pumping issue using SAM GETEM

I'm trying to run hypothetical EGS scenarios in SAM GETEM for varying temperatures and depths. However, in certain scenarios (e.g. depth of 5,000m, temperature of 155C, default inputs for injectivity index (3,000) and productivity index (2,500), production well flow rate of 60 kg/s per well) I run in to issues with the injection pump costs. These inputs result in a negative value for injection pump size, which propagates into injection pump costs of NaN that in turn results in a LCOE of NaN.

Should I manually overwrite the pump costs to avoid this 'NaN' issue, by using the calculated costs and adding injection pump cost as $0?

Also, intuitively, I'd assumed this error was arising because the BHP of the production well exceeded the hydrostatic pressure (i.e. it was an artesian well). However, an artesian production well would result in zero pumps in the production well and pumps only in the injection wells - the opposite of what is occurring here. Can you explain why only the injection pumps are affected, as this will greatly affect the economics based on the ratio of production to injection wells used?

Thanks in advance.

Hi Megan,

SAM should be setting the injection pump size to zero when the input parameters result in a negative value. We'll fix that for the next update to SAM, which we plan to release later this spring. In the meantime, your workaround should at least address the cost implications.

As for how well the model works for an artesian production well, I'm waiting for a reply from my colleague on the geothermal team and will follow up when I hear back.

Thanks,
Paul.

Hi Megan, 

On your question about how the GETEM in SAM handles artesian wells, we do not have an explicit case for artesian wells. The assumptions from the original Excel GETEM model stipulate:

• A production pumping requirement for low-to-medium grade systems (200C or less) as binary plants that require the wells to be pumped to meet a fixed power sale (or net power).
• No production pumping for flash systems and allowing for two-phase flow in the wellbore if reservoir pressure is not sufficient (since the fluid still gets flashed at the surface).

The artesian well might be closer to the assumption for a flash plant. However, in reality, you can tie an artesian well to a binary power cycle at the surface as long as you don’t develop two phase flow in the wellbore or at the surface. This will require the bottomhole pressure (reservoir/hydrostatic pressure – drawdown) minus frictional pressure drop in the well to be equal or above the fluid-dependent sum of the saturation (vapor) pressure and the excess pressure required to prevent cavitation and exsolution of non-condensable gases. On the other hand, you’ll need to compensate for the pressure drop at the surface by an injection pump in order to maintain reservoir pressure. Therefore, it is not physical to have a negative injection pressure for an artesian well (except there is an active pressure support from a neighboring reservoir that is compensating for the depleted pressure in the main reservoir).

Dayo