A LABORATORY EXPERIMENT TO CHARACTERIZE CHANGES IN POROSITY, PERMEABILITY, AND WORMHOLE FORMATION IN A CARBONATE ROCK

Acid treatment is an important means of extracting hydrocarbons held within tight pores of carbonate rocks. However, the intricate geometry of pores and mineralogical heterogeneities of rocks make the design and implementation of acid treatment programs very challenging. The aim of this work is to perform acid treatment of plug samples under laboratory conditions, and to fully characterize resulting changes within the samples. Changes in porosity and permeability, as well as wormhole geometry in three-dimensions, that are all pertinent to the success of acid stimulation programs in the field will be fully evaluated. Three plug samples of a carbonate rock were prepared and characterized using X-ray diffraction and scanning electron microscope - energy dispersive spectroscopy techniques. The samples were then individually scanned using a conventional X-ray computed tomography (X-ray CT) instrument to obtain pre-treatment images. The permeability of the plug samples were measured using a brine solution, and were then subjected to acid treatment in a core flooding unit. The permeability of the samples were again measured after acid treatment, and scanned to acquire post-treatment images. The pre and post-treatment images of the plug samples were registered against each other to orient corresponding voxel locations to the same position in space in three-dimensions. The changes in grayscale at every voxel location within the images were analyzed and used to determine the change in porosity and the 3-D geometry of the wormholes resulting from matrix acidizing. Results showed the formation a single principal wormhole within each sample and a change in porosity of about 2 %, however, the change in permeability in all the samples were over 100 %. The length of the wormholes were directly proportional to the change in permeability. Data generated from this work provide good inputs for acid simulation programs that can be up-scaled to field dimensions for optimum results.