Artesian Heating and Cooling In Christchurch

Christchurch’s aquifers are ideally suited to ground-sourced energy extraction because of the highly permeable strata, consistent temperature of the source, and high thermal attenuation upon reinjection. As such, there is increasing utilisation of aquifers beneath the Christchurch Central Business District (CBD) during the earthquake rebuild to source energy.

Groundwater is pumped from deeper aquifers at approximately 12 to 15 °C and run through heat exchange plant to extract or dispose of heat energy, which can be used to heat or cool a building. The groundwater is then reinjected back into shallower aquifers or discharged into waterways at up to an 8 °C temperature change, resulting in a non-consumptive or hydrogeologically neutral “take”.

Reinjection wells are commonly drilled into Aquifer 1 (Riccarton Gravel Formation), found at about a depth of 25 to 45 m below ground level. Groundwater extraction generally occurs from the deeper aquifers (Aquifers 2 to 4), at about 65 to 140 m depth. The vertical distribution of the aquifers limits hydraulic connectivity and recirculation of the discharged water from the abstraction wells, and allows pairs of reinjection and abstraction wells to be positioned in close proximity to each other. The positioning of wells is a critical factor in space-constrained urban sites, and ensuring access for drilling rigs to maintain the wells becomes a key planning consideration in developing these systems and sites.

Reinjection wells are designed in a similar manner to normal pumping wells with the addition of a grout sealed annulus to prevent short-circuiting around the well casing under pressure. Flow and pressure testing are completed at the same time as the testing of the abstraction wells, and projects to date indicate that Aquifer 1 has a remarkably high capacity for reinjection.

Key issues include excessive pressures in the aquifer where reinjection occurs, reinjection pressures within the reinjection wells and interference from surrounding reinjection wells that are hydraulically connected within Aquifer 1. Wider pressure increases in Aquifer 1 have the potential to develop due to reinjection, and approaches to avoid the unintended emergence of groundwater seepage at surface or around build environment assets is required.

The paper discusses technical highlights of ground-source well development in the Christchurch CBD and makes suggestions for the design of resilient and sustainable ground-source supply and reinjection infrastructure.