Hamid Nick | Imperial College London (original) (raw)

Papers by Hamid Nick

$Research paper thumbnail of A geometrically based method for predicting stress-induced fracture aperture and flow in discrete fracture networks$

A B S T R A C T Modeling of fluid flow in naturally fractured reservoirs is often done through mo... more A B S T R A C T Modeling of fluid flow in naturally fractured reservoirs is often done through modeling and upscaling of discrete fracture networks (DFNs). The two-dimensional fracture geometry required for DFNs is obtained from subsurface and outcropping analog data. However, these data provide little information on subsurface fracture aperture, which is essential for quantifying porosity and permeability. Apertures are difficult to obtain from either out-cropping or subsurface data and are therefore often based on fracture size or scaling relationships, but these do not consider the orientation and spatial distribution of fractures with respect to the in situ stress field. Using finite-element simulations, mechanical aperture can be modeled explicitly, but because changes in fracture geometry require renewed meshing and simulating, this approach is not easily integrated into subsurface DFN modeling workflows. We present a geometrically based method for calculating the shear-induced hydraulic aperture, that is, an aperture of up to 0.5 mm (0.02 in.) that can result from shear displacement along irregular fracture walls. The geometrically based method does not require numerical simulations, but it can instead be directly applied to DFNs using the fracture orientation and spacing distributions in combination with an estimate of the regional stress tensor and orientation. The frequency distribution of hydraulic aperture from the geometrically based method is compared with finite-element models constructed from five real fracture networks , digitized from outcropping pavements. These networks cover a wide range of possible geometries and spatial distributions. The geometrically based method predicts the average hydraulic aperture and equivalent permeability of fractured porous media with error margins of less than 5%.

$Research paper thumbnail of Modeling seawater intrusion and the associated reactive solute transport in fractured coastal aquifers$

In coastal aquifers seawater and terrestrial water get into contact and the reactive mixing betwe... more In coastal aquifers seawater and terrestrial water get into contact and the reactive mixing between these water bodies controls the water quality of submarine groundwater discharge. The rates of such mixing controlled reactions are depending not only on the properties of the reactive species but also on the density driven flow dynamics and the resulting transport patterns. A prediction of these flow and transport processes and thus of the fate of reactive species is specifically challenged in fracture aquifers as it depends on the focusing of the flow and the local balance of viscous and gravitational forces. To study the influence of fractures on mixing and reactive transport in coastal aquifers we present a reactive discrete fracture and matrix (DFM) model using unstructured spatially adaptively refined finite-element meshes. This model is developed by coupling the Complex System Modelling Platform (CSMP++) utilizing a hybrid FEFV scheme, and a Biogeochemical Reaction Network Simu...

This study introduces PoreFlow, a pore-network modeling tool capable of simulating fluid flow and... more This study introduces PoreFlow, a pore-network modeling tool capable of simulating fluid flow and multi-component reactive and adsorptive transport under saturated and variably saturated conditions. PoreFlow includes a variety of modules, such as: pore network generator, drainage simulator, calculation of pressure and velocity distributions, and modeling of reactive solute transport accounting for advection and diffusion. The pore space is represented using a multi-directional pore-network capable of capturing the random structure of a given porous media with user-defined directional connectivities for anisotropic pore structures. The chemical reactions can occur within the liquid phase, as well as between the liquid and solid phases which may result in an evolution of porosity and permeability. Under variably saturated conditions the area of interfaces changes with degree of the fluid saturation. PoreFlow uses complex formulations for more accurate modeling of transport problems in presence of the nonwetting phase. This is done by refining the discretization within drained pores. An implicit numerical scheme is used to solve the governing equations, and an efficient substitution method is applied to considerably minimize computational times. Several examples are provided, under saturated and variably saturated conditions, to demonstrate the model applicability in hydrogeology problems and petroleum fields. We show that PoreFlow is a powerful tool for upscaling of flow and transport in porous media, utilizing different pore scale information such as various interfaces, phase distributions and local fluxes and concentrations to determine macro scale properties such as average saturation, relative permeability, solute dispersivity, adsorption coefficients, effective diffusion and tortuosity. Such information can be used as constitutive relations within continuum scale governing equations to model physical and chemical processes more accurately at the larger scales.

$Research paper thumbnail of Flow and transport simulation with embedded discontinuity in fractured porous media$

ABSTRACT Todate, the complexity of fractured porous media still precludes the direct incorporatio... more ABSTRACT Todate, the complexity of fractured porous media still precludes the direct incorporation of small-scale features into field scale modelling. These features, however, can be instrumental in shaping and triggering coarsening instabilities and other emerging behaviours which need to be considered by field scale models. We develop numerical simulation methods for this purpose and demonstrate their superior performance with single/two phase models of heterogeneous porous media. Material discontinuities in fractured porous media strongly influence single/multiphase fluid flow. When continuum methods are used to model transport across material interfaces, they smear out jump discontinuities of concentration or saturation. To overcome this problem, we split the finite-element models with complementary node-centred finite-volumes along the material interfaces, developing a transport scheme that realistically represents the dependent variable discontinuities arising at these interfaces. The main advantage of this new scheme is its ability to embed discontinuities into continuum models. We have found that using the discontinuous scheme is crucial to capture the emerging patterns due to the interaction of heterogeneity, and flow and transport. Hence, we suggest that discontinuity should be embedded in single/multi phase flow modelling as a default for material interfaces in fractured porous media.

$Research paper thumbnail of Influence of fracture aperture on solute transport in porous media$

International Journal of …, 2012

The reactive mixing between seawater and terrestrial water in coastal aquifers influences the wat... more The reactive mixing between seawater and terrestrial water in coastal aquifers influences the water quality of submarine groundwater discharge. While these waters come into contact at the seawater groundwater interface by density driven flow, their chemical components dilute and react through dispersion. A larger interface and wider mixing zone may provide favorable conditions for the natural attenuation of contaminant plumes. It has been claimed that the extent of this mixing is controlled by both, porous media properties and flow conditions. In this study, the interplay between dispersion and reactive processes in coastal aquifers is investigated by means of numerical experiments. Particularly, the impact of dispersion coefficients, the velocity field induced by density driven flow and chemical component reactivities on reactive transport in such aquifers is studied. To do this, a hybrid finite-element finite-volume method and a reactive simulator are coupled, and model accuracy and applicability are assessed. A simple redox reaction is considered to describe the degradation of a contaminant which requires mixing of the contaminated groundwater and the seawater containing the terminal electron acceptor. The resulting degradation is observed for different scenarios considering different magnitudes of dispersion and chemical reactivity. Three reactive transport regimes are found: reaction controlled, reaction–dispersion controlled and dispersion controlled. Computational results suggest that the chemical components' reactivity as well as dispersion coefficients play a significant role on controlling reactive mixing zones and extent of contaminant removal in coastal aquifers. Further, our results confirm that the dilution index is a better alternative to the second central spatial moment of a plume to describe the mixing of reactive solutes in coastal aquifers.► Developing a multicomponent reactive transport model for density driven flow regimes. ► The kinetics of the degradation and dispersion determines the extent of mass removal. ► Three distinct dispersion- or/and reaction-controlled transport regimes are shown.

$Research paper thumbnail of Modelling stress-dependent permeability in fractured rock including effects of propagating and bending fractures$

The influence of in-situ stresses on flow processes in fractured rock is investigated using a nov... more The influence of in-situ stresses on flow processes in fractured rock is investigated using a novel modelling approach. The combined finite-discrete element method (FEMDEM) is used to model the deformation of a fractured rock mass. The fracture wall displacements and aperture changes are modelled in response to uniaxial and biaxial stress states. The resultant changes in flow properties of the rock mass are investigated using the Complex Systems Modelling Platform (CSMP++). CSMP++ is used to model single-phase flow through fractures with variable aperture and a permeable rock matrix. The study is based on a geological outcrop mapping of a low density fracture pattern that includes the realism of intersections, bends and segmented features. By applying far-field (boundary) stresses to a square region, geologically important phenomena are modelled including fracture-dependent stress heterogeneity, the re-activation of pre-existing fractures (i.e. opening, closing and shearing), the propagation of new fractures and the development of fault zones. Flow anisotropy is investigated under various applied stresses and matrix permeabilities. In-situ stress conditions that encourage a closing of fractures together with a more pervasive matrix-dominated flow are identified. These are compared with conditions supporting more localised flow where fractures are prone to dilatational shearing and can be more easily exploited by fluids. The natural fracture geometries modelled in this work are not perfectly straight, promoting fracture segments that dilate as they shear. We have demonstrated the introduction of several realistic processes that have an influence on natural systems: fractures can propagate with wing cracks; there is the potential for new fractures to connect with existing fractures, thus increasing the connectivity and flow; blocks can rotate when bounded by fractures, bent fractures lead to locally different aperture development; highly heterogeneous stress distributions emerge naturally. Results presented in this work provide a mechanically rigorous demonstration that a change in the stress state can cause reactivation of pre-existing fractures and channelling of flow in critically stressed fractures.

$Research paper thumbnail of Role of geomechanically grown fractures on dispersive transport in heterogeneous geological formations$

We benchmark a family of hybrid finite element–node-centered finite volume discretization methods... more We benchmark a family of hybrid finite element–node-centered finite volume discretization methods (FEFV) for single- and two-phase flow/transport through porous media with discrete fracture representations. Special emphasis is placed on a new method we call DFEFVM in which the mesh is split along fracture–matrix interfaces so that discontinuities in concentration or saturation can evolve rather than being suppressed by nodal averaging of these variables. The main objective is to illustrate differences among three discretization schemes suitable for discrete fracture modeling: (a) FEFVM with volumetric finite elements for both fractures and porous rock matrix, (b) FEFVM with lower dimensional finite elements for fractures and volumetric finite elements for the matrix, and (c) DFEFVM with a mesh that is split along material discontinuities. Fracture discontinuities strongly influence single- and multi-phase fluid flow. Continuum methods, when used to model transport across such interfaces, smear out concentration/saturation. We show that the new DFEFVM addresses this problem producing significantly more accurate results. Sealed and open single fractures as well as a realistic fracture geometry are used to conduct tracer and water-flooding numerical experiments. The benchmarking results also reveal the limitations/mesh refinement requirements of FE node-centered FV hybrid methods. We show that the DFEFVM method produces more accurate results even for much coarser meshes.

… in porous media, Jan 1, 2009

A two-dimensional numerical model is used to study the nonlinear behavior of density gradients on... more A two-dimensional numerical model is used to study the nonlinear behavior of density gradients on transverse dispersion. Numerical simulations are conducted using d 3f, a computer code for simulation of density-dependent flow in porous media. Considering a density-stratified horizontal flow in a heterogeneous porous media, a series of simulations is carried out to examine the effect of the density gradient on macro-scale transverse dispersivity. Changing salt concentration significantly affects fluid properties. This physical behavior of the fluid involves a non-linearity in modeling the interaction between salt and fresh water. It is concluded that the large-scale transport properties for high density flow deviate significantly from the tracer case due to the spatial variation of permeability, described by statistical parameters, at the local-scale. Indeed, the presence of vertical flow velocities induced by permeability variations is responsible for the reduction of the mixing zone width in the steady state in the case of a high density gradient. Uncertainties in the model simulations are studied in terms of discretization errors, boundary conditions, and convergence of ensemble averaging. With respect to the results, the gravity number appears to be the controlling parameter for dispersive flux. In addition, the applicability and limitations of the nonlinear model of Hassanizadeh (1990) and Hassanizadeh and Leijnse (1995) (Adv Water Resour 18(4):203–215, 1995) in heterogeneous porous media are investigated. We found that the main cause of the nonlinear behavior of dispersion, which is the interaction between density contrast and vertical velocity, needs to be explicitly accounted for in a macro-scale model.

$Research paper thumbnail of Upscaling two-phase flow in naturally fractured reservoirs$

AAPG bulletin, Jan 1, 2009

$Research paper thumbnail of Simulation of solute transport through fractured rock: A higher-order accurate finite-element finite-volume method permitting large time steps$

Transport in porous media, Jan 1, 2010

Discrete-fracture and rock matrix (DFM) modelling necessitates a physically realistic discretisat... more Discrete-fracture and rock matrix (DFM) modelling necessitates a physically realistic discretisation of the large aspect ratio fractures and the dissected material domains. Using unstructured spatially adaptively refined finite-element meshes, we find that the fastest flow often occurs in the smallest elements. Flow velocity and element size vary over many orders of magnitude, disqualifying global Courant number (CFL)-dependent transport schemes because too many time steps would be necessary to investigate displacements of interest. Here, we present a higher-order accurate implicit pressure–(semi)-implicit transport scheme for the advection–diffusion equation that overcomes this CFL limitation for DFM models. Using operator splitting, we solve the pressure and the transport equations on finite-element, node-centred finite-volume meshes, respectively, using algebraic multigrid methods. We apply this approach to field data-based DFM models where the fracture flow velocity and mesh refinement is 2–4 orders of magnitude greater than that of the matrix. For a global CFL of ≤10,000, this implies sub-CFL, second-order accurate behaviour in the matrix, and super-CFL, at least first-order accurate, transports in fast-flowing fractures. Their greater refinement, however, largely offsets this numerical dispersion, promoting a highly accurate overall solution. Numerical and fracture-related mechanical dispersions are compared in the realistic DFM models using second-order accurate runs as reference cases. With a CFL histogram, we establish target error criteria for CFL overstepping. This analysis indicates that for extreme fracture heterogeneity, only a few transport steps can be sufficient to analyse macro-dispersion. This makes our implicit method attractive for quick analysis of transport properties on multiple realisations of DFM models.

Vadose Zone Journal, Jan 1, 2011