[Daumic (OP)]

There is a great hope to obtain ethanol with cellulosic materials. This hope is based on the depolymerization of cellulose into sugars. Sugars are then fermented with yeasts to produce ethanol.

Some processes are studied to obtain cellulose depolymerization:
-   thermohydrolyse where cellulose is treated by hot water under pressure (1),
-   acidic hydrolyse where cellulose is treated with hot diluted acid,
-   alkaline hydrolyse where cellulose is treated with ammonia,
-   enzymatic hydrolyse where cellulose is treated with enzymes.

The simplest process remains the treatment by hot water under pressure. But high temperature and high pressure need a pressurized furnace. This apparatus makes the process costly. To reduce this cost, it can be possible to use the vertical part of depleted wells of shale mining. Since 2005, hydraulic fracturing has permit the exploitation of gas and oil confined in shale deposits. This extraction lasts some years, after that the well is neutralized with cement. Instead of this neutralization, the depleted well can be transformed into a reactor to hydrolyse cellulosic material. For that:
-   the deep end of vertical well is clogged,
-   a smaller pipe is inserted in the pipe in place.

The treatment of cellulosic material with the modified well can be:
-   the cellulosic material is grinded in water to form a mud,
-   this mud is injected in the inner pipe of the well,
-   the mud fills the pipe until the deep end,
-   the mud rises to the ground in the space between inner pipe and outer pipe,
-   the mud is filtered to separate untreated cellulose and soluble sugar,
-   the solution of sugar is sent to fermenting plant.

The depolymerization of cellulosic material is obtained by:
-   the hydrostatic pressure of the column of mud,
-   the temperature of the surrounding rocks heated by geothermal energy.

The maximal values of pressure and temperature depend of the deep of well:

Deep in km   Temperature of deep end in °C   Pressure of deep end in bar
 2                    80                                                   200
 3                    110                                                 300
 4                    140                                                 400
 5                    170                                                 500

Recycling depleted shale wells as natural reactors can reduce the cost of cellulose depolymerization.

Perhaps the mining of shale deposits isn’t the enemy of energy transition.

(1)https://www.academia.edu/44565223/Conversion_of_lignocellulosics_pretreated_with_liquid_hot_water_to_ethanol