Abstract
This paper introduces a 3D model for chimney formations in tight rocks in sedimentary basins. This is an adaption of a model for hydraulic fracturing in an anisotropic stress field by fluid injection (fracking). The model assumes that a chimney formation is triggered and sourced by overpressure build-up in permeable units, such as reservoirs or aquifers. Cells in the numerical models fracture when the fluid pressure exceeds the least compressive stress and a random rock strength. Chimney growth is represented by chains of cells (branches) that emanate from the base of the cap rock. The branches have an enhanced permeability during ascension, because the fluid pressure in the fracture network is greater than the least compressive stress. When the branches reach the hydrostatic surface, the fluid pressure drops below the fracture pressure and the fracture network closes. The reservoir is drained by the branches in the closed fracture network that reaches the seafloor. The model produces pipe-like structures and chimneys as accumulations of branches that reach the surface. The degree of random rock strength controls how pipe-like the chimneys become. Chimney formation stops when the rate of fluid leakage through the chimneys surpasses the production of excess fluid by the overpressure-building process. A “low” permeability of the chimney branches produces wide chimneys with many branches, and a “high” permeability gives narrow chimneys made of just a few branches. The model is demonstrated in a setup that could be relevant for the chimneys observed in the cap rock over the Utsira aquifer in the North Sea. By using the proposed model, the permeability of such chimneys is estimated to be of the order of 10 μD.
Highlights
The rapid increase in the atmospheric CO2 concentrations over the last 100 years is likely the reason for current climate change (Bryant, 1997)
A physical model has been proposed here for the formation of the chimneys and pipe-like structures commonly observed in tight rocks in sedimentary basins
The chimney model is an extension of a model for hydraulic fracturing of tight rock by fluid injection, and builds on concepts of invasion percolation
Summary
The rapid increase in the atmospheric CO2 concentrations over the last 100 years is likely the reason for current climate change (Bryant, 1997). Chimneys have been interpreted as localised porous channels for fluid flow, and can often be traced to a reservoir formation (Løseth et al, 2009, 2011; Kartens and Berndt, 2015; Kartens et al, 2017; Räss et al, 2019) These vertical structures serve as leakage pathways through the seal for reservoir fluids, and are important with respect to the seal integrity of the reservoir units used for storing CO2. Iyer et al (2017) recently developed a finite-element model for hydrothermal venting in sedimentary basins driven by heat from magmatic intrusions This approach is similar to the model introduced here, with fractured elements occurring where the fluid pressure exceeds the least compressive stress. This paper proposes a model for the formation of chimneys and pipe-like structures that builds on a recent model for hydraulic fracturing and damage of low-permeability rocks by fracking operations (Wangen, 2017, 2019). Chimney development based on a reference case is discussed, and how the chimney structure depends on the chimney permeability is demonstrated, before the chimney development is related to the volume balance of the pore fluid
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
