking engineering.

Dynamic propagation behaviors of hydraulic fracture networks considering hydro-mechanical coupling effects in tight oil and gas reservoirs: A multi-thread parallel computation method

Hydrofracturing is a remarkable technology for unconventional oil and gas exploitation in tight reservoirs, in which the fluid-driven propagation of hydraulic fractures in a porous medium rock reservoir is an extremely complex physical process, for which there are great challenges in numerical simulation, such as multiphysical field coupling, multiple field evolution on engineering-scale reservoirs, and multi-scale fracture propagation processes. This complex physical process renders it difficult to obtain reliable solutions rapidly and efficiently. Understanding the behavior and mechanism of hydrofracturing is complicated. A multi-thread parallel computation scheme for solid and fluid analysis was developed to improve computational efficiency in large-scale models and multi-physical field coupling (hydro-mechanical) problems, and fracture criteria via a dual bilinear cohesive zone model was introduced. Considering the hydro-mechanical coupling and leak-off effects, the combined finite element-discrete element-finite volume approach was introduced and implemented successfully, and a multi-thread parallel computation method and global procedure was proposed. This study provides valuable results for parallel computation of dynamic fluid-driven propagation of hydraulic fractures in porous elastic rock mass and lays the foundation for further development of efficient and reliable simulations for dynamic multiscale complex fractures under multiphysical field coupling.

Analysis of Coupled Seepage and Temperature Fields of Fractured Porous Rock Mass under Brine–Liquid Nitrogen Freezing

The existence of fracture flow has an undesirable effect on the creation of the frozen wall. Brine and liquid nitrogen combined freezing technology can ensure the safety of freezing engineering, reduce the construction period and save cost. Considering the permeability of the rock matrix, fluid exchange and Darcy–Stokes coupling effect between the rock matrix and fracture, a thermo-hydraulic model of the fractured porous rock mass under water seepage is herein established. The interfacial seepage field characteristics of fractured rock mass under different fluid flow models and interface conditions are compared. The numerical simulations of the initial brine freezing and liquid nitrogen reinforcement freezing are carried out. The results show that the overall permeability of fractured rock mass computed by free flow considering the Darcy–Stokes effect is greater than that computed by the Cubic law. The limit seepage velocity of the intact rock mass in brine freezing is 2.5 m/d, and that of fractured rock mass decreases to 1 m/d. The fracture aperture and groundwater seepage velocity are directly proportional to the closure time of the frozen wall. Liquid nitrogen freezing can seal water quickly and shorten the closure time of the frozen wall when the seepage velocity of the fractured rock mass is greater than the limit seepage velocity, and the rapid cooling of the upstream region plays an important role in the formation of the frozen wall in fractured rock mass.

Finite element for Richards’ equation in porous rocks

The interaction processes of water movement and environment have several aspects important to engineering and geology, which motivate a large amount of research all over the world. As computers with larger capacity become available and affordable, several simplifications used in the past have become meaningless, and nonlinear complex phenomena like unsaturated flow can be processed in affordable machines. The reduction of water flow capacity of filters and drains in embankments due to unsaturated zones, and the environmental problems of remediation of contaminated zones in porous rock masses at the vadose zone are examples of special problems in which advanced modeling should be applied to avoid misleading conclusions or dreadful consequences. The unsaturated flux can also be important in understanding the influence of humidity in the bowing phenomenon in calcitic marbles, since laboratory tests have shown different evolutions of this pathology in partially saturated samples and in completely wet or dry samples. Moreover, this formulation also has important application in building envelope modeling, like in evaporative roofs which enhance thermal comfort in hot weather. To increase the understanding of unsaturated problems, this paper presents a finite element formulation for applications in unsaturated flow in stiff porous media, like sedimentary rocks or concrete. The formulation is based on Richards’ equation for one-dimensional flows and it includes a linear interpolation of suction, as well as an arbitrary implicit parameter.

Permeability Characteristics of Porous Rock with Conduits under Stokes–Brinkman–Darcy Coupling Model

Conduits, the main channels within a rock mass, can significantly affect the permeability characteristics of rock masses. This paper presents a Stokes–Brinkman–Darcy coupling model to study the permeability characteristics of porous rock masses with conduits. In this model, the Navier–Stokes equation and the Brinkman-extended Darcy equation are adopted to describe the fluid motions in the conduit and porous rock matrix, respectively. The analytical solutions of both the velocity distribution and the equivalent permeability coefficient, which is the average permeability for the complete rock volume, are derived under the stress jump boundary condition for a rock mass with a conduit. The results of the sensitivity analysis show that the equivalent permeability coefficient of the rock mass with a conduit is positively correlated with the permeability of the rock mass, the relative aperture of the conduit, and the porosity and is negatively correlated with the stress jump coefficient; the aperture of the conduit plays a dominant role in the equivalent permeability coefficient. In addition, the analytical solution of the equivalent permeability coefficient is verified by a seepage test.

INVESTIGATION OF THE EFFECT OF THE DISCONTINUITY DIRECTION ON FLUID FLOW IN POROUS ROCK MASSES ON A LARGE-SCALE USING HYBRID FVM-DFN AND STREAMLINE SIMULATION

Understanding the fluid behaviour in rock masses is of great importance in various rock mass-related engineering projects, such as seepage in tunnels, geothermal reservoirs, and hazardous waste disposal. Different approaches have been implemented to study the flow pattern in fractured porous rock masses. Laboratory experiments can provide good information regarding this issue, but high expenses aside, they are time-consuming and suffer the lack of ability to study field scale mediums. Numerical methods are beneficial in simulating such mediums with the Discrete Fracture Network (DFN) method in terms of costs and time as they offer sufficient flexibility and creativity. In this paper, a Matlab code was extended to study the flow regime in a Dual Permeability Media (DPM) with two point sources in the right and left side of the model as an injector and a producer well, respectively. A high permeability discontinuity with different angles was embedded in a very low-permeability limestone matrix. Pressure equations were solved implicitly with a two-point flux approximation scheme of the Finite Volume Method (FVM). Streamlines were traced in the medium and used to analyse the model’s hydraulic behaviour with the aid of Time Of Flight (TOF) for each point. The results show that the FVM-DFN hybrid method can be used as a fast method for fluid flow in DPM with the aid of streamline simulation to study the fluid flow in a large model with discontinuity.

Evaluation of Hydraulic Fracturing and Overcoring Methods to Determine and Compare the In Situ Stress Parameters in Porous Rock Mass

There are various methods to determine in situ stress parameters, but each method is having its own advantages and limitations. Among the methods available, the hydraulic fracturing method is the most commonly adopted procedure for in situ stress measurements because of its simplicity, reliability and economic viability. But, the limitations with this technique are that some assumptions implicit for this method such as that the rock mass is continuous and elastic. Moreover, the reliability and validity of this method becomes questionable in porous rocks that may encounter in underground tunnels. These limitations are experienced ever since the introduction of this method, especially in porous rocks. In general, the recognition of fracture initiation in the hydraulic fracturing test has not proved difficult. The relatively slow rates of pressurisation have ensured that when fracture initiation occurs, the sudden increase in volume has led to a marked drop in pressure in the porous section, which is easily recognised from the pressure record (Sjoberg et al. in Int J Rock Mech 40:999–1010, 2003). But the drop-in pressure in a porous rock is far more difficult to recognise. This is since pressure cannot be developed if the rate of leakage in the formation is equal to or higher than the flow rate applied for fracture initiation. This problem of non-generation of water pressure can be tackled by use of a high viscosity fluid. Stress measurement were conducted by hydraulic fracturing method by using high viscous liquid in porous rocks. The stresses evaluated by this technique were correlated with stress measured by overcoring method. The stress measured by overcoring method was used as bench-mark as this method does not suffer from the presence of porosity of the rock. This new technique will be helpful in conducting the stress measurements in porous rocks, which will be highly beneficial to both mining and hydropower related excavation.

Estimation of equivalent permeability tensor for fractured porous rock masses using a coupled RPIM-FEM method

PurposeUnderstanding the fluid flow through rock masses, which commonly consist of rock matrix and fractures, is a fundamental issue in many application areas of rock engineering. As the equivalent porous medium approach is the dominant approach for engineering applications, it is of great significance to estimate the equivalent permeability tensor of rock masses. This study aims to develop a novel numerical approach to estimate the equivalent permeability tensor for fractured porous rock masses.Design/methodology/approachThe radial point interpolation method (RPIM) and finite element method (FEM) are coupled to simulate the seepage flow in fractured porous rock masses. The rock matrix is modeled by the RPIM, and the fractures are modeled explicitly by the FEM. A procedure for numerical experiments is then designed to determinate the equivalent permeability tensor directly on the basis of Darcy’s law.FindingsThe coupled RPIM-FEM method is a reliable numerical method to analyze the seepage flow in fractured porous rock masses, which can consider simultaneously the influences of fractures and rock matrix. As the meshes of rock matrix and fracture network are generated separately without considering the topology relationship between them, the mesh generation process can be greatly facilitated. Using the proposed procedure for numerical experiments, which is designed directly on the basis of Darcy’s law, the representative elementary volume and equivalent permeability tensor of fractured porous rock masses can be identified conveniently.Originality/valueA novel numerical approach to estimate the equivalent permeability tensor for fractured porous rock masses is proposed. In the approach, the RPIM and FEM are coupled to simulate the seepage flow in fractured porous rock masses, and then a numerical experiment procedure directly based on Darcy’s law is introduced to estimate the equivalent permeability tensor.

Experimental Investigation on the Law of Grout Diffusion in Fractured Porous Rock Mass and Its Application

Because of the limitation of mining techniques and economic conditions, large amounts of residual coal resources have been left in underground coal mines around the world. Currently, with mining technology gradually developing, residual coal can possibly be remined. However, when residual coal is remined, caving areas might form, which can seriously affect the safety of coal mining. Hence, grouting technology is put forward as one of the most effective technologies to solve this problem. To study the grouting diffusion in fractured rock mass, this paper developed a visualization platform of grouting diffusion and a three-dimensional grouting experimental system that can monitor the grout diffusion range, diffusion time and grout pressure; then, a grouting experiment is conducted based on this system. After that, the pattern of the grouting pressure variation, grout flow and grout diffusion surface are analyzed. The relationship among some factors, such as the grouting diffusion radius, compressive strength of the grouted gravel, porosity, water-cement ratio, grouting pressure, grouting time, permeability coefficient and level of grout, is quantitatively analyzed by using MATLAB. The study results show that the flow pattern of the grout in fractured porous rock mass has a parabolic shape from the grouting hole to the bottom. The lower the level is, the larger the diffusion range of the grout is. The grouting pressure has the greatest influence on the grouting diffusion radius, followed by the grouting horizon and water-cement ratio. The grouting permeability coefficient has the least influence on the grouting diffusion radius. The grout water-cement ratio has the greatest influence on the strength of the grouted gravel, followed by the grouting permeability. The grouting pressure coefficient has the least amount of influence on the grouting diffusion radius. According to the results, the grouting parameters are designed, and a layered progressive grouting method is proposed. Finally, borehole observation and a core mechanical property test are conducted to verify the application effect. This grouting technology can contribute to the redevelopment and efficient utilization of wasted underground coal resources.

New Experimental Equipment Recreating Geo-Reservoir Conditions in Large, Fractured, Porous Samples to Investigate Coupled Thermal, Hydraulic and Polyaxial Stress Processes

Use of the subsurface for energy resources (enhanced geothermal systems, conventional and unconventional hydrocarbons), or for storage of waste (CO2, radioactive), requires the prediction of how fluids and the fractured porous rock mass interact. The GREAT cell (Geo-Reservoir Experimental Analogue Technology) is designed to recreate subsurface conditions in the laboratory to a depth of 3.5 km on 200 mm diameter rock samples containing fracture networks, thereby enabling these predictions to be validated. The cell represents an important new development in experimental technology, uniquely creating a truly polyaxial rotatable stress field, facilitating fluid flow through samples, and employing state of the art fibre optic strain sensing, capable of thousands of detailed measurements per hour. The cell’s mechanical and hydraulic operation is demonstrated by applying multiple continuous orientations of principal stress to a homogeneous benchmark sample, and to a fractured sample with a dipole borehole fluid fracture flow experiment, with backpressure. Sample strain for multiple stress orientations is compared to numerical simulations validating the operation of the cell. Fracture permeability as a function of the direction and magnitude of the stress field is presented. Such experiments were not possible to date using current state of the art geotechnical equipment.

Microseismic monitoring and stability analysis of the right bank slope at Dagangshan hydropower station after the initial impoundment

The right bank slope of the Dagangshan hydropower station in China has suffered from reservoir water pressure and seepage effects since the initial impoundment process. This paper describes the microseismic monitoring system and impoundment process of the Dagangshan hydropower station. Microseismic activity was detected in the right bank slope after the initial impoundment by the microseismic monitoring system. The spatiotemporal characteristics of the microseismic events and the piezometer monitoring results are used to confirm the conductive seepage pathways, the porous rock mass and the water recharge source in the right bank slope. Based on the geological conditions of the porous rock mass identified by microseismic and piezometer monitoring, a simplified two-dimensional (2D) finite element model is developed. A 2D finite element analysis is then applied to explore the damage mechanisms of local regions of the right bank slope after the initial impoundment. In addition, the load/unload response ratio theory is used to predict the failure of the slope based on the seismic source information obtained by the microseismic monitoring system. According to the actual geological conditions of the right bank slope, a simplified 3D numerical model is developed for the stability analysis of the right bank slope after the initial impoundment. Based on the porous rock mass identified by the microseismic monitoring, the effective stress is considered in the stability analysis. The safety factor of the right bank slope including the effective stress is 1.645, which indicates that the slope was stable after the initial impoundment. The microseismic monitoring technique can record disturbances caused by seepage effects in deep rock masses after the initial impoundment process in real time.

Effect of buoyancy-driven natural convection in a rock-pit mine air preconditioning system acting as a large-scale thermal energy storage mass

Underground mining is among the most energy-intensive industries and ventilation comprises a significant portion of the energy demands of this important industry. Using the vast volume of broken rock, left in a decommissioned mine pit, as a thermal energy storage mass has enormous potential to lower ventilation-related energy costs in deep underground mines. This approach facilitates moderating seasonal air temperature variations. Seasonal thermal energy storage is a cost-effective solution to improve cooling and heating process efficiencies, thereby reducing associated costs. Temperature gradients observed in the proposed storage system suggest the presence of a natural convection heat transfer mechanism that is buoyancy-driven. The effect of natural convection and a variety of heat transfer mechanisms were modeled and simulation results and field-data measurements were compared. The conjugate heat transfer and fluid flow model that was developed considers the porous rock mass in the rock-pit along with the air (i.e. fluid) blanketing the top surface. The effects of rock size, permeability and porosity were studied. It was observed that, for the range of porosities (from 0.45 to 0.20), these parameters have a small effect on the outlet air temperature and the performance of thermal storage phenomenon. The novel model compares forced (from ventilation fan) and natural (result of buoyancy) convection. Further, it incorporates the effect of design factors, such as air trench positions and flow rate of ventilated air, on energy savings.

Analysis of pore scale fluid migration in a porous medium- application to coal rock seam

PurposeThe purpose of this study is to digitize the porous structure and reconstruct the geometry of the rock by using the image processing software photoshop (PS) and ant colony algorithm coded with compiler Fortran PowerStation (fps) 4.0 based on the microscopic image of a typical rock mass.Design/methodology/approachThe digital model of the microstructure of the porous coal rock was obtained, and imported into the numerical simulation software to build the finite element model of microstructure of the porous coal rock. Creeping flow equations were used to describe the fluid flow in the porous rock.FindingsThe simulation results indicate that the method utilized for reconstructing the microstructure of the porous coal rock proposed in this work is effective. The results demonstrate that the transport of fluid in a porous medium is significantly influenced by the geometric structure of the pore and that the heterogeneous porous structure would result in an irregular flow of the fluid.Research limitations/implicationsThe authors did not experience a limitation.Practical implicationsThe existence of the pores with dead ends would hinder the fluid to flow through the coal rock and reduce the efficiency of extracting fluid from the porous coal rock. It is also shown that the fluid first enters the large pores and subsequently into the small pore spaces.Social implicationsThe paper provides important and useful results for several industries.Originality valueImage processing technology has been utilized to incorporate the micro image of the porous coal rock mass, based on the characteristics of pixels of the micro image. The ant colony algorithm was used to map out the boundary of the rock matrix and the pore space. A FORTRAN code was prepared to read the micro image, to transform the bmp image into a binary format, which contains only two values. The digital image was obtained after analyzing the image features. The geometric structure of the coal rock pore was then constructed. The flow process for the micro fluid in the pore structure was illustrated and the physical process of the pore scale fluid migration in the porous coal seam was analyzed.

A Conjugate Natural Convection Model for Large Scale Seasonal Thermal Energy Storage Units: Application in Mine Ventilation

Ventilation plays an important role in energy demands of mining industry. Using the immense volume of waste rock as thermal energy storage mass to shave seasonal air temperature oscillations is a genuine and practical solution to save on ventilation energy demands. The extensive size and considerable temperature gradients exhibited in such systems point to the importance of buoyancy-driven natural convection heat transfer mechanism. The present study investigates the effect of natural convection and identifies the order of significance of various heat transfer mechanisms. A Conjugate heat transfer and fluid flow model is proposed which includes both the porous rock mass placed in the rock-pit and the air positioned above it. The results of the model are compared and validated against the existing Non-Conjugate models. Using this novel model, significance of forced convection, driven by ventilation fan, and natural convection, driven by buoyancy, are compared. The model is also used to study how design parameters, such as position of intake air trenches and flow rate of fresh air, will affect energy savings.

A numerical manifold method model for analyzing fully coupled hydro-mechanical processes in porous rock masses with discrete fractures

In this study, a numerical manifold method (NMM) model was developed for fully coupled analysis of hydro-mechanical (HM) processes in porous rock masses with discrete fractures. Using an NMM two-cover-mesh system of mathematical and physical covers, fractures are conveniently discretized by dividing the mathematical cover along fracture traces to physical cover, resulting in a discontinuous model on a non-conforming mesh. In this model, discrete fracture deformation (e.g. open and slip) and fracture fluid flow within a permeable and deformable porous rock matrix are rigorously considered. For porous rock, direct pore-volume coupling was modeled based on an energy-work scheme. For mechanical analysis of fractures, a fracture constitutive model for mechanically open states was introduced. For fluid flow in fractures, both along-fracture and normal-to-fracture fluid flow are modeled without introducing additional degrees of freedom. When the mechanical aperture of a fracture is changing, its hydraulic aperture and hydraulic conductivity is updated. At the same time, under the effect of coupled deformation and fluid flow, the contact state may dynamically change, and the corresponding contact constraint is updated each time step. Therefore, indirect coupling is realized under stringent considerations of coupled HM effects and fracture constitutive behavior transfer dynamically. To verify the new model, examples involving deformable porous media containing a single and two sets of fractures were designed, showing good accuracy. Last, the model was applied to analyze coupled HM behavior of fractured porous rock domains with complex fracture networks under effects of loading and injection.

Fully coupled hydro-mechanical numerical manifold modeling of porous rock with dominant fractures

Coupled hydro-mechanical (HM) processes are significant in geological engineering such as oil and gas extraction, geothermal energy, nuclear waste disposal and for the safety assessment of dam foundations and rock slopes, where the geological media usually consist of fractured rock masses. In this study, we developed a model for the analysis of coupled hydro-mechanical processes in porous rock containing dominant fractures, by using the numerical manifold method (NMM). In the current model, the fractures are regarded as different material domains from surrounding rock, i.e., finite-thickness fracture zones as porous media. Compared with the rock matrix, these fractured porous media are characterized with nonlinear behavior of hydraulic and mechanical properties, involving not only direct (poroelastic) coupling but also indirect (property change) coupling. By combining the potential energy associated with mechanical responses, fluid flow and solid–fluid interactions, a new formulation for direct HM coupling in porous media is established. For indirect coupling associated with fracture opening/closure, we developed a new approach implicitly considering the nonlinear properties by directly assembling the corresponding strain energy. Compared with traditional methods with approximation of the nonlinear constitutive equations, this new formulation achieves a more accurate representation of the nonlinear behavior. We implemented the new model for coupled HM analysis in NMM, which has fixed mathematical grid and accurate integration, and developed a new computer code. We tested the code for direct coupling on two classical poroelastic problems with coarse mesh and compared the results with the analytical solutions, achieving excellent agreement, respectively. Finally, we tested for indirect coupling on models with a single dominant fracture and obtained reasonable results. The current poroelastic NNM model with a continuous finite-thickness fracture zone will be further developed considering thin fractures in a discontinuous approach for a comprehensive model for HM analysis in fractured porous rock masses.

Pipe network model for unconfined seepage analysis in fractured rock masses

An efficient unconfined seepage analysis method is developed for analysing fluid flow in fractured rock mass. The proposed method discretises both porous rock matrix (a low permeability medium) and discrete fracture networks (a high permeability medium) as permeable pipe networks. A finite volume based approach is used to determine the equivalent flow parameters in the undirected pipes for the porous rock matrix such that seepage analysis in a fractured rock mass is carried out in a consistent mathematical framework which gives excellent computational accuracy and efficiency. Using an auxiliary boundary method, pipe networks in the vicinity of the phreatic surface can be adapted in unconfined seepage problems. Validated by finding the phreatic surface in dam structures, it has been found that the fracture orientation, hydraulic aperture, fracture set number and fracture distribution can affect the phreatic surface significantly in a fractured rock mass. The proposed method is applied to investigate the effectiveness of the water curtain system in a water-sealed underground crude oil storage cavern. Grouting effect can also be conveniently investigated by the unified pipe network system. Results show that the proposed method is robust in seepage analysis for both fractured and porous rock masses.

A 2D, fully-coupled, hydro-mechanical, FDEM formulation for modelling fracturing processes in discontinuous, porous rock masses

This paper presents a new, fully-coupled, hydro-mechanical (HM) formulation for a finite-discrete element method computer code. In the newly-developed, hydraulic solver, fluid flow is assumed to occur through the same triangular mesh used for the mechanical calculations. The flow of a viscous, compressible fluid is explicitly solved based on a cubic law approximation. The implementation is verified against closed-form solutions for several flow problems. The approach is then applied to a field-scale simulation of fluid injection in a jointed, porous rock mass. Results show that the proposed method can be used to obtain unique geomechanical insights into coupled HM phenomena.