A fast method for homogeneous dissolution of chalk specimens for laboratory experiments - documentation by X-ray CT scanning and scanning electron microscopy

In chalk reservoirs, acid injection for well stimulation purposes as well as certain EOR methods, such as CO₂ injection, causes dissolution of the chalk matrix due to the acidic nature of fluid injected. The effects of the chalk dissolution e.g. on rock mechanical properties are currently not well understood. This is partly due to challenges with interpretation of results from laboratory experiments in the context of in situ reservoir conditions as laboratory experiments most often result in inhomogeneous dissolution of chalk specimens. To facilitate this challenge, we present a method for fast homogeneous dissolution of chalk specimens by a so-called retarded or heat activated acid (Acidgen^TM FG) – and subsequent documentation by X-ray CT-scanning and Field Emission Gun Scanning Electron Microscopy (FEG-SEM) imaging.
The method was validated through dissolution tests using a large number of Stevns outcrop chalk specimens. Dissolution of the chalk was performed with either of two acidic or acid generating solutions: 1) Acidgen TM FG or 2) acetic acid. In both cases, the homogeneity of the chalk dissolution was investigated by image analysis of X-ray CT-scanning images, and for the Acidgen^TM FG case also of FEG-SEM images.
By application of Acidgen^TM FG it is possible to create homogeneous dissolution in the outcrop chalk specimens with a resulting absolute porosity change of up to at least 3.5% Likewise, the retarded acid creates homogeneous dissolution of reservoir chalk specimens. For comparison, the application of acetic acid mostly creates fast inhomogeneous dissolution from the outside of the chalk specimens. Combined image analysis of X-ray CT images and SEM images has proven a strong tool for evaluation of the homogeneity of the chalk plug specimens.