Hydro-mechanical Coupling Effect Research Articles

Adaptive finite element-discrete element method for numerical analysis of the multistage hydrofracturing of horizontal wells in tight reservoirs considering pre-existing fractures, hydromechanical coupling, and leak-off effects

Multistage fracturing is widely used to improve gas production in tight hydrocarbon reservoirs. As a sequential process that interacts with pre-existing fractures, multistage fracturing must be properly addressed in numerical simulations, which are important in evaluating and optimising fracturing technology. The challenges of accurately representing the complex fracture structures and physical properties of naturally fractured reservoirs cause that the interaction between hydraulic and pre-existing fractures has not yet been resolved satisfactorily. In this study, we adopt an adaptive finite element-discrete element method to simulate the multistage fracturing of a naturally fractured reservoir by improving the mesh auto-refinement and identification of multiple fracture propagation. The numerical model considers interactions among hydraulic fractures, pre-existing fractures, and microscale pores, and integrates the nonlinear Carter leak-off criterion to describe fluid leak-off and hydromechanical coupling effects during multistage fracturing. The proppant transport equation for idealised parallel plate flow in fractures is introduced, and Darcy's law is adopted to analyse the seepage flow in the fracture network and determine the gas recovery. The fracture network and consequent fluid flow induced by the hydrofracturing of unfractured and naturally fractured models are compared to assess the influence of pre-existing fractures on multistage fracturing behaviour and gas production.

A multiscale multi-permeability poroplasticity model linked by recursive homogenizations and deep learning

Many geological materials, such as shale, mudstone, carbonate rock, limestone and rock salt are multi-porosity porous media in which pores of different scales may co-exist in the host matrix. When fractures propagate in these multi-porosity materials, these pores may enlarge and coalesce and therefore change the magnitude and the principal directions of the effective permeability tensors. The pore-fluid inside the cracks and the pores of host matrix may interact and exchange fluid mass, but the difference in hydraulic properties of these pores often means that a single homogenized effective permeability tensor field is insufficient to characterize the evolving hydraulic properties of these materials at smaller time scale. Furthermore, the complexity of the hydro-mechanical coupling process and the induced mechanical and hydraulic anisotropy originated from the micro-fracture and plasticity at grain scale also makes it difficult to propose, implement and validate separated macroscopic constitutive laws for numerical simulations. This article presents a hybrid data-driven method designed to capture the multiscale hydro-mechanical coupling effect of porous media with pores of various different sizes. At each scale, data-driven models generated from supervised machine learning are hybridized with classical constitutive laws in a directed graph that represents the numerical models. By using sub-scale simulations to generate database to train material models, an offline homogenization procedure is used to replace the up-scaling procedure to generate cohesive laws for localized physical discontinuities at both grain and specimen scales. Through a proper homogenization procedure that preserves spatial length scales, the proposed method enables field-scale simulations to gather insights from meso-scale and grain-scale micro-structural attributes. This method is proven to be much more computationally efficient than the classical DEM–FEM or FEM2 approach while at the same time more robust and flexible than the classical surrogate modeling approach. Due to the usage of bridging-scale technique, the proposed model may provide multiple opportunities to incorporate different types of simulations and experimental data across different length scales for machine learning. Numerical issues will also be discussed.

Effect of hydro mechanical coupling on natural fracture network formation in sedimentary basins

Effect of hydro mechanical coupling on natural fracture network formation in sedimentary basins

Numerical Study on the Evolution of Mesoscopic Properties and Permeability in Sandstone Under Hydromechanical Coupling Conditions Involving Industrial Internet of Things

In an attempt to study microcracks development, coordination number, volumetric strain, and permeability evolution of sandstone, the hydromechanical coupling mesoscopic simulation was carried out using Particle Flow Code 2D software based on experimental results of permeability evolution in the complete stress–strain process obtained via triaxial compression tests. A numerical model conforming to the laboratory experiments was established, and the critical mesoscopic mechanism was analyzed. The studied results have included big data as one of the examples for the application of Industrial Internet of Things. In particular, the following were investigated in details in this paper: the stress–strain curves, the distribution of pore pressure, the characteristics of crack growth, the coordination number, the volumetric strain, and the evolution characteristics of permeability under hydromechanical coupling conditions. A discussion is also provided, based on the numerical simulation and analysis of hydromechanical coupling effect, on the synergistic action among changes of mesoscopic structure, accumulated damage, and permeability evolution of the sandstone under hydro-mechanical coupling.

Capturing the two‐way hydromechanical coupling effect on fluid‐driven fracture in a dual‐graph lattice beam model

SummaryFluid‐driven fractures of brittle rock is simulated via a dual‐graph lattice model. The new discrete hydromechanical model incorporates a two‐way coupling mechanism between the discrete element model and the flow network. By adopting an operator‐split algorithm, the coupling model is able to replicate the transient poroelasticity coupling mechanism and the resultant Mandel‐Cryer hydromechanical coupling effect in a discrete mechanics framework. As crack propagation, coalescence and branching are all path‐dependent and irreversible processes, capturing this transient coupling effect is important for capturing the essence of the fluid‐driven fracture in simulations. Injection simulations indicate that the onset and propagation of fractures is highly sensitive to the ratio between the injection rate and the effective permeability. Furthermore, we show that in a permeable rock, the borehole breakdown pressure, the pressure at which fractures start to grow from the borehole, depends on both the given ratio between injection rate and permeability and the Biot coefficient.

The Unsaturated Hydromechanical Coupling Model of Rock Slope Considering Rainfall Infiltration Using DDA

Water flow and hydromechanical coupling process in fractured rocks is more different from that in general porous media because of heterogeneous spatial fractures and possible fracture-dominated flow; a saturated-unsaturated hydromechanical coupling model using a discontinuous deformation analysis (DDA) similar to FEM and DEM was employed to analyze water movement in saturated-unsaturated deformed rocks, in which the Van-Genuchten model differently treated the rock and fractures permeable properties to describe the constitutive relationships. The calibrating results for the dam foundation indicated the validation and feasibility of the proposed model and are also in good agreement with the calculations based on DEM still demonstrating its superiority. And then, the rainfall infiltration in a reservoir rock slope was detailedly investigated to describe the water pressure on the fault surface and inside the rocks, displacement, and stress distribution under hydromechanical coupling conditions and uncoupling conditions. It was observed that greater rainfall intensity and longer rainfall time resulted in lower stability of the rock slope, and larger difference was very obvious between the hydromechanical coupling condition and uncoupling condition, demonstrating that rainfall intensity, rainfall time, and hydromechanical coupling effect had great influence on the saturated-unsaturated water flow behavior and mechanical response of the fractured rock slopes.

The Role of Surface Infiltration in Hydromechanical Coupling Effects in an Unsaturated Porous Medium of Semi-Infinite Extent

Rainfall infiltration into an unsaturated region of the earth’s surface is a pervasive natural phenomenon. During the rainfall-induced seepage process, the soil skeleton can deform and the permeability can change with the water content in the unsaturated porous medium. A coupled water infiltration and deformation formulation is used to examine a problem related to the mechanics of a two-dimensional region of semi-infinite extent. The van Genuchten model is used to represent the soil-water characteristic curve. The model, incorporating coupled infiltration and deformation, was developed to resolve the coupled problem in a semi-infinite domain based on numerical methods. The numerical solution is verified by the analytical solution when the coupled effects in an unsaturated medium of semi-infinite extent are considered. The computational results show that a numerical procedure can be employed to examine the semi-infinite unsaturated seepage incorporating coupled water infiltration and deformation. The analysis indicates that the coupling effect is significantly influenced by the boundary conditions of the problem and varies with the duration of water infiltration.

Transient pulse test and morphological analysis of single rock fractures

Transient pulse tests were performed on single rock fractures at different confining pressures. A new data analysis method based on polynomial fitting was introduced to investigate the relationship between flow velocity and hydraulic gradient. 3D laser scanning was used to quantify the morphological changes of the fracture surface after the transient pulse test or under the hydro-mechanical coupling effect. The results show that the flow velocity versus hydraulic gradient data gradient shows a nonlinear relationship at very low hydraulic, possibly due to strong solid-water interaction, but becomes approximately linear after the hydraulic gradient is high enough. The permeability of a single fracture is sensitive to the confining pressure. As the confining pressure increases, the permeability first remarkably decreases when the confining pressure is lower than a certain value and then decreases at a much lower speed when the confining pressure is higher than that value. Both mechanical and hydraulic apertures decrease at a decreasing rate with the increase in confining pressure. The effect of fracture roughness on the permeability is related to the magnitude of the confining pressure. Rougher fractures have lower permeability at low confining pressures; but the opposite can be true when the confining pressure is high enough. Roughness is no longer critical to permeability when the confining pressure is over a certain value.

Sediment–well interaction during depressurization

Depressurization gives rise to complex sediment–well interactions that may cause the failure of wells. The situation is aggravated when high depressurization is imposed on sediments subjected to an initially low effective stress, such as in gas production from hydrate accumulations in marine sediments. Sediment–well interaction is examined using a nonlinear finite element simulator. The hydro-mechanically coupled model represents the sediment as a Cam-Clay material, uses a continuous function to capture compressibility from low to high effective stress, and recognizes the dependency of hydraulic conductivity on void ratio. Results highlight the critical effect of hydro-mechanical coupling as compared to constant permeability models: A compact sediment shell develops against the screen, the depressurization zone is significantly smaller than the volume anticipated assuming constant permeability, settlement decreases, and the axial load on the well decreases; in the case of hydrates, gas production will be a small fraction of the mass estimated using a constant permeability model. High compressive axial forces develop in the casing within the production horizon, and the peak force can exceed the yield capacity of the casing and cause its collapse. Also tensile axial forces may develop in the casing above the production horizon as the sediment compacts in the depressurized zone and pulls down from the well. Well engineering should consider: slip joints to accommodate extensional displacement above the production zone, soft telescopic/sliding screen design to minimize the buildup of compressive axial force within the production horizon, and enlarged gravel pack to extend the size of the depressurized zone.

Hydro-mechanical coupling effect on surficial layer stability of unsaturated expansive soil slopes

Rain-induced surficial slope failures in expansive soils are frequently reported in the literature. These failures are primarily attributed to the shear strength reduction due to the loss of suction associated with the water infiltration. For this reason, slope stability is conventionally evaluated taking account of the hydraulic response of the soil to infiltration of water. The unsaturated expansive soil swells significantly upon wetting; the associated hydro-mechanical coupling effect on the slope stability of expansive soils is not considered in conventional geotechnical engineering practice. This paper aims to numerically evaluate the coupling effect or swelling on the hydraulic response as well as the stability of surficial layer of a typical expansive soil slope. Both hydro-mechanical (coupled) and hydraulic (uncoupled) responses of the slope to a low intensity prolonged rainfall are modeled using the commercial software SIGMA/W and SEEP/W, respectively. Subsequently, the infinite slope formulation is used to compute the factors of safety (FS) profiles extending both coupled (SIGMA/W) and uncoupled (SEEP/W) analysis. The results of the study presented in this paper show that coupled analysis (considering swelling) leads to different suction (negative pore water pressure (PWP)) and FS profiles within the surficial layer from those resulting from uncoupled analysis at the same elapsed time. The wetting fronts in the PWP profile from coupled analysis advance at a relatively faster rate in comparison to uncoupled analysis contributing to more critical FS values or failure conditions. The study highlights the coupled hydro-mechanical behavior of expansive soil has an adverse effect on the slope stability and hence should be taken into account in practice for estimating reliable values of FS.

Creep behavior of boom clay

Several creep tests (lasting more than one year) were performed to study the delayed mechanical behavior of Boom clay under the hydro-mechanical coupling effect. To prevent the soil from swelling as much as possible during re-saturation, the samples were submitted to a confining pressure close to the in situ effective mean stress (2.5MPa) at a room temperature of 21°C. However, certain swelling still exists at the beginning of the saturation. Creep tests further highlight the creep potential of Boom clay. Delayed behavior became more and more significant as the deviatoric stress increased. A deviatoric stress threshold (approximately 1.0MPa), below which only primary creep occurred, was proved to exist from the development of secondary and tertiary creep phases during the creep tests. If we introduce a quasi-steady state creep rate, i.e., the average creep rate after the creep deformation becoming stable, it can be found that the quasi-steady state creep rate of Boom clay is on the order of 10−6ε/h under low deviatoric stress (1.5MPa) in the laboratory, which is on the same order as the average creep rate of the in situ measurements in the second year (1988). However, in situ measurements show that steady creep state of the host rock was not reached even after five years. The in situ quasi-steady state diameter reduction rate calculated from the average of 10 years (1996–2006) of stable deformation of the tunnel linings is on the order of 10−8ε/h.

Improving fracture initiation predictions of a horizontal wellbore in laminated anisotropy shales

Increasingly prominent unconventional petroleum resources tend to occur in formations with laminated heterogeneity. The predominant laminated heterogeneity in tight gas shale reservoirs results in considerable variability in the elastic properties along the orientations parallel and perpendicular to the bedding direction.By incorporating the anisotropic elastic deformation and pore hydro-mechanical coupling effects, a 3D numerical model of fracture initiation from a perforated horizontal wellbore is established. A sensitivity analysis is proposed to evaluate the effects of the anisotropic mechanical behavior and in situ stress conditions on the fracture initiation pressure (FIP) and location of an initial rupture. Comprehensive analysis results revealed elastic anisotropy results in the complex near-wellbore stress concentrations, proved by the fact that the near wellbore fracture tortuosity increases as the perforation azimuth increases, which is not observed in traditional isotropic rocks. Furthermore, perforation parameters including perforation density, perforation diameter, and perforation depth, are also analyzed assuming elastic anisotropy conditions. In addition, a stronger Young's modulus anisotropy causes lower fracture initiation pressure and lower fracture tortuosity at the wellbore face, while the impact of the Poisson's ratio anisotropy is relatively small. Changes in the in situ stress conditions have a significant effect on the fracture initiation pressure whether the rock is an isotropic or anisotropic formation. Numerical simulation results indicate that near-wellbore modeling in horizontal completions in an anisotropic shale is a necessary step for predicting and controlling the potential problems during fracturing treatment and avoiding erroneous completion decisions based on traditional isotropic models.

Numerical modeling and investigation of fracture propagation with arbitrary orientation through fluid injection in tight gas reservoirs with combined XFEM and FVM

Hydraulic fracturing is widely used in the petroleum engineering to enhance the reservoir conductivity. The most 3D simulators for modeling hydraulic fracturing assume that the created fracture propagates perpendicular to the minimum horizontal stress. In the reality, the fracture orientation is more or less affected by many factors, e.g. the reservoir heterogeneity. If the fracture is strongly reoriented, then such 3D simulators could not be used. Therefore, it is essential to investigate the orientation problem. In this paper, a 2D numerical approach to model hydraulic-driven fractures with arbitrary orientation in tight gas reservoirs is presented. The approach is based on the elasticity and lubricant theory with consideration of fully hydro-mechanical coupling effects. It was solved using a combination of the extended finite element and the finite volume methods. The approach was verified by modeling the dynamical growth of a KGD fracture (a model developed by Kristnovitch-Geertsma-Daneshy). Three numerical examples are illustrated. The first example is hydraulic fracturing in a heterogeneous reservoir. The hydraulic fracture propagated asymmetrically with approx. 7 m or 1.5° deviation in the direction of maximum principal stress. In the second example, a two stage multiple hydraulic fracturing was modeled. The influences of the perforation and the first fracture on the second fracture were observed from the modeling results. In the third example, a re-fracturing operation after 1 year of production was modeled. Due to the irregular stress change, an almost 90° fracture reorientation was observed.

Numerical investigation of a low-efficient hydraulic fracturing operation in a tight gas reservoir in the North German Basin

Abstract In this paper, a hydraulic fracturing operation in the tight gas reservoir LZN in the North Germany Basin was numerically investigated to find out the reason for the weak productivity after the fracturing operation. For the investigation, a self-developed numerical model was used, in which rock formation, pore and fracture systems are coupled together. The hydro-mechanical coupling effect, the proppant transport and settling as well as their influences on the fracture closure and contact were fully considered. In the numerical modeling of the in situ operation, the whole process of the data and the main fracturing including fracture propagation, closure, contact and proppant transport were simulated. The modeling was based on the history matching of the derived bottom hole pressure (BHP) from the measured treating pressure. According to the final placement of the proppant and the width distribution after 1500 min, it was found that the middle part of the fracture around the perforation was fully closed after shut in. It indicates that the operation is unsuccessful for the production enhancement; because the connection between the borehole and the created fracture is weak. If the perforation would have been set 15 m lower, then a successful operation with a better connectivity between the borehole and the propped fracture would have been achieved. The precondition to move down the perforation is that the lower fracture barrier must be thick and has a much higher minimum horizontal stress to prevent the fracture going through it. This requirement is also fulfilled in the case study.

A hybrid-dimensional approach for an efficient numerical modeling of the hydro-mechanics of fractures

Characterization of subsurface fluid flow requires accounting for hydro-mechanical coupling between fluid-pressure variations and rock deformation. Particularly, flow of a compressible fluid along compliant hydraulic conduits, such as joints, fractures, or faults, is strongly affected by the associated deformation of the surrounding rock. We investigated and compared two alternative numerical modeling approaches that describe the transient fluid-pressure distribution along a single deformable fracture embedded in a rock matrix. First, we analyzed the coupled hydro-mechanical problem within the framework of Biot's poroelastic equations. Second, in a hybrid-dimensional approach, deformation characteristics of the surrounding rock were combined with a one-dimensional approximation of the fluid-flow problem to account for the high aspect ratios of fractures and the associated numerical problems. A dimensional analysis of the governing equations reveals that the occurring physical phenomena strongly depend on the geometry of the hydraulic conduit and on the boundary conditions. For the analyzed geometries, hydro-mechanical coupling effects dominate and convection effects can be neglected. Numerical solutions for coupled hydro-mechanical phenomena were obtained and compared to field data to characterize the fractured rock in the vicinity of an injection borehole. Either approach captures convection, diffusion, and hydro-mechanical effects, yet the hybrid-dimensional approach is advantageous due to its applicability to problems involving high-aspect-ratio features. For such cases, the modeling of pumping tests by means of the hybrid-dimensional approach showed that the observed inverse-pressure responses are the result of the coupling between the fluid flow in the fracture and the rock deformation caused by fluid-pressure variations along the fracture. Storage capacity as a single parameter of a fracture is insufficient to address all aspects of the coupling.

Hydro-mechanical coupling mechanism on joint of clay core-wall and concrete cut-off wall

The joint of clay core-wall and concrete cut-off wall is one of the weakest parts in high earth and rockfill dams. A kind of highly plastic clay is always fixed on the joint to fit the large shear deformation between clay core-wall and concrete cut-off wall, so the hydro-mechanical coupling mechanisms on the joint under high stress, high hydraulic gradient, and large shear deformation are of great importance for the evaluation of dam safety. The hydro-mechanical coupling characteristics of the joint of the highly plastic clay and the concrete cut-off wall in a high earth and rockfill dam in China were studied by using a newly designed soil-structure contact erosion apparatus. The experimental results indicate that: 1) Shear failure on the joint is due to the hydro-mechanical coupling effect of stress and seepage failure. The seepage failure will induce the final shear failure when the ratio of deviatoric stress to confining pressure is within 1.0–1.2; 2) A negative exponential permeability empirical model for the joint denoted by a newly defined principal stress function, which considers the coupling effect of confining pressure and axial pressure on the permeability, is established based on hydro-mechanical coupling experiments. 3) The variation of the settlement before and after seepage failure is very different. The settlement before seepage failure changes very slowly, while it increases significantly after the seepage failure. 4) The stress-strain relationship is of a strain softening type. 5) Flow along the joint still follows Darcian flow rule. The results will provide an important theoretical basis for the further evaluation on the safety of the high earth and rockfill dam.

Experimental Researches on Hydro-Mechanical Properties of Altered Rock Under Confining Pressures

Altered rock, as the abutment materials of Xiaowan Hydropower Station in China, is a kind of geological defective rock mass. It is loosely structured and its strength is low, with some development of pores and cavities. Research on the hydro-mechanical coupling of the altered rock are of important significance to hydropower projects. In this study, the advanced fully automatic triaxial fluid flow-rheological test servo system is employed to study the hydro-mechanical coupling characteristics of the altered rock, and the water pressures and confining pressures in the laboratory tests are set to simulate the conditions of excavation and impoundment of Xiaowan Hydropower Station. Based on the test results, the stress–strain laws of the rock specimens under the effect of complete hydro-mechanical coupling, as well as the lateral strain and volumetric strain characteristics, are studied. The fluid flow laws of the rock specimens and the effects of the confining pressures on the fluid flow are analyzed. The fluid flow failure characteristic under the effect of the complete hydro-mechanical coupling is discussed. The research achievements show that with the change of the stress states, the permeability of the rock also changes, and the permeability evolution shows the phase characteristic during the process of stress and strain. The impacts of the confining pressures on the strength and deformation and permeability of the altered rock are obvious. The failure behaviours of the rock specimens under the effect of coupling relates to the confining pressures, including two kinds of splitting failure and shear failure. The fluid flow failure characteristic of the rock specimens depend upon the initiation, growth and coalesce of micro-cracks, heterogeneity, confining pressures and properties of the rock.

A new numerical 3D-model for simulation of hydraulic fracturing in consideration of hydro-mechanical coupling effects

A new approach for simulating hydraulic fracture propagation, which treats fracture propagation in a 3D geometric model under 3D stress state with fully hydro-mechanical coupling, is introduced and integrated into the software FLAC3D. In the new modeling approach, the mechanical behavior of the rock formation is based on continuum mechanics. A modified tensile failure model has been used to describe fracture enlargement and closure. Meanwhile, a simplified fracture flow equation derived from the general Navier–Stokes equation and Darcy law, has been developed and used to describe fluid flow, both in the fracture and in the matrix. Fluid leakoff is no longer controlled by semi-analytical models, such as the Carter model, but resolved by means of numerical methods. In order to describe fracture propagation, the tensile failure criterion has been applied. To verify the new modeling approach, simulation of a laboratory test has been carried out. Numerical and measured results have been compared and found to be in agreement. After verification of the new modeling approach, a calculation, based on real data for a tight gas sandstone reservoir from the Northern German Basin, has been applied and graphically illustrated. Unlike conventional models, the new modeling approach not only considers the propagation of a single fracture, but also its influence on the adjacent rock formations and the neighboring fractures.

Experimental study on the hydro-mechanical behavior of Boom clay

In the context of research into deep nuclear waste disposal, Boom clay, a stiff soil from the Underground Research Laboratory in Mol, Belgium, has been studied in various works. In this paper, containing X-ray diffraction tests, oedometer tests, consolidated undrained shearing (CUS) tests and scanning electron microscopic (SEM) tests, a series of experimental studies intended to study the hydro-mechanical behavior and micro fabric of Boom clay are presented. X-ray diffraction tests and SEM tests further highlight the high content of clay mineral of Boom clay (about 60% in average). Preconsolidation pressure, which is about 5.0MPa, was determined by oedometer tests. High swelling property under hydro-mechanical coupling effect has been investigated on the clay samples during the CUS test, and also high viscous potential has been represented under shearing rate of 20μm/min. SEM tests show that the microstructure of Boom clay arranged more directional and denser, and it can be observed clearly through the microstructure variation of pores and fissures.

Excavation-induced hydraulic conductivity reduction around a tunnel – Part 1: Guideline for estimate of ground water inflow rate

Field observations indicate that current engineering practice does not consistently estimate ground water flows into unlined rock excavated tunnels due to various factors that analytical solutions do not take into account. These factors include significant geological features, groundwater drawdown, inadequate estimates of hydraulic conductivity from packer tests, and stress-induced rock-mass permeability reduction in the vicinity of tunnel (lining-like zone). A key variable that is not properly accommodated in current practice, is the hydro-mechanical interaction within the joints in the surrounding rock mass. The significance of this variable is discussed in the 1st part of the paper which presents an analytical solution assessing ground water inflow rate into a tunnel using a mathematical derivation that takes into account the excavation-induced rock permeability reduction in the vicinity of a tunnel based on hydro-mechanical coupling effect. In the 2nd part of the paper, results from numerical analysis are presented to verify the proposed analytical solution for estimating ground water inflow rate into a tunnel. Further studies are currently underway to identify other key variables and their impact on the behavior of unlined tunnels and hydrological flow regime in the surrounding fractured rock mass using a distinct element method program which can fully consider hydro-mechanical coupled behavior of joints.