Characterization of rocks from geothermal reservoirs is crucial to better understand their behavior. Los Humeros geothermal field contains one of the most important reservoirs in Mexico. This reservoir contains volcanic rocks product of various cooling processes. In the present study, we use four different samples, three distinct andesites and one highly altered volcanic rock from Los Humeros reservoir. We acquired 3D images using X-rays microtomography to visualize and estimate porosity in these rocks. In the three andesites, we used the maximum possible resolution of 0.5 micros/voxel. In the altered sample, we used four different resolutions of 0.5, 3, 14 and 53 microns/voxel and took physical and digital subsamples at various locations in the original core plug to compare and identify its heterogeneity. From the 3D images, image porosity after segmentation and fractal-scaling porosity were estimated. From the core plug, the experimental porosity was obtained by a collaborative lab (IPICYT). The fractal-scaling porosity approximates the fractal porosity under the assumption that all pores have the same size approximately equal to the box size, used in the box counting method. The calculations were done in all images at all resolutions. We found that the image porosity at the highest resolution is generally closer to the experimental porosity. However, the altered sample, even at this resolution, presents some variations. The image porosity of this sample at the other resolutions shows values far from the experimental porosity, as the pore space could not be fully capture. As a matter of fact, the fractal dimension increases from the lower resolution to the higher resolution as a mainly magnification effect, as well as the porosity. The fractal-scaling porosity, which uses the fractal dimension, the average spherical equivalent diameter, and the size of the image gives estimates of porosity very close to the image porosity. This demonstrates the fractal character of all these samples, implying that the pore structure in these rocks can be better evaluated using Fractal geometry. The different results obtained in the altered sample corroborates its diverse processes. To complete this study, we are working on finding potential correlations between these porous structures and permeability, and adding more samples.