Dual porosity flow crash
Hello,
I'm trying to set up a EGS simulation where 120 °C water is injected into a 270 °C reservoir. The aim is to get the system up and running with TOUGHREACT but since those simulations keep on crashing after approximately 10 day simulation time I decided to get the flow conditions correct first and then re-visit the TOUGHREACT model. For the time being I have also turned off all flow of heat into the model. I have thought about this problem for quite a few days now and would appreciate any constructive criticism and comments.
The mesh I use is the following. It is 1 km x 10 m x 1 m. The x is divided into 100 ten meter long segments. There are two layers, a 5 mm wide fracture (horizontal) which has porosity of 0.99 and permeability of 7.33e-10 m2. The matrix layer is therefore 99.5 cm and has the porosity of 0.1 and permeability of 1e-13. The main part of water is therefore flowing through a fracture with cross section perpendicular to the water flow of 0.005m x 10m. At each of the X boundary there is a cell that has very large volume (volume factor of 1e50) and therefore fixes the pressure in the system.
I inject a 0.13 kg/s at x= 255 meters into the fracture cell and produce from a cell at x=745 meters. Following advice I got previously I'm not producing a constant mass out of the production cell but rather I use well on delivery. The initial pressure in the system is 100 bar and I maintain the pressure of the producing cell at 98 bar.
I calculate the productivity index according to equations 29 and 30 in the TOUGH manual (p. 64) and assume a well with diameter of 0.125 m. As mentioned above k is 7.33e-10 m2, deltaZ is 0.005m, A is 10m2 and therefore the PI is (assuming no skin factor) 1.07e-11.
The above conditions result in a run that crashes after 35 days of sim time. What I can't figure out is why. The pressure does not build up and below is a cross section of the pressure and temperature along the center of the fracture after 30 days (see below picture). The scale for pressure is 1e7 to 1.018e7 Pa and for temperature 120-280 °C
The figure also shows the setup of the model. The flow in and out are through the red cells.
Does anyone see what I could be doing wrong with my setup?
Many thanks, Bergur
1 reply
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Probably it is best that I answer myself since the problem has been sorted. When the injection fluid moved close to the left boundary the mesh, the simulation tended to crash. Here are the following changes I did to the model:
1. Increase the size of the mesh to x = 10 km and y and z = 10 meters. The injection and production are therefore further away from the boundary of the model. I however maintain the width of the fracture the same. I guess this is the most important amendment
2. Decrease DELTMX to 10 hours.
3. RE1 is now 1e-6, was 1e-5.
4. Introduce pore compressibility and expandability to the rocks (however these do not affect if this specific simulation crashes).
5. I now produce constant mass out of the production cell as it seems to run faster in most model setups I run.