Ludovic Räss

Spontaneous formation of fluid escape pipes from subsurface reservoirs

Ubiquitous observations of channelised fluid flow in the form of pipes or chimney-like features in sedimentary sequences provide strong evidence for significant transient permeability-generation in the subsurface. Understanding the mechanisms and dynamics for spontaneous flow localisation into fluid conductive chimneys is vital for natural fluid migration and anthropogenic fluid and gas operations, and in waste sequestration. Yet no model exists that can predict how, when, or where these conduits form. Here we propose a physical mechanism and show that pipes and chimneys can form spontaneously through hydro-mechanical coupling between fluid flow and solid deformation. By resolving both fluid flow and shear deformation of the matrix in three dimensions, we predict fluid flux and matrix stress distribution over time. The pipes constitute efficient fluid pathways with permeability enhancement exceeding three orders of magnitude. We find that in essentially impermeable shale (10−19 m2), vertical fluid migration rates in the high-permeability pipes or chimneys approach rates expected in permeable sandstones (10−15 m2). This previously unidentified fluid focusing mechanism bridges the gap between observations and established conceptual models for overcoming and destroying assumed impermeable barriers. This mechanism therefore has a profound impact on assessing the evolution of leakage pathways in natural gas emissions, for reliable risk assessment for long-term subsurface waste storage, or CO2 sequestration.

M2Di: Concise and efficient MATLAB 2-D Stokes solvers using the Finite Difference Method

Recent development of many multiphysics modeling tools reflects the currently growing interest for studying coupled processes in Earth Sciences. The core of such tools should rely on fast and robust mechanical solvers. Here we provide M2Di, a set of routines for 2-D linear and power law incompressible viscous flow based on Finite Difference discretizations. The 2-D codes are written in a concise vectorized MATLAB fashion and can achieve a time to solution of 22 s for linear viscous flow on 10002 grid points using a standard personal computer. We provide application examples spanning from finely resolved crystal-melt dynamics, deformation of heterogeneous power law viscous fluids to instantaneous models of mantle flow in cylindrical coordinates. The routines are validated against analytical solution for linear viscous flow with highly variable viscosity and compared against analytical and numerical solutions of power law viscous folding and necking. In the power law case, both Picard and Newton iterations schemes are implemented. For linear Stokes flow and Picard linearization, the discretization results in symmetric positive-definite matrix operators on Cartesian grids with either regular or variable grid spacing allowing for an optimized solving procedure. For Newton linearization, the matrix operator is no longer symmetric and an adequate solving procedure is provided. The reported performance of linear and power law Stokes flow is finally analyzed in terms of wall time. All MATLAB codes are provided and can readily be used for educational as well as research purposes. The M2Di routines are available from Bitbucket and the University of Lausanne Scientific Computing Group website, and are also supplementary material to this article.

Shear-induced Dilation and its Implications for Chimney Flow in Porous Rocks

new model of shear-induced dilation and shear-enhanced compaction in brittle (elastic) and ductile (viscous) rocks is proposed. The essential feature of the model is the dependence of the porosity equation on the equivalent shear stress. This allows dilation of the pore space even at nominally compressive effective pressures in agreement with experimental data. The implications for the formation of fluid- or gas-filled chimneys are considered. Spontaneous self-localization of Darcy flow in a deforming porous rock due to preferential dilation of the pore space is a viable mechanism for chimney formation.

The Importance of Poromechanics for CO2 Storage Integrity

Understanding deformation related to CO2 injection into subsurface reservoirs is crucial to ensure storage integrity, but our understanding is hampered by the lack of models that capture the non-linear interaction between fluid flow and complexly deforming rocks. We developed new fully coupled (i.e. the solid feels the fluid pressure and fluid flow is affected by solid stresses and deformation of the porous rock) models that allow the simulation of CO2 injection and flow in a stressed crust that may deform visco-elastically, including non-linear rheology leading to weakening. These simulations reproduce features observed in CO2 injection operations, such as localization of flow into chimneys or channels and flow that is (locally and periodically) faster than predicted by Darcy flow. The model also allows investigating the effect of injection on the over-, under- and sideburden of the reservoir and to predict the effect of stress changes in the neighbouring formations. Comparison of a fully coupled with an incompletely coupled simulation illustrates the necessity of proper coupling.