Masuda's sandstone core hydrate dissociation experiment revisited

Numerical simulation of hydrate dissociation in porous media is important to investigate future hydrate fuel extraction strategies and/or the impacts of climate change on the long-term stability of vulnerable near-surface hydrate deposits. The core-scale hydrate dissociation experiment of Masuda et al. (1999) represents an important experimental data set that can be used for benchmarking numerical simulators for this purpose. Data collected includes gas production, water production, boundary pressure and temperature from three internal observation points. At least six modeling studies exist within the literature seeking to simulate the gas production data and the temperature data. However, the pressure data and water production data are generally overlooked. In this article we present a set of numerical simulations capable of reconciling the Masuda et al. (1999) data set in its entirety. Improvements on existing modeling studies are achieved by: (1) using improved estimates of the initial hydrate saturation; (2) obtaining relative permeability parameters, a hydrate stability depression temperature and a convective heat transfer coefficient by calibration with the observed data; and (3) applying a new critical threshold permeability model, specifically to reconcile a relatively fast gas production with a relatively slow far-field boundary pressure response. A subsidiary finding is that permeability is significantly reduced in the presence of very low hydrate saturations. But more importantly, the multi-faceted effectiveness of the data set from Masuda’s experiment is clearly demonstrated for numerical simulation benchmarking in the future

Simulation of three-component two-phase flow in porous media using method of lines

Numerical simulation of compositional flow problems commonly involves the use of 1st- or 2nd-order Euler time stepping. Method of lines (MOL), using highly accurate and efficient ODE solvers, is an alternative technique which, although frequently applied to the solution of two-phase, two-component flow problems, has generally been overlooked for problems concerning more than two components. This article presents the development of a numerical simulator for 1D, compressible, two-phase, three-component, radially symmetric flow using the method of lines (MOL) and a 3rd-order accurate spatial discretization using a weighted essentially non-oscillatory (WENO) scheme. The MOL implementation enables application of the MATLAB ODE solver, ODE15s, for time integration. Simulation examples are presented in the context of CO2CO2 injection into a reservoir containing a mixture of CH4CH4 and H2OH2O. Following an assumption of constant equilibrium ratios for CO2CO2 and CH4CH4, a ternary flash calculator is developed providing closed-form relationships for exact interpolation between equations of state for CO2CO2–H2OH2O and CH4CH4–H2OH2O binary mixtures. The numerical code is successfully tested and verified for a range of scenarios by comparison with an existing analytical solution

Application of Hydrogeological parameters for evaluating the thermal resource potential for deep groundwater systems

Geothermal energy has significant global potential as a clean non-intermittent energy resource. Exploiting geothermal energy uses water which either flows naturally or is stimulated to flow in the sub-surface within deep aquifers or fractured basement. Therefore, it is necessary to understand fluid flow in the upper crust of the Earth (0–5 km depth). Fluid flow could be through waterbearing porous and permeable media (e.g. sandstones and limestones), fractured dry rocks or fluid filled fault zones. The UK has low to medium temperature geothermal resources related to past intrusive igneous activity. A thorough understanding of these low to medium temperature systems is particularly important, because their usefulness will only be realised by optimising site conditions from a geological and engineering standpoint. It is necessary not only to examine the temperatures at depth but to ensure that fluid flow is sufficient so that the geothermal resource is not quickly depleted. Conversely, we also need to ensure that any fluids removed for heat extraction can be re-injected elsewhere in the system to prevent discharge of warm, chemically unsuitable fluids to surface water courses. The requirement to understand these systems is critical for the UK because economic exploitation of a marginally productive resource relies upon the interplay of several finely balanced factors. This paper presents a hydrogeological evaluation of two geothermal case studies, one from north-east England and one from the North Sea. The applicability of these two case studies to other marginally productive geothermal areas is then discussed

A trigonometric interpolation approach to mixed-type boundary problems associated with permeameter shape factors

[1] Hydraulic conductivity is a fundamental hydrogeological parameter, whose in situ measurement at a local scale is principally performed through injection tests from screened probes or using impermeable packers in screened wells. The shape factor F [L] is a proportionality constant required to estimate conductivity from observed flow rate to injection head ratios, and it depends on the geometric properties of the flow field. Existing approaches for determination of F are either based on geometric or mathematical simplifications and are limited to particular assumptions about the flow domain's external boundaries. The present work presents a general semianalytical solution to steady state axisymmetric flow problems, where external boundaries may be nearby and of arbitrary combinations of impermeable and constant head type. The inner boundary along the probe or well may consist of an arbitrary number of impermeable and constant head intervals resulting in a mixed-type boundary value problem, for which a novel and direct solution method based on trigonometric interpolation is presented. The approach is applied to generate practical nondimensional charts of F for different field and laboratory situations. Results show that F is affected by less than 5% if a minimum distance of 10 probe or well diameters is kept between the injection screen and a nearby boundary. Similarly, minimum packer lengths of two well diameters are required to avoid increasing F by more than 10%. Furthermore, F is determined for laboratory barrel experiments giving guidelines for achieving equal shape factors as in field situations without nearby boundaries. F for the theoretical case of infinitely short packers is shown to be infinitely large

A statistical analysis of well production rates from UK oil and gas fields – Implications for carbon capture and storage

The number of wells required to dispose of global CO2 emissions by injection into geological formations is of interest as a key indicator of feasible deployment rate, scale and cost. Estimates have largely been driven by forecasts of sustainable injection rate from mathematical modelling of the CO2 injection process. Recorded fluid production rates from oil and gas fields can be considered an observable analogue in this respect. The article presents statistics concerning Cumulative average Bulk fluid Production (CBP) rates per well for 104 oil and gas fields from the UK offshore region. The term bulk fluid production is used here to describe the composite volume of oil, gas and water produced at reservoir conditions. Overall, the following key findings are asserted: (1) CBP statistics for UK offshore oil and gas fields are similar to those observed for CO2 injection projects worldwide. (2) 50% probability of non-exceedance (PNE) for CBP for oil and gas fields without water flood is around 0.35 Mt/yr/well of CO2 equivalent. (3) There is negligible correlation between reservoir transmissivity and CBP. (4) Study of net and gross CBP for water flood fields suggest a 50% PNE that brine co-production during CO2 injection could lead to a 20% reduction in the number of wells required

Analytical solution for clay plug swelling experiments

Clay swelling experiments frequently involve monitoring the one-dimensional displacement with time of an initially dry clay plug as it imbibes water from a supply at its base. This article presents a new analytical solution for interpreting such experiments based on Richards' equation for flow in a partially saturated porous medium combined with a linear empirical function relating moisture ratio with void ratio. The analytical solution is described by just two parameter groups. The first parameter group describes the swelling potential of the clay. The second parameter group describes the rate at which the swelling plug reaches equilibrium, which is controlled by permeability and capillary pressure. Application of the analytical solution is demonstrated by calibration to one-dimensional displacement data from clay swelling experiments for an illite and bentonite clay

On the importance of relative permeability data for estimating CO2 injectivity in brine aquifers

Performance assessment of possible CO2 storage schemes is often investigated through numerical simulation of the CO2 injection process. An important criterion of interest is the maximum sustainable injection rate. Relevant numerical models generally employ a multi-phase extension to Darcy's law, requiring data concerning the evolution of relative permeability for CO2 and brine mixtures with increasing CO2 saturation. Relative permeability data is acutely scarce for many geographical regions of concern and often cited as a major source of uncertainty. However, such data is expensive and time consuming to acquire. With a view to improving our understanding concerning the significance of relative permeability uncertainty on injectivity, this article presents a sensitivity analysis of sustainable CO2 injection rate with respect to permeability, porosity and relative permeability. Based on available relative permeability data obtained from 25 sandstone and carbonate cores discussed in the literature, injectivity uncertainty associated with relative permeability is found to be as high as ±57% for open aquifers and low permeability closed aquifers (100 mD), aquifer compressibility plays a more important role and the uncertainty due to relative permeability is found to reduce to ±6%

Recent Developments in Algorithmic Teaching

Abstract. The present paper surveys recent developments in algorith-mic teaching. First, the traditional teaching dimension model is recalled. Starting from the observation that the teaching dimension model some-times leads to counterintuitive results, recently developed approaches are presented. Here, main emphasis is put on the following aspects derived from human teaching/learning behavior: the order in which examples are presented should matter; teaching should become harder when the memory size of the learners decreases; teaching should become easier if the learners provide feedback; and it should be possible to teach infinite concepts and/or finite and infinite concept classes. Recent developments in the algorithmic teaching achieving (some) of these aspects are presented and compared.

Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV

Results are presented from a search for a W' boson using a dataset corresponding to 5.0 inverse femtobarns of integrated luminosity collected during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV. The W' boson is modeled as a heavy W boson, but different scenarios for the couplings to fermions are considered, involving both left-handed and right-handed chiral projections of the fermions, as well as an arbitrary mixture of the two. The search is performed in the decay channel W' to t b, leading to a final state signature with a single lepton (e, mu), missing transverse energy, and jets, at least one of which is tagged as a b-jet. A W' boson that couples to fermions with the same coupling constant as the W, but to the right-handed rather than left-handed chiral projections, is excluded for masses below 1.85 TeV at the 95% confidence level. For the first time using LHC data, constraints on the W' gauge coupling for a set of left- and right-handed coupling combinations have been placed. These results represent a significant improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe

Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV

A search for a Higgs boson decaying into two photons is described. The analysis is performed using a dataset recorded by the CMS experiment at the LHC from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross section of the standard model Higgs boson decaying to two photons. The expected exclusion limit at 95% confidence level is between 1.4 and 2.4 times the standard model cross section in the mass range between 110 and 150 GeV. The analysis of the data excludes, at 95% confidence level, the standard model Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The largest excess of events above the expected standard model background is observed for a Higgs boson mass hypothesis of 124 GeV with a local significance of 3.1 sigma. The global significance of observing an excess with a local significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is estimated to be 1.8 sigma. More data are required to ascertain the origin of this excess.Comment: Submitted to Physics Letters