A helium prospect without a trap

After the initial hype of helium exploration in the Rukwa basin in Tanzania, things went quiet for 18 months. This is partially because Helium One’s exploration permit expired, and they had to apply and wait for a mining li­cence for the next phase of their opera­tions. No doubt a lot of work was done behind the scenes, but now things are moving again at the wellsite, too.

During the exploration cam­paign, it took Helium One multiple wells to get things right and dis­cover the best play type to produce helium from. Thanks to well Itumbula West-1 (ITW-1), the company found that drilling straight into basement faults and the surrounding damage zones is the best approach. Quite simply, this is because helium uses the faults as mi­gration pathways. Helium forms in the Pre-Cambrian basement as a by-product of radioactive decay. The elevated heat flow provided by the upwelling man­tle in the East African rift setting then liberates the helium from the uranium and thorium-bearing minerals, and it migrates up section. The helium subse­quently accumulates in shallower aqui­fers, without there being a structural or stratigraphic trap. It eventually reaches the surface, as evidenced by salt ponds in the area with active seeps that contain up to 10.2 % helium.

The Itumbula discovery is a fault-bounded basement horst with a thin sedimentary cover. ITW-1 tested two major faults and found helium in two aquifers: fluvial-deltaic sediments of the Karoo Formation and in weathered basement. Water samples from both aquifers contained over 5 % helium. Helium One is planning a five-well campaign to further appraise the area.

Helium One sourced an Electric Submersible Pump (ESP) to produce the basement aquifer. A 20 day test at the start of the year demonstrat­ed a six-fold increase in flow rate to 15,000 bbls/day compared to the nat­ural flow rate during the extended well test. The produced fluid had a Gas-Wa­ter ratio (GWR) of 0.06 m³/m³. The ESP was set at 1,061 m MD within the fractured basement, with a packer in the lowermost Karoo Formation to reduce comingled flow. However, the 90⁰ C water temperature and measured salin­ity suggest basement isolation was not fully achieved. The contribution of the Karoo aquifer is thought to have low­ered the GWR.

There is no plan to reinject the produced water, and Helium One is evaluating disposal options. The waste­water could possibly be used for geo-thermal energy and / or irrigation. Nitrogen is the only other gas present and can be safely vented to the atmos­phere. Helium One has now embarked on a farmout process to find a suitable industry partner.