(484h) Imaging the Dissolution of Chalk during HCl Injection

Geological carbon storage is a promising climate change mitigation technology based on the injection of CO₂ into geological formations such as depleted oil and gas reservoirs, which prevents it from attributing to global warming as it is locked out of the atmosphere [1, 2]. In Denmark, depleted oil and gas fields are composed of chalk, which presents a challenge due to the chemical reactions initiated by CO₂-acidified brines and reservoir rocks [3]. The chemical dissolution of chalk could lead to the weakening of the reservoir, subsidence, and subsequently a failure to store the injected CO₂ [4]. Since the infrastructure for injecting and monitoring is already in place, a CS program in chalk could be economically viable [3]. It is, therefore, essential to study the dissolution of the chalk to determine its viability of it as a storage medium.

In this work, we present a systematic study of the dissolution of Danish North Sea chalk in the presence of acidic solutions. North Sea chalk is characterised by small pore spaces, low permeabilities, and high porosities. Experimental studies were performed in a novel triaxial flow cell that allows for the simulation of reservoir temperatures and pressure and the application of geomechanical tests; the central part of the cell is made of aluminium to maintain X-ray transparency. We used X-ray computed tomography (CT) to monitor the dissolution process across the chalk sample during core flooding. An example of a CT of a chalk sample can be seen in Fig. 1, along with a photo of the flow cell. The chemical compositions of the effluents were analysed using ion chromatography (IC) techniques, and the continuous measurements of differential pressures across the core provided information on the changes in permeability.

We present two case studies of chalk dissolution at two HCl concentrations. In the first study, HCl (7 x 10-4 mol/kg) was injected while in the second study, we injected a higher concentration of HCl (5 x 10-3 mol/kg). The concentration of HCl in the first experiment was chosen to simulate the pH of CO₂-acidified brine at reservoir conditions. In this experiment, we saw no discernible change in the permeability or porosity of the core after injecting over a short period indicating that chalk might be a suitable storage medium.

In the second experiment, with the more concentrated acid, we imaged dissolution of chalk as it occurred. We clearly resolved the dissolution pattern (see Fig. 2) and show the development of wormholes in the core. This figure only shows a tenth of the CT scans we performed during the experiment, which means we can track the development of the channels with a high temporal resolution. The development of tunnels seen in Fig. 2 results in a 61% increase in permeability, despite the porosity only increasing by 0.3%. IC measurements shown in Fig. 3 track the changes in of calcium, sodium, chloride and sulphate in the effluent during the saturation of the core sample using de-ionized water and the subsequent injection of HCl.

The combination of X-ray CT with effluent analysis and permeability and porosity measurements allows us to study chalk dissolution in detail. Furthermore, the triaxial capabilities of our flow cell makes it possible to quantify the weakening of chalk after dissolution. This is valuable data that feed into the risk analysis of CS and can be used as input for modelling and reservoir simulations.