An injection well can be pumped or free-flowing:

*A pumped injection well*: (for all mass flowrates:)

- the fluid has to be pumped into the well
- the wellhead pressure is positive
- the fluid is liquid throughout the well.

Pumped wells are covered in earlier tutorials.

*A free-flowing injection well*: (perhaps only at low mass flowrates):

- usually has a deep water level when closed
- the fluid flows freely into the well
- the wellhead pressure is zero or close to zero
- at the top of the well, there is a section where the fluid may boil and form a mixture of liquid and steam, depending on its temperature; below this, the fluid is liquid.

This tutorial covers free-flowing wells. It assumes you are familiar with earlier tutorials about injection and injectivity simulations for pumped wells.

To work with free-flowing wells in WellSim, you must measure the depth of the top of the liquid while the fluid is flowing into the well. This is the `Start depth`

for injection simulations. For free-flowing wells, `Start depth`

is greater than zero (for pumped wells, `Start depth`

is zero).

For the mass flowrates that do not need to be pumped, enter the measured depth of the top of the liquid in the well at that flowrate (while the fluid is flowing into the well). The measured wellhead pressure is zero or close to zero at these flowrates.

Here is example measured injectivity data for a free-flowing well. For the 80 t/hr mass flowrate, the measured depth of the top of the liquid is 285 m and wellhead pressure is (close to) zero:

Here is an injection simulation using the measurements from the above measured injectivity curve at 80 t/hr, where the fluid does not need to be pumped into the well:

*Note*:

- Always enter a wellhead pressure of zero. For some wells, particularly if the wellhead fluid temperature is above 100 C, the fluid flows freely into the well but the wellhead pressure is slightly above zero, perhaps a few bar. In these cases, WellSim will calculate the correct wellhead pressure.
- The start depth is the depth of the top of the liquid in the well, 285 m.
- WellSim only calculates fluid parameters below the top of the liquid in the well, 285 m.
- The finish depth is the depth of the deepest feedzone, as always.

Here is a linear TestData injectivity simulation using the measurements from the measured injectivity curve above:

For such injectivity simulations, the start depth is the wellhead, because the simulation needs to be over the entire length of the well. And the finish depth is the depth of the deepest feedzone, as always.

Here's the graph of this injectivity simulation, with the testdata (red) and the calculated data (black):

To change the series and point colours and shapes, go to `Graph > Graph settings`

and click `Schema`

at the bottom left of the settings window.