Simulation of multiphase flow in fractured reservoirs using a fracture-only model with transfer functions

We present a fracture-only reservoir simulator for multiphase flow: the fracture geometry is modeled explicitly, while fluid movement between fracture and matrix is accommodated using empirical transfer functions. This is a hybrid between discrete fracture discrete matrix modeling where both the fracture and matrix are gridded and dual-porosity or dual-permeability simulation where both fracture and matrix continua are upscaled. The advantage of this approach is that the complex fracture geometry that controls the main flow paths is retained. The use of transfer functions, however, simplifies meshing and makes the simulation method considerably more efficient than discrete fracture discrete matrix models. The transfer functions accommodate capillary- and gravity-mediated flow between fracture and matrix and have been shown to be accurate for simple fracture geometries, capturing both the early- and late-time average behavior. We verify our simulator by comparing its predictions with simulation results where the fracture and matrix are explicitly modeled. We then show the utility of the approach by simulating multiphase flow in a geologically realistic fracture network. Waterflooding runs reveal the fraction of the fracture–matrix interface area that is infiltrated by water so that matrix imbibition can occur. The evolving fraction of the fracture–matrix interface area turns out to be an important characteristic of any particular fracture system to be used as a scaling parameter for capillary driven fracture–matrix transfer.     

Numerical Simulation and Observational Analysis of the Bora of Pag’s Ribs

The severe bora case that lasted from 13 to 15 November 2004 has been selected for the analysis of the bora of Pag’s ribs, which occurs in the northern part of the eastern Adriatic coast over the Pag island area (Croatia). According to the measurements from automatic stations, the MM5 numerical model is successful in the 10-min mean wind speed prediction at 10-m height. The vertical analysis of the wind speed and potential temperature also gave satisfactory results. At the commencement of the bora the modelled wind had a magnitude of 20ms⁻¹ at 10-m height in the Pag island area, which sharply attenuated in the cross direction and to the open sea. In this way the model has proved successful in predicting the characteristics of the bora of Pag’s ribs. Potential vorticity (PV) at 600m has lower values within PV banners than during the developed bora. The consequence is a strong jet emanating from the nearest gap. The vertical cross-sections through the centre of the gap point out a permanent hydraulic-like flow. At the time of the bora of Pag’s ribs the highest modelled turbulent kinetic energy is found in the jump-like region above the inversion and within the boundary layer along the lower boundary, ranging from 6–8m² s⁻². It is concluded that the dissipation in the hydraulic jumps and the wave breaking regions are the reasons for PV generation.     

Atmosphere–Surface Fluxes of CO<Subscript>2</Subscript> using Spectral Techniques

Different flux estimation techniques are compared here in order to evaluate air–sea exchange measurement methods used on moving platforms. Techniques using power spectra and cospectra to estimate fluxes are presented and applied to measurements of wind speed and sensible heat, latent heat and CO₂ fluxes. Momentum and scalar fluxes are calculated from the dissipation technique utilizing the inertial subrange of the power spectra and from estimation of the cospectral amplitude, and both flux estimates are compared to covariance derived fluxes. It is shown how even data having a poor signal-to-noise ratio can be used for flux estimations.     

Sensitivity of North Atlantic subpolar gyre and overturning to stratification-dependent mixing: response to global warming

We use a reduced complexity climate model with a three-dimensional ocean component and realistic topography to investigate the effect of stratification-dependent mixing on the sensitivity of the North Atlantic subpolar gyre (SPG), and the Atlantic meridional overturning circulation (AMOC), to idealized CO₂ increase and peaking scenarios. The vertical diffusivity of the ocean interior is parameterized as κ ∼ N ^−α, where N is the local buoyancy frequency. For all parameter values 0 ≤ α ≤ 3, we find the SPG, and subsequently the AMOC, to weaken in response to increasing CO₂ concentrations. The weakening is significantly stronger for α ≥ α_cr ≈ 1.5. Depending on the value of α, two separate model states develop. These states remain different after the CO₂ concentration is stabilized, and in some cases even after the CO₂ concentration has been decreased again to the pre-industrial level. This behaviour is explained by a positive feedback between stratification and mixing anomalies in the Nordic Seas, causing a persistent weakening of the SPG.     

Available energy and energy balance closure at four coniferous forest sites across Europe

The available energy (AE), driving the turbulent fluxes of sensible heat and latent heat at the earth surface, was estimated at four partly complex coniferous forest sites across Europe (Tharandt, Germany; Ritten/Renon, Italy; Wetzstein, Germany; Norunda, Sweden). Existing data of net radiation were used as well as storage change rates calculated from temperature and humidity measurements to finally calculate the AE of all forest sites with uncertainty bounds. Data of the advection experiments MORE II (Tharandt) and ADVEX (Renon, Wetzstein, Norunda) served as the main basis. On-site data for referencing and cross-checking of the available energy were limited. Applied cross checks for net radiation (modelling, referencing to nearby stations and ratio of net radiation to global radiation) did not reveal relevant uncertainties. Heat storage of sensible heat J _H, latent heat J _E, heat storage of biomass J _veg and heat storage due to photosynthesis J _C were of minor importance during day but of some importance during night, where J _veg turned out to be the most important one. Comparisons of calculated storage terms (J _E, J _H) at different towers of one site showed good agreement indicating that storage change calculated at a single point is representative for the whole canopy at sites with moderate heterogeneity. The uncertainty in AE was assessed on the basis of literature values and the results of the applied cross checks for net radiation. The absolute mean uncertainty of AE was estimated to be between 41 and 52 W m^?2 (10–11 W m^?2 for the sum of the storage terms J and soil heat flux G) during mid-day (approximately 12% of AE). At night, the absolute mean uncertainty of AE varied from 20 to about 30 W m^?2 (approximately 6 W m^?2 for J plus G) resulting in large relative uncertainties as AE itself is small. An inspection of the energy balance showed an improvement of closure when storage terms were included and that the imbalance cannot be attributed to the uncertainties in AE alone.     

Relative Kinematics of the Leading Edge and the Prominence in Coronal Mass Ejections

We present a statistical analysis of the relationship between the kinematics of the leading edge and the eruptive prominence in coronal mass ejections (CMEs). We study the acceleration phase of 18 CMEs in which kinematics was measured from the pre-eruption stage up to the post-acceleration phase. In all CMEs, the three part structure (the leading edge, the cavity, and the prominence) was clearly recognizable from early stages of the eruption. The data show a distinct correlation between the duration of the leading edge (LE) acceleration and eruptive prominence (EP) acceleration. In the majority of events (78%) the acceleration phase onset of the LE is very closely synchronized (within ± 20 min) with the acceleration of EP. However, in two events the LE acceleration started significantly earlier than the EP acceleration (> 50 min), and in two events the EP acceleration started earlier than the LE acceleration (> 40 min). The average peak acceleration of LEs (281 m s⁻²) is about two times larger than the average peak acceleration of EPs (136 m s⁻²). For the first time, our results quantitatively demonstrate the level of synchronization of the acceleration phase of LE and EP in a rather large sample of events, i.e., we quantify how often the eruption develops in a “self-similar” manner.