Acoustic and petrophysical properties of mechanically compacted overconsolidated sands: part 1 – experimental results

This paper part one is set out to lay primary observations of experimental compaction measurements to form the basis for rock physics modelling in paper part two. P- and S-wave velocities and corresponding petrophysical (porosity and density) properties of seven unconsolidated natural sands with different mineralogical compositions and textures are reported. The samples were compacted in a uniaxial strain configuration from 0.5 up to 30 MPa effective stresses. Each sand sample was subjected to three loading cycles to study the influence of stress reduction on acoustic velocities and rock physical properties with the key focus on simulating a complex burial history with periods of uplift. Results show significant differences in rock physical properties between normal compaction and overconsolidation (unloaded and reloaded). The differences observed for total porosity, density, and P- and S-wave velocities are attributed to irrecoverable permanent deformation. Microtextural differences affect petrophysical, acoustic, elastic and mechanical properties, mostly during normal consolidation but are less significant during unloading and reloading. Different pre-consolidation stress magnitudes, stress conditions (isotropic or uniaxial) and mineral compositions do not significantly affect the change in porosity and velocities during unloading as a similar steep velocity–porosity gradient is observed. The magnitude of change in the total porosity is low compared to the associated change in P- and S-wave velocities during stress release. This can be explained by the different sensitivity of the porosity and acoustic properties (velocities) to the change in stress. Stress reduction during unloading yields maximum changes in the total porosity, P- and S-wave velocities of 5%, 25%, and 50%, respectively. These proportions constitute the basis for the following empirical (approximation) correlations: Δϕ ∼ ±5 ΔV_P and ΔV_P ∼ ±2ΔV_S. The patterns observed in the experiments are similar to well log data from the Barents Sea. Applications to rock physics modelling and reservoir monitoring are reported in a companion paper.     

Pressure Buildup and Brine Migration During CO2 Storage in Multilayered Aquifers

Carbon dioxide injection into deep saline formations may induce large‐scale pressure increases and migration of native fluid. Local high‐conductivity features, such as improperly abandoned wells or conductive faults, could act as conduits for focused leakage of brine into shallow groundwater resources. Pressurized brine can also be pushed into overlying/underlying formations because of diffuse leakage through low‐permeability aquitards, which occur over large areas and may allow for effective pressure bleed‐off in the storage reservoirs. This study presents the application of a recently developed analytical solution for pressure buildup and leakage rates in a multilayered aquifer‐aquitard system with focused and diffuse brine leakage. The accuracy of this single‐phase analytical solution for estimating far‐field flow processes is verified by comparison with a numerical simulation study that considers the details of two‐phase flow. We then present several example applications for a hypothetical CO₂ injection scenario (without consideration of two‐phase flow) to demonstrate that the new solution is an efficient tool for analyzing regional pressure buildup in a multilayered system, as well as for gaining insights into the leakage processes of flow through aquitards, leaky wells, and/or leaky faults. This solution may be particularly useful when a large number of calculations needs to be performed, that is, for uncertainty quantification, for parameter estimation, or for the optimization of pressure‐management schemes.     

Particle tracking in the vicinity of Helgoland,North Sea: a model comparison

Station Helgoland Roads in the south-eastern North Sea (German Bight) hosts one of the richest long-term time series of marine observations. Hydrodynamic transport simulations can help understand variability in the local data brought about by intermittent changes of water masses. The objective of our study is to estimate to which extent the outcome of such transport simulations depends on the choice of a specific hydrodynamic model. Our basic experiment consists of 3,377 Lagrangian simulations in time-reversed mode initialized every 7 h within the period Feb 2002–Oct 2004. Fifty-day backward simulations were performed based on hourly current fields from four different hydrodynamic models that are all well established but differ with regard to spatial resolution, dimensionality (2D or 3D), the origin of atmospheric forcing data, treatment of boundary conditions, presence or absence of baroclinic terms, and the numerical scheme. The particle-tracking algorithm is 2D; fields from 3D models were averaged vertically. Drift simulations were evaluated quantitatively in terms of the fraction of released particles that crossed each cell of a network of receptor regions centred at the island of Helgoland. We found substantial systematic differences between drift simulations based on each of the four hydrodynamic models. Sensitivity studies with regard to spatial resolution and the effects of baroclinic processes suggest that differences in model output cannot unambiguously be assigned to certain model properties or restrictions. Therefore, multi-model simulations are needed for a proper identification of uncertainties in long-term Lagrangian drift simulations.