Spontaneous formation of fluid escape pipes from subsurface reservoirs

Abstract

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.

Pockmarks in the Witch Ground Basin, Central North Sea

“…It may act as a trap focusing and mixing migrating microbial methane from the Quaternary succession with thermogenic methane from deeper sources. Thermogenic fluids may migrate through the overburden from deeper layers (Figure b), for example, Kimmeridge clay (Judd et al, ), along faults (Chand et al, ), pipes, or chimneys (Karstens & Berndt, ; Räss et al, ). This is, however, not corroborated by our geochemical results and the seismic data do not resolve the depth extent of the pipe structures underneath the class 1 pockmarks to the reservoir rocks (e.g., Montrose and Piper sands), but this may be due to imperfect imaging.…”

Resolving thermomechanical coupling in two and three dimensions: spontaneous strain localization owing to shear heating

“…The significant advantage of the PT method is that it can be extended for high-resolution 3-D numerical simulations without significant modification of the 2-D algorithm and without a drastic increase in memory requirements as the latter scales linearly with the number of grid cells. We show that the PT method is suitable to perform high-resolution 3-D simulations of thermomechanically activated shear localization and other relevant coupled physics in geodynamics (Räss et al 2018). Further, the efficiency of the thermomechanical codes makes it suitable for systematic analysis of the parameters that control the dynamics of shear zone development.…”

Experimental Poroviscoelasticity of Common Sedimentary Rocks

“…A few models have been developed to describe the poroviscoelastic behavior of rock (e.g., Abousleiman et al, ; Coussy, ), and the principles behind them have recently been reviewed (Yarushina & Podladchikov, ). The influence of creep on fluid‐saturated rock has been shown to play an important role in the long‐term storage of nuclear waste (Belmokhtar et al, ; Gasc‐Barbier et al, ) and CO 2 (Le Guen et al, ; Liteanu et al, ; Räss et al, ). In general, geomaterials have a nonlinear rheology even when only partially saturated with water, and the time‐dependent behavior of reservoir rock is a function of the combination of the mean stress, deviatoric stress, pore pressure, pore fluid chemistry, temperature, and microstructural properties of the rock, among others (e.g., Brantut et al, ; Bürgmann & Dresen, ; Sone & Zoback, ; Yang et al, ; Zhang et al, ).…”