2nd EAGE Workshop on Fluid Flow in Faults and Fracture - Modelling, Uncertainty and Risk

Not Provided

A large cross-disciplinary research project funded by a mining company in the Galilee Basin (Queensland) was undertaken from August 2020 to December 2021. This project involved six individual researchers / practitioners with expertise in geology, geophysics, structural geology, hydrogeology and geomechanics. The purpose of the project was to investigate the potential for vertical transfer of groundwater through the Rewan Formation (regarded as a regional aquitard) via preferential flow paths within faults, fractures or connected lithological units. Using multiple lines of evidence from different disciplines, the project was able to determine that: (a) the critical deformation mechanism within Triassic sedimentation is layer parallel shear with no need to invoke significant faulting; (b) there are no faults with demonstrable displacement; (c) the juxtaposition of lithologies across a fault plane is overwhelmingly self-juxtaposition; (d) a complete breach of the Rewan Formation by a fault is impossible; (e) on acoustic scans, there are only rare occurrences of discontinuities that could be potential conduits with enhanced flow; (f) acoustic scans show no lateral continuity of fractures between holes only 30 m apart; (g) in situ fractures will be generally compressible and mechanically weak due to relatively low elastic moduli; (h) geochemical analysis confirms that ecologically important springs have a shallow source (above the Rewan Formation); and (i) the Rewan Formation is confirmed to be dominantly argillaceous.

Modelling of single- and multi-phase flow in fractured subsurface systems is critical in applications relating to the environment, energy, and resource extraction. However, techniques for quantifying the effect of fracture characteristics such as roughness and wettability, as well as fluid flow regimes, on hydraulic properties (e.g., relative permeability) are not described in the literature. Often fractures are approximated as parallel plates (at some level of locality), ignoring intrinsic roughness and decorrelation between surfaces. This leads to the well-known Cubic or Local Cubic Law for single-phase flow, of which many modifications have been proposed to cater for various pore-scale flow phenomena. Although there is a wide range of available literature working within such a methodology, the realisation of a general, robust model based on expected properties of the fracture surface remains a challenge. In comparison, two-phase fracture flows see significantly less development in the literature, with only few studies characterising permeability-saturation curves as a function of fracture properties. Roughness and wettability impact pore-scale flow and can lead to earlier transition between various displacement regimes and govern the relative permeability of the fluid phases present in a fracture.

This work aims to develop a framework for studying the impact of fracture-scale phenomena and upscaling it to the level of Discrete Fracture Networks, and then into field-scale analysis. This talk will provide examples of the studies conducted for single- and two-phase flow at the fracture-scale and discuss the methodology of upscaling the observed behavior while quantifying the uncertainty introduced by fracture characteristics (e.g., topology, wettability).

Not Provided

Measured in-situ stress data represents the stress states in a particular location and, hence, are considered as pointwise data. However, in most geomechanical analysis we need stress information outside of stress measurement domains (i.e., stress data gaps). There are different methods to estimate the stress state in these data gap regions. In this study we use a 3D geomechanical-numerical approach to predict a continuous description of stress state throughout a large rock volume. The best-fit geomechanical model contains all parameters of the 3D stress tensor for the northern Bowen Basin and is used to characterize different fault behaviours (such as dilation and slip tendency) in the present-day stress regime.

An automated workflow to convert static geological models with non-vertical faults into groundwater flow models is presented. Faults in the numerical model are defined geometrically and parametrically. The proposed method is tested with a multi-layer geological model around a complex faulting system, proving its efficiency to simulate anisotropic groundwater flows due to the presence of non-vertical faults.

Formation dry‐out in fracture‐dominated geological reservoirs may alter the fracture space, impair rock absolute permeability, and cause a significant decrease in well injectivity, which may have consequences for the feasibility of projects related to geological storage of CO2 or CO2-based enhanced geothermal systems. The potential physico‐chemical interactions between dry supercritical CO2, the reservoir fluid, and reservoir rocks may cause formation dry‐out, whereby minerals precipitate due to continuous evaporation of water into the CO2 stream. Depending on the spatial distribution of the mineral precipitate within fracture space, the absolute permeability can be significantly impaired.

In this study, we numerically model the dry‐out processes occurring during supercritical CO2 injection into single brine‐filled fractures and evaluate the potential for salt precipitation under increasing effective normal stresses in the evaporative regime. We use an open‐source, parallel finite‐element framework to numerically model two‐ phase flow through 2D fracture planes with aperture fields taken from naturally fractured granite cores at the Grimsel Test Site in Switzerland. Our results reveal a displacement front and a subsequent dry‐out front in all simulated scenarios, where higher effective stresses result in flow channelling, higher rates of water evaporation, and larger volumes of salt precipitate. However, despite the larger salt volumes, the permeability impairment was lower at higher effective normal stresses. We conclude that the spatial distribution of the salt precipitated in fractures with heterogeneous aperture fields, strongly affects the absolute permeability impairment caused by formation dry‐out. The numerical simulations assist in understanding the behaviour of the injectivity in fractures and fracture networks during subsurface applications that involve CO2 injection into brine.