Viktoriya M. Yarushina

(De)compaction of porous viscoelastoplastic media: Solitary porosity waves

Buoyancy-driven flow in deformable porous media is important for understanding sedimentary compaction as well as magmatic and metamorphic differentiation processes. Here mathematical analysis of the viscoplastic compaction equations is used to develop an understanding of the porosity wave instability and its sensitivity to the choice of rheological model. The conditions of propagation, size, speed, and shape of the porosity waves depend strongly on the properties of the solid rock frame. Whereas most of the previous studies on porosity waves were focused on viscous or viscoelastic mode, here we consider the ability of a solid matrix to undergo simultaneous plastic (rate-independent) and viscous (rate-dependent) deformation in parallel. Plastic yielding is identified as a cause of compaction-decompaction asymmetry in porous media—this is known to lead to a strong focusing of porous flow. Speed and amplitude of a porosity wave are given as functions of material parameters and a volume of a source region. Formulation is applicable to fluid flow in sedimentary rocks where viscous deformation is due to pressure solution as well as in deep crustal or upper mantle rocks deforming in a semibrittle regime.

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.

Water and CO2-permeability of Intact and Split Shale Core

We present new measurements of the bedding-parallel permeability of a single shale core to water and to supercritical CO2, under confined conditions. Furthermore, measurements were carried out on the same core plug after this had been split along its bedding. Our results show that with increasing effective confining pressure, permeability decreases due to (permanent) compaction, with both an instantaneous and a time-dependent component. Furthermore, measurements performed on the core after splitting only show an effect of the crack at low effective confining pressure, whereas at higher confining pressures there was no significant effect. Finally, a first measurement performed using CO2 rather than water as the transport fluid gave a four times higher permeability. However, more measurements will be performed to confirm this preliminary observation. Our results show that (time-dependent) compaction is an important factor controlling shale permeability. Furthermore, it is demonstrated that shale permeability may not be (permanently) affected by fracturing under confined conditions, as compaction may lead to crack closure. Accurate measurements of shale compaction and permeability are required to assess caprock integrity.

High Permeability Pathways Triggered by Subsurface Storage Operations

High permeability fluid flow pathways are widely observed in nature, and their formation occurs on both geological and human timescales. Outcrop study as well as interpretation of seismic cross-section show clear evidences of these multi-scale vertical pipe features, where unconsolidated to loose material is present inside. Even if well documented, their formation process remains still not answered yet. We propose a physically consistent 3D two-phase model of focusing fluid flow that allow the formation of such vertical high permeability channels. Viscous or creep rheology is the key feature to explain this formation process. Our result show that the proposed mechanism triggers pipe formation where permeability increases over two orders of magnitude in impermeable shale, and with propagation speed close to 2 meters per year in these usual ceiling rocks. Our results are in good accordance with the values needed to explain the fast vertical breakthrough of CO2 plume in the layered Sleipner saline aquifer.

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.