Real-time Neutron Radiographic and X-Ray Tomographic Analysis of Acid Dissolution of Limestone
Abstract
While the importance of porosity and pore-evolution has long been recognized, quantitative assessment of how porosity and permeability evolve in reactive carbonates remains elusive. A typical pH range for CO2-acidified brine, for example, is 3 to 4.5, which represents a substantial perturbation of typical brines (pH 6 to 8). At core-scale the flow-rate, permeability and reactivity define the dissolution structure (e.g. wormholing). Limestones and dolostones are not, however, impermeable materials that simply react with the fluid. They contain nano- to macroscale porosity comprised of cracks, grain boundaries, single pores, vugs and networks of pores of random shapes and orientations. These control pore accessibility and the evolution of the pore network during reaction.
A combination of neutron and X-ray imaging is well suited to studies of fluid flow. Neutron radiography can directly image water or hydrocarbons flow without the aid of an added contrast medium that could modify interfacial tension and fluid/fluid interactions, and pore-reaction XCT provides higher-resolution 3D imagery of the dissolution structure. In order to understand the reaction of acidified fluids with limestone we have used a combination of neutron radiography and X-ray tomography to study the uptake and reaction of water and an acidic fluid (pH 0, 1, 2, 4 HCl) with two samples of Indiana limestone, one with a permeability of 2-4 mD, the other with a permeability of 70 mD. Water and acid were allowed to permeate the sample based solely on capillary uptake, and the latter was also introduced under forced flow at 1 and 10 ml/min. The results clearly show that the preexisting porous network plays an important role in controlling the dissolution process as fluid flow through these systems helps determine the ultimate result. Small and ultrasmall angle neutron scattering have been used after the experiment to evaluate changes in pore size distribution as a function of distance along the core. Image shows entrance of a 1.5" OD core of 70 mD Indiana Limestone after reaction with pH = 0 HCl at 10 cc/min.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H33B..02A
- Keywords:
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- 1832 Groundwater transport;
- HYDROLOGYDE: 1859 Rocks: physical properties;
- HYDROLOGYDE: 1878 Water/energy interactions;
- HYDROLOGYDE: 1895 Instruments and techniques: monitoring;
- HYDROLOGY