Effects of mineralogy on wormhole propagation in carbonate acidizing

Abstract

Matrix acidizing is a common stimulation method for wells in carbonate reservoirs. Typically, 15% HCl in water, and some additives for specific purposes, is injected into carbonate rocks to create wormholes to improve hydrocarbon flow. These wormholes are formed by acid dissolution of the carbonate rock. The path by which the wormholes are formed is normally of lowest resistance. The path of rock dissolution is governed by permeability heterogeneity and normally follows the path of highest permeability. There are other aspects that could influence the path of the wormhole, that is mineralogy heterogeneity. When an acid is exposed to an insoluble mineral, it is expected that it will change its path and could deviate, even from the path of highest permeability. Effects of this process on the stimulation treatment are expected to vary from one rock type to another. The objective of this study was to compare the path of the wormhole in rocks with homogenous mineralogy with that of rocks with heterogeneous mineralogy. In both cases, the permeability distribution in the rock was analyzed and compared to the path of the wormhole. The ultimate goal was to define whether the presence of acid insoluble minerals affects the path of the wormhole. Micro-CT scanning at various resolutions was utilized to visualize the pore space connectivity. Thin section analysis of rock samples was used, in combination with micro-CT images, to identify mineralogy at various locations. This was conducted with the support of scanning electron microscope (SEM) analysis. Thin section analysis was conducted for the same rock samples that were scanned and the results were compared so that CT-images can be calibrated and used to identify mineralogy of other similar rock samples. The results were beneficial to the research team as more information about the rock properties from CT-scan images were obtained.