Commit fa9f85c2 authored by Holger Class's avatar Holger Class

update in the fractures description

parent fc99ebe2
......@@ -23,7 +23,7 @@ Hydraulic fracturing typically requires a horizontally deviated wellbore which i
The stimulation period, where fractures are generated in the shale layers, is relatively short, a few hours, for example, while the subsequent gas production period can continue for a long time. Potentially hazardous events related to fracking are manifold and many of them do not need our flow and transport models to be prevented and managed, like spilling of fluids and accidents at the surface, not properly sealed wells, etc. However, there are hazards that are inherently related to subsurface flow processes and hydrogeological background, and these are those which are associated with enormous uncertainties. This makes risk assessment a difficult task and a delicate issue for communication to stakeholders and public opinion. It is precisely the short period of stimulation when these hazards are initiated and triggered. They can roughly be distinguished in three categories: (i) The release of fracking fluid in the subsurface which may harm groundwater resources, (ii) the uncontrolled migration of methane released from the shale into the overburden, eventually also into aquifers or even to the surface, and (iii) induced seismicity, which means microseismic events, small earthquakes, triggered by the high pressure during the stimulation period.
The example we have here in this exercise is strongly simplified and shows only in principile the effects of the two-phase flow system in the low-permeable matrix and the high-permeable fractures with high pressure gradients during the injection and buoyancy (methane is much less dense than water) as the main driving forces.
The example we have here in this exercise is strongly simplified and shows only in principle the effects of the two-phase flow system in the low-permeable matrix and the high-permeable fractures with high pressure gradients during the injection and buoyancy (methane is much less dense than water) as the main driving forces. Thus, the example addresses only the second above-mentioned category - methane migration.
The example assumes that we have a shale where water is injected at the bottom over a certain time (see input-file). The fracture network is pre-existing (which would actually be generated only during the injection!).
\subsection*{Questions}
......@@ -41,4 +41,7 @@ The example assumes that we have a shale where water is injected at the bottom o
permeability values, etc.
In particular take care for the injection pressures at the bottom. Pressures higher
than 100 MPa not realistic to achieve.
\item Advanced: Change the boundary conditions such that you model a scenario with gas production through an
assumed horizontal well at the bottom of the domain. Try to set a Dirichlet pressure condition at the bottom boundary
to apply underpressure.
\end{enumerate}
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