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8 October 2020

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Researching the impact of CO2 storage in rock

Carbon dioxide is a greenhouse gas which contributes to global warming. Its concentration is rising in atmosphere and directly increasing extreme weather events. The gas also dissolves into the ocean, causing ocean acidification.

To mitigate the effects of excess CO2, it can be trapped within reservoirs of rock. One way to do this is to dissolve CO2 into the brine which is within pores of rock under the sea. However, there are concerns on the effect of the stored CO2 on the reservoir rock, given the need to ensure the long term integrity of the storage reservoir. Injecting CO2 stresses the rock and affects its fatigue behaviour.

Engineers in Edinburgh and Wolverhampton are investigating how the behaviour of CO2 in rock is controlled by temperature, pressure and saturation conditions. As part of their investigations they are using Windmill software and the Microlink 751 multi-function data acquisition unit.

Map of core sample locations
Map showing from where the core rock samples were collected in North East Scotland

The experimental set up held the rock sample, brine and CO2 at 35 oC during fatigue testing. A pressure transducer measured and logged the pressure on the sample. The Microlink 751 recorded displacement data from LVDTs and sent the readings to Windmill software running on a PC to display and save the data. The researchers set the Microlink at 15 bits per channel with each channel measuring +/- 10 V dc. They chose a resolution of 0.8 mV giving a precision of +/- 0.05%.

Axial and lateral displacements were recorded per second during the entire fatigue and Windmill displayed the data on the screen. The readings were processed to yield the axial and lateral strain, volumetric strain, fatigue rates, fatigue duration, bulk modulus and compressibility.

Fatigue Curve
Fatigue Curves for different deviatoric stresses

The Microlink can measure temperature, strain, pressure, voltage and current through 16 analogue input channels. It can switch up to 32 digital outputs and monitor up to 32 digital inputs. It can also count events with up to 8 counters. You can connect eight Microlinks to one PC giving 128 analogue inputs and 256 digital inputs and outputs. Use Windmill software to choose from four analogue input ranges or automatic ranging to let the software match the input signal as closely as possible. You can also use Windmill to select the resolution from 7 options: choose high throughput or high resolution. Read more

Further reading
Ameh Peter et al, Static fatigue of saline rocks under different CO2 phase conditions, Journal of Petroleum Science and Engineering, Volume 195, December 2020, 107940 Advance Article
The Microlink 751 multi-function data acquisition unit
Hugh Baker et al, Rising CO2 may increase dangerous weather extremes, whatever happens to global temperatures, University of Oxford