Simulations of thermal upgrading methods for oil shale reservoirs

This paper analyzes reaction and thermal front development in porous reservoirs with reacting flows, such as those encountered in shale oil extraction. A set of dimensionless parameters and a 3D code are developed in order to investigate the important physical and chemical variables of such reservoirs when heated by in situ methods. This contribution builds on a 1D model developed for the precursor study to this work. Theory necessary for this study is presented, namely shale decomposition chemical mechanisms, governing equations for multiphase flow in porous media and necessary closure models. Plotting the ratio of the thermal wave speed to the fluid speed allows one to infer that the reaction wave front ends where this ratio is at a minimum. The reaction front follows the thermal front closely, thus allowing assumptions to be made about the extent of decomposition solely by looking at thermal wave progression. Furthermore, this sensitivity analysis showed that a certain minimum permeability is required in order to ensure the formation of a traveling thermal wave. It was found that by studying the non-dimensional governing parameters of the system one can ascribe characteristic values for these parameters for given initial and boundary conditions. This allows one to roughly predict the performance of a particular method on a particular reservoir given approximate values for initial and boundary conditions. Channelling and flow blockage due to carbon residue buildup impeded each method's performance. Blockage was found to be a result of imbalanced heating. Copyright 2012, Society of Petroleum Engineers.

Hydrodynamic pressures behind flexible and rigid dams

During earthquakes, hydrodynamic pressures are generated by the impounded reservoir on the dam face. The magnitude and distribution of the hydrodynamic pressures vary with factors such as frequency and intensity of earthquake-induced ground motion, depth of impounded reservoir, stiffness of dam and geological conditions. It is difficult to obtain experimental data on hydrodynamic pressures from the field owing to uncertainties associated with earthquake loading. This paper aims at using dynamic centrifuge modelling to measure hydrodynamic pressures behind both relatively stiff and flexible model dams. Comparisons of the experimental data with theoretical hydrodynamic pressures show that Westergaard's equation gives a conservative estimation of hydrodynamic pressures. Comparison with Chopra's method revealed that it underpredicts hydrodynamic pressures for low reservoir depths but gives reasonably good predictions for higher depths of reservoir. It is concluded that dynamic centrifuge modelling may be an effective experimental method to estimate the hydrodynamic pressures acting on a dam. © 2010 Thomas Telford Ltd.

Electricity Storage Using a Thermal Storage Scheme

The increasing use of renewable energy technologies for electricity generation, many of which have an unpredictably intermittent nature, will inevitably lead to a greater demand for large-scale electricity storage schemes. For example, the expanding fraction of electricity produced by wind turbines will require either backup or storage capacity to cover extended periods of wind lull. This paper describes a recently proposed storage scheme, referred to here as Pumped Thermal Storage (PTS), and which is based on "sensible heat" storage in large thermal reservoirs. During the charging phase, the system effectively operates as a high temperature-ratio heat pump, extracting heat from a cold reservoir and delivering heat to a hot one. In the discharge phase the processes are reversed and it operates as a heat engine. The round- trip efficiency is limited only by process irreversibilities (as opposed to Second Law limitations on the coefficient of performance and the thermal efficiency of the heat pump and heat engine respectively). PTS is currently being developed in both France and England. In both cases, the schemes operate on the Joule-Brayton (gas turbine) cycle, using argon as the working fluid. However, the French scheme proposes the use of turbomachinery for compression and expansion, whereas for that being developed in England reciprocating devices are proposed. The current paper focuses on the impact of the various process irreversibilities on the thermodynamic round-trip efficiency of the scheme. Consideration is given to compression and expansion losses and pressure losses (in pipe-work, valves and thermal reservoirs); heat transfer related irreversibility in the thermal reservoirs is discussed but not included in the analysis. Results are presented demonstrating how the various loss parameters and operating conditions influence the overall performance.

Thermodynamic analysis of pumped thermal electricity storage

The increasing use of renewable energy technologies for electricity generation, many of which have an unpredictably intermittent nature, will inevitably lead to a greater need for electricity storage. Although there are many existing and emerging storage technologies, most have limitations in terms of geographical constraints, high capital cost or low cycle life, and few are of sufficient scale (in terms of both power and storage capacity) for integration at the transmission and distribution levels. This paper is concerned with a relatively new concept which will be referred to here as Pumped Thermal Electricity Storage (PTES), and which may be able to make a significant contribution towards future storage needs. During charge, PTES makes use of a high temperature-ratio heat pump to convert electrical energy into thermal energy which is stored as ‘sensible heat’ in two thermal reservoirs, one hot and one cold. When required, the thermal energy is then converted back to electricity by effectively running the heat pump backwards as a heat engine. The paper focuses on thermodynamic aspects of PTES, including energy and power density, and the various sources of irreversibility and their impact on round-trip efficiency.

Increased gas production from hydrates by combining depressurization with heating of the wellbore

To extract gas from hydrate reservoirs, it has to be dissociated in situ. This endothermic dissociation process absorbs heat energy from the formation and pore fluid. The heat transfer governs the dissociation rate, which is proportional to the difference between the actual temperature and the equilibrium temperature. This study compares three potential gas production schemes from hydrate-bearing soil, where the radial heat transfer is governing. Cylindrical samples with 40% pore-filling hydrate saturation were tested. The production tests were carried out over 90 min by dissociating the hydrate from a centered miniature wellbore, by either lowering the pressure to 6, 4, or 6 MPa with simultaneous heating of the wellbore to 288 K. All tests were replicated by a numerical simulation. With additional heating at the same wellbore pressure, the gas production from hydrates could, on average, be increased by 1.8 and 3.6 times in the simulation and experiments, respectively. If the heat influx from the outer boundary is limited, a simulation showed that the specific heat of the formation is rapidly used up when the wellbore is only depressurized and not heated. © 2012 American Chemical Society.

Imaging the failure of a rock-fill dam following liquefaction

Rock-fill dams are popular in developing countries due to their ease of construction and use of local materials. They are used to store water and to provide flood defences. The presence of such dams in earthquake-prone regions poses risks, particularly from ground liquefaction. In this paper, results from physical model tests on dams with different configurations are presented. Model dams with impermeable cores including sheet pile walls and clay cores were tested and the effect of reservoir water was investigated. High-speed photography was used to capture the response of the model dams allowing the movement of foundation soil below the dam to be established. It is concluded that the stiffness of the impermeable core has a significant influence on the ultimate deformation of the dam. The presence of reservoir water led to increased downstream movements of the dam and differential settlements between the upstream and downstream sides.

Current research in embankment dam engineering at imperial college London

This paper gives a short introduction to two research streams in embankment dam engineering underway at Imperial College London. The first is the modelling of embankment dam behaviour during earthquakes and the second is an investigation into the susceptibility of granular filters to internal erosion. The research motivation, methods and expected outcomes of each stream are discussed.