Jishan Liu

FRACTURE SPACING IN LAYERED MATERIALS: A NEW EXPLANATION BASED ON TWO-DIMENSIONAL FAILURE PROCESS MODELING

Opening-mode fractures in layered materials, such as sedimentary rocks, pavement, functionally graded composite materials or surface coating films, often are periodically distributed with spacings scaled to the thickness of the fractured layer. The current general explanation is that when the fracture spacing to layer thickness ratio changes from greater than to less than critical values the normal stress acting perpendicular to the fractures changes from tensile to compressive. This stress state transition is believed to preclude further infilling of fractures, and the critical fracture spacing to layer thickness ratio at this point defines a lower limit, called fracture saturation. To better understand the controls on fracture spacing, we have investigated the problem using a progressive fracture modeling approach that shares many of the natural kinematic features, such as fracture nucleation, fracture infilling and fracture termination. As observed experimentally, our numerical simulations demonstrate that fracture spacing initially decreases as extensional strain increases in the direction perpendicular to the fractures, and at a certain ratio of fracture spacing to layer thickness, no new fractures nucleate (saturated). Beyond this point, the additional strain is accommodated by further opening of existing fractures: the spacing then simply scales with layer thickness, creating fracture saturation. An important observation from our fracture modeling is that saturation may also effectively be achieved by the interface delamination and throughgoing fracturing, which inhibit additional layer-confined fracturing. We believe that these processes may serve as another mechanism to accommodate additional strain for a fracture saturated layer. Because interface debonding stops the transition of stress from the neighboring layers to the embedded central layer, which may preclude further infilling of new fractures, our fracture modeling approach predicts a larger critical length scale of fracture spacing than that predicted by a stress analysis approach based on stress transition theory. Numerical simulations also show that the critical value of the fracture spacing to layer thickness ratio is strongly dependent on the mechanical disorder in the fractured layer. The spacing to thickness ratio decreases with increasing heterogeneity of the mechanical properties.

Impact of Gas Adsorption Induced Coal Matrix Damage on the Evolution of Coal Permeability

It has been widely reported that coal permeability can change from reduction to enhancement due to gas adsorption even under the constant effective stress condition, which is apparently inconsistent with the classic theoretical solutions. This study addresses this inconsistency through explicit simulations of the dynamic interactions between coal matrix swelling/shrinking induced damage and fracture aperture alteration, and translations of these interactions to permeability evolution under the constant effective stress condition. We develop a coupled coal–gas interaction model that incorporates the material heterogeneity and damage evolution of coal, which allows us to couple the progressive development of damage zone with gas adsorption processes within the coal matrix. For the case of constant effective stress, coal permeability changes from reduction to enhancement while the damage zone within the coal matrix develops from the fracture wall to further inside the matrix. As the peak Langmuir strain is approached, the decrease of permeability halts and permeability increases with pressure. The transition of permeability reduction to permeability enhancement during gas adsorption, which may be closely related to the damage zone development in coal matrix, is controlled by coal heterogeneity, external boundary condition, and adsorption-induced swelling.

Damage and Plastic Deformation Modeling of Beishan Granite Under Compressive Stress Conditions

Based on experimental investigations, we propose a coupled elastoplastic damage model to simulate the mechanical behavior of granite under compressive stress conditions. The granite is taken from the Beishan area, a preferable region for China’s high-level radioactive waste repository. Using a 3D acoustic emission monitoring system in mechanical tests, we focus on the cracking process and its influence on the macroscopic mechanical behavior of the granite samples. It is verified that the crack propagation coupled with fractional sliding along the cracks is the principal mechanism controlling the failure process and nonlinear mechanical behavior of granite under compressive stress conditions. Based on this understanding, the coupled elastoplastic damage model is formulated in the framework of the thermodynamics theory. In the model, the coupling between damage and plastic deformation is simulated by introducing the independent damage variable in the plastic yield surface. As a preliminary validation of the model, a series of numerical simulations are performed for compressive tests conducted under different confining pressures. Comparisons between the numerical and simulated results show that the proposed model can reproduce the main features of the mechanical behavior of Beishan granite, particularly the damage evolution under compressive stress conditions.

Impact of various parameters on the production of coalbed methane

Coalbed-methane (CBM) reservoirs are naturally fractured formations, comprising both permeable fractures and matrix blocks. The interaction between fractures and matrix presents a great challenge for the forecast of CBM reservoir performance. In this work, a dual-permeability model was applied to study the parameter sensitivity on the CBM production, because the dual-permeability model incorporates not only the influence from matrix and fractures but also that between adjacent matrix blocks. The mass exchange between two systems is defined as a function of desorption time constant at the standard condition, coal matrix porosity, and the difference of gas pressure between two systems. Correspondingly, gas diffusivity in matrix is considered as a variable and represented by a function of shape factor, gas desorption time, and reservoir pressure. These relations are integrated into a fully coupled numerical model of coal geomechanical deformation and gas desorption/gas flow in both systems. This numerical approach demonstrates the important nonlinear effects of the complex interaction between matrix and fractures on CBM production behaviors that cannot be recovered without rigorously incorporating geomechanical influences. This model was then used to investigate the sensitivity of CBM extraction behavior to different controlling factors, including gas desorption time constant, initial fracture permeability, fracture spacing, swelling capacity, desorption capacity, production pressure, and fracture and matrix porosities. Modeling results show that the peak magnitudes of gasproduction rate increase with initial fracture permeability, sorption and swelling capacities, and matrix porosity, and decrease with gas desorption time constant and production pressure. These results also show dramatic increase in gas-production efficiency with decreasing magnitudes of fracture spacing. The comparison of the transient contributions of the desorbed gas and the free phase gas from the matrix system to gas production shows that the free phase gas plays the dominant role at the early stage, but diminishes when the adsorption phase gas takes over the dominant role, indicating the necessity of incorporating free phase gas impact in simulation models. The numerical model was also applied to match the history data from a gas-production well. A better matching result than that for the single-permeability model demonstrates the potential capability of the dual-permeability model for the forecast of CBM production.

Evaluation of pore water pressure fluctuation around an advancing longwall face

Large deformations that accompany longwall mining result in complex spatial and temporal distributions of changes in undrained pore fluid pressures around the advancing face. These seemingly anomalous changes are recorded in the rapid water level response of undermined and adjacent wells, and may be explained in the short-term as a undrained poroelastic effect. A three-dimensional finite element model is applied to define anticipated pore fluid response both around the advancing mining face, at depth, and in the near surface region. The results are carefully verified against the response recorded at three well-instrumented longwall sites. Pore pressure changes are indexed directly to volumetric strains defining zones of significant depressurization in the caving zone and in zones of extension adjacent to the subsidence trough on the ground surface. Overpressurization occurs in the abutment region, at panel depth, and in the surface compressive zone immediately inside the angle-of-draw. These results are confirmed with available, short-term water level response data, defining the strongly heterogeneous spatial response and the significance of well depth on anticipated water level response.

The Influence of Fracturing Fluids on Fracturing Processes: A Comparison Between Water, Oil and SC-CO2

Conventional water-based fracturing treatments may not work well for many shale gas reservoirs. This is due to the fact that shale gas formations are much more sensitive to water because of the significant capillary effects and the potentially high contents of swelling clay, each of which may result in the impairment of productivity. As an alternative to water-based fluids, gaseous stimulants not only avoid this potential impairment in productivity, but also conserve water as a resource and may sequester greenhouse gases underground. However, experimental observations have shown that different fracturing fluids yield variations in the induced fracture. During the hydraulic fracturing process, fracturing fluids will penetrate into the borehole wall, and the evolution of the fracture(s) then results from the coupled phenomena of fluid flow, solid deformation and damage. To represent this, coupled models of rock damage mechanics and fluid flow for both slightly compressible fluids and CO2 are presented. We investigate the fracturing processes driven by pressurization of three kinds of fluids: water, viscous oil and supercritical CO2. Simulation results indicate that SC-CO2-based fracturing indeed has a lower breakdown pressure, as observed in experiments, and may develop fractures with greater complexity than those developed with water-based and oil-based fracturing. We explore the relation between the breakdown pressure to both the dynamic viscosity and the interfacial tension of the fracturing fluids. Modeling demonstrates an increase in the breakdown pressure with an increase both in the dynamic viscosity and in the interfacial tension, consistent with experimental observations.

Simulations of a coupled hydro-chemo-mechanical system in rocks

A coupled hydro-chemo-mechanical numerical model is developed for these coupled phenomena in many engineering fields. The model has been applied to predicting the response of a stressed rockmass column to an injected reactive fluid (reagent) flow. The response includes evolutions of porosity, permeability, reagent and mineral concentrations during dissolution. In the model, the progress of dissolution is defined by the change in porosity ratio and the porosity increases with dissolution assuming there is no precipitation. The numerical evolutions of porosity, permeability, reagent and mineral concentrations during dissolution are validated against steady state solutions. The model results show that these evolutions are regulated to a certain extent by the applied external loadings: an applied extensional stress enhances the progress of the dissolution process while an applied compression stress slows the progress of the dissolution process.

Impact of thermal processes on CO2 injectivity into a coal seam

The objective of this study is to investigate how thermal gradients, caused by CO2 injection, expansion and adsorption, affect the permeability and adsorption capacity of coal during CO2 sequestration. A new permeability model is developed in which the concept of elastic modulus reduction ratio is introduced to partition the effective strain between coal matrix and fracture. This model is implemented into a fully coupled mechanical deformation, gas flow and heat transport finite element simulator. To predict the amount of CO2 sequested, the extended Langmuir sorption model is used, with parameters values taken from the literature. The coupled heat and gas flow equations, are solved in COMSOL using the finite element method. The simulation results for a constant volume reservoir demostrate that thermal strain acts to significantly reduce both CO2 injectivity and adsorption capacity. These impacts need to be considered in the calculation of the optimum injection rate and the total sequestration capacity.

The Evolution of Permeability in Natural Fractures - The Competing Roles of Pressure Solution and Free-Face Dissolution

Abstract Fracture permeabilities are shown surprisingly sensitive to mineral dissolution at modest temperatures (c. 20°–80°C) and flow rates. Net dissolution may either increase or decrease permeability, depending on the prevailing ambient THMC conditions. These behaviours have important ramifications for constitutive laws for flow and transport. Flow-through tests are completed on a natural fracture in novaculite at temperatures of 20°C, 80°C, 120°C, and 150°C, and on an artificial fracture in limestone at 20°C. Measurements of fluid and dissolved mass fluxes, concurrent X-ray CT and imaging, and post-test sectioning and SEM are used to constrain the progress of mineral dissolution and its effect on transport properties. For the novaculite, under constant effective stress, fracture permeability decreased monotonically with an increase in temperature, with fracture permeability reducing by two-orders-of-magnitude over the 900 h test. For the limestone, an initial decrease in permeability over the first 935h of the test, switched to a net increase in permeability as distilled water was subsequently circulated for the final 500h of the test.

Yield stress and microstructure of washed oxide suspensions at the isoelectric point: experimental and model fractal structure

The yield stress and microstructure of washed, relatively monodisperse spherical zirconia (ZrO2) and titania (TiO2) suspensions at the isoelectric point (pI) were characterised. The yield stress was found to be dependent upon the particle size. At a given solid concentration, the finer suspensions produced a larger yield stress due to the higher particle concentration and hence, a greater density of attractive interaction. At pI, only the van der Waals force is in play. Vitrified fractal microstructures of these suspensions at pI were captured by cryo-SEM. A power law relationship described the (maximum) yield stress–volume fraction data for both oxides which is consistent with the prediction of scaling theory. An exponent value of ∼3 was obtained for both oxides. The fractal dimension (Df) extracted from this exponent value of the scaling law for large aggregate cluster interaction in the slow flow regime was ∼2.3. The theoretical fractal structure with the same Df constructed from monodisperse spherical particles based on the off-lattice variable-Df model showed strong resemblance to the cryo-SEM imagery of the vitrified structure for both oxides.