Christopher McDermott

Application of hybrid numerical and analytical solutions for the simulation of coupled thermal, hydraulic, mechanical and chemical processes during fluid flow through a fractured rock

In many subsurface engineering geoscience applications the impact of thermal, hydraulic, mechanical and chemical (THMC) processes needs to be evaluated. Coupled process models require solution of the partial differential equations describing energy or mass balance. Ignoring the coupling of these processes can lead to a significant oversimplification which may not adequately represent the systems being modelled. Incorporation of coupled processes and associated phenomena inevitably leads to numerical stability issues due to very different scales in terms of spatial distribution, time and parametrical heterogeneity. One approach to simplify the computational demands is to integrate analytical and physical models into standard numerical modelling techniques (in this case finite elements), effectively adding sub-grid scale and sub-time scale information to the model. We present such an approach for the simulation of fluid flow through a fracture validated against experimental data and cross comparison with results of other modelling teams within the DECOVALEX 2015 (development of coupled models and their validation against experiments) project (http://www.decovalex.org). By replacing the mechanical behaviour and chemical transport processes with physical models, and by utilising the static nature of the temperature changes, only the hydraulic system required numerical solution in a highly coupled problem. Physical models for fracture closure due to pressure solution, fracture opening due to chemical dissolution, the development of channel flow and a change in the reactive transport characteristics with time were implemented and are described here. The main features of the experimental data could be replicated, although lying outside of the parameter range suggested by the literature. Comparison with other teams using different modelling approaches indicated internal consistency.

A synthesis of approaches for modelling coupled thermal–hydraulic–mechanical–chemical processes in a single novaculite fracture experiment

Abstract The geological formation immediately surrounding a nuclear waste disposal facility has the potential to undergo a complex set of physical and chemical processes starting from construction and continuing many years after closure. The DECOVALEX project (DEvelopment of COupled models and their VALidation against EXperiments) was established and maintained by a variety of waste management organisations, regulators and research organisations to help improve capabilities in experimental interpretation, numerical modelling and blind prediction of complex coupled systems. In the present round of DECOVALEX (D-2015), one component of Task C1 has considered the detailed experimental work of Yasuhara et al. (Earth Planet Sci Lett 244:186–200, 2006), wherein a single artificial fracture in novaculite (micro- or crypto-crystalline quartz) is subject to variable fluid flows, mechanical confining pressure and different applied temperatures. This paper presents a synthesis of the completed work of six separate research teams. A range of approaches are presented including 2D and 3D high-resolution coupled thermo–hydro–mechanical–chemical models. The results of the work show that while good, physically plausible representations of the experiment can be obtained using a range of approaches, there is considerable uncertainty in the relative importance of the various processes, and that the parameterisation of these processes can be closely linked to the interpretation of the fracture surface topography at different spatial scales.

Development of approaches for modelling coupled thermal–hydraulic–mechanical–chemical processes in single granite fracture experiments

The geological formation immediately surrounding a nuclear waste disposal facility has the potential to undergo a complex set of physical and chemical processes starting from construction and continuing many years after closure. The DECOVALEX project (DEvelopment of COupled models and their VALidation against EXperiments) was established and maintained by a variety of waste management organizations, regulators and research organizations to help improve capabilities in experimental interpretation, numerical modelling and blind prediction of complex coupled systems. In the present round of DECOVALEX (D-2015), one component of Task C1 has considered the detailed experimental work of Yasuhara et al. (Appl Geochem 26:2074–2088, 2011), wherein three natural fractures in Mizunami granite are subject to variable fluid flows, mechanical confining pressure and different applied temperatures. This paper presents a synthesis of the completed work of six separate research teams, building on work considering a single synthetic fracture in novaculite. A range of approaches are presented including full geochemical reactive transport modelling and 2D and 3D high-resolution coupled thermo–hydro–mechanical–chemical (THMC) models. The work shows that reasonable fits can be obtained to the experimental data using a variety of approaches, but considerable uncertainty remains as to the relative importance of competing process sets. The work also illustrates that a good understanding of fracture topography, interaction with the granite matrix, a good understanding of the geochemistry and the associated multi-scale THMC process behaviours is a necessary pre-cursor to considering predictive models of such a system.

Evaluating the importance of different coupled thermal, hydraulic, mechanical and chemical process simulations during fluid flow experiments in fractured novaculite and fractured granite

Abstract Fluid migration in the subsurface has the potential to induce changes in fluid pressure distribution, temperature distribution, mechanical stresses and the chemistry of both the fluid and the natural geological material it is flowing through. In many situations, the change in all of these processes gives a coupled response, in that one process feeds back to another. When trying to understand fluid flow through naturally and artificially fractured systems, it is important to be able to identify the relative importance of the processes occurring and the degree of interactions between them. Modelling of such highly nonlinear coupled flow is complex. Current and predicted computational ability is not able to simulate discretely all the known and physically described processes operating. One approach to coping with this complexity is to identify the relative importance and impact of relevant processes, dependent on the application of interest. Addressing such complexity can be particularly important when the characteristics of natural and disturbed geological materials are being evaluated in the context of disposal of radioactive waste or other geo-engineering systems where an understanding of the long-term evolution is required. Based on a series of coupled (THMC—Thermal, Hydraulic, Mechanical and Chemical) experimental investigations on the flow of fluid through fractured novaculite and granite crystalline rock samples, several couplings are examined where there is both a significant kinetic chemical control as well as mechanical and temperature control on the fluid flow behaviour. These interactions can be shown both in the literature and experimentally to have a significant effect on the rate of fluid flow through fractures. A new discrete numerical approach and a new homogenous approach are used to model the experimental results of coupled flow through fractures. The results of these modelling approaches are benchmarked both against one another and against the experimental results, and then the processes included in the approaches are ranked in order of impact.

Wastewater from hydraulic fracturing in the UK: assessing the viability and cost of management

The safe and effective management of wastewaters from unconventional hydrocarbon production using the hydraulic fracturing (fracking) process poses a major challenge. Exploitation of unconventional hydrocarbons, such as shale gas, remains controversial in the UK primarily due to concerns surrounding the hydraulic fracturing process required to extract the resource. The key issue of how waste fluids produced by hydraulic fracturing in the UK will be safely managed has yet to be adequately addressed, and the capacity for the specialist treatment required is currently uncertain. To address this critical knowledge gap we review, for the first time, the available management options for these waste fluids in the UK. We find that these are limited in comparison to the options available in the U.S., due to uncertainty surrounding whether wastewater injection wells will be permitted in the UK. Consequently, it is highly probable that these fluids will need to be treated and safely disposed of at the surface. In order to constrain the composition of wastewater which will require treatment in the UK, we analyse the only existing data set of returned waters from hydraulic fracturing (n = 31). We supplement this with measurements of wastewater from UK conventional onshore hydrocarbon (n = 3), and offshore hydrocarbon (n = 14), operations which produce water from similar formations as those currently targeted for shale gas exploration. Comparison of this limited UK data to the more extensive unconventional production dataset from the United States (n = 3092) provides confidence in our projected UK wastewater compositions. We find that the high level of salinity and concentration of naturally occurring radioactive material (NORM) in UK wastewaters will be problematic to treat for disposal into a freshwater environment. We use our data compilation to estimate costs of treating such wastewaters in a number of relevant scenarios. We find that the projected salinity in FP waters from UK hydraulic fracturing operations can be treated at a cost of between

Mathematical Modeling: Approaches for Model Solution

2701 (∼£2000) and

Natural Analogue Studies

1 376 093 (∼£1 047 000) per well, requiring between 2 and 26% of expected revenue. Additional costs, specific to the UK of up to £163 450 per well, will be incurred due to the legislative requirement for disposal of NORM concentrated sludge in permitted landfill sites. We find that existing capacity to receive NORM waste at currently permitted UK treatment facilities is limited, and that this will pose management problems if wastewaters are generated from multiple unconventional wells simultaneously.

Risk Management for CO 2 Geological Storage Projects

The governing equations and mathematical models describing CO2 spreading and trapping in saline aquifers and the related hydro-mechanical and chemical processes were described in Chapt. 3. In this chapter, the focus is on methods for solving the relevant equations. The chapter gives an overview of the different approaches, from high-fidelity full-physics numerical models to more simplified analytical and semi-analytical solutions . Specific issues such as modeling coupled thermo-hydro-mechanical-chemical processes and modeling of small-scale processes , such as convective mixing and viscous fingering , are also addressed. Finally, illustrative examples of modeling real systems, with different types of modeling approaches, are presented.

Fault Seal Analysis of a Natural CO2 Reservoir

Lessons learned from sites where CO2 has naturally been stored for long geologic periods of time provides valuable information for assessing proposed anthropogenic storage sites. This chapter discusses the natural CO2 storage analogue sites and looks at them worldwide to determine which geological characteristics are preferable for natural CO2 storage and which are not. Following this, an approach is presented based on geomechanical facies, for a comparative assessment of storage sites, accounting for features observed in the natural analogue sites. Finally, a number of anthropogenic storage sites are classified according to the characterization criteria and a detailed description of a number of natural and anthropogenic storage sites are presented.

Multiphase Flow Processes

A number of key challenges relating to potential CO2 reservoir capacity, injectivity and confinement need to be overcome when validating the performance of a storage system for its lifecycle. In the case of a failure of a storage operation, the environment, investments, and human health and safety, may be at risk. It is therefore important to use risk management methods to ensure that the project will meet its objectives in all aspects. The aims of risk management are both to identify and evaluate all the risks that could impact the project objectives, and to establish treatment, monitoring actions and plans to reduce the impact of risks thereby ensuring the project performance. This Chapter discusses the implementation of risk management for a CO2 geological storage project.