Simulating a Controlled CO2 Release Experiment in a Shallow Fault Zone in Western Australia

Geological and dynamic modelling studies are routinely performed to design and interpret field test experiments. These studies help with the planning of well location, well operations and monitoring strategies, as well as with a range of different test activities. After the test, the new interpretations of the experiment observations are then integrated with previous knowledge and reconciled with the assumption that was first tested. Often, the results of a test are significantly different from what was anticipated (hence the purpose of testing). In this work, we examine such an example and put in perspective the impact of the first modelling study on the interpretation of the test. The CSIRO In-Situ Laboratory Project (In-Situ Lab) is located at the eastern edge of the South West Hub CCS Flagship project (SW Hub) in Western Australia, which has been identified as a potential area for commercial-scale CO2 storage. A first test at the In-situ Lab has evaluated the ability to monitor and detect unwanted leakage of CO2 from a storage complex in the shallow subsurface. The In-Situ Lab is also collocated within a major fault zone. Prior to a controlled-release test, a numerical simulation study was undertaken to inform the optimal location for a monitoring well, the vertical location of the downhole instrumentation, and to ensure breakthrough of CO2 occurred within the project timeframe. Based on the likely extent of the CO2 plume for an injection volume of 40 tonnes, the observation well ISL OB-1 was located at 7 m from the injection well to maximise the likelihood the CO2 plume would be intersected. The CO2 plume was predicted to migrate vertically for at least 20 m above the injection interval. Due to the resolution of the existing seismic and limited well data, large uncertainty existed regarding the detailed fault zone geometry, and, more specifically, how disaggregation and drag has impacted the contiguity and slope of lithostratigraphic layers within the fault zone. In other words, it was not clear to what extent the relatively low-permeability paleosols would form baffles for the migration of CO2. Also, it was not known whether sub-seismic faults, if present, would act as vertical barriers or conduits to CO2 migration. During a controlled-release test, 38 tonnes of gaseous CO2 were injected in February 2019, and the gas was monitored by a wide range of downhole and surface monitoring technologies. A higher than expected bottomhole pressure was observed. CO2 reached the ISL OB-1 monitoring well after approximately 1.5 days. Observations suggest that the fault zone did not alter the CO2 migration along bedding at the scale and depth of the controlled release experiment. No vertical CO2 migration was detected beyond the perforated injection interval. The injectivity was lower than predicted from core measurements and prior modelling. Also, contrary to prior model predictions, the plume did not migrate vertically across the paleosol overlying the injection interval. Post-injection reservoir simulations calibrated to the observed pressure and CO2 distribution data confirm that (1) a vertical segregation exists just above the injection interval, and (2) local overpressure are due to a combination of wellbore damage, mud invasion at the injection well and, to a lesser extent, reservoir compartmentalisation.