Influence of Acid Type on Wormhole Generation and Carbonate Rock Properties

Matrix acidizing is one of the most effective techniques to restore formation permeability and increase production rate. It is typically performed using different types of treatments, such as mineral acids, organic acids or chelating agents. These stimulation treatments tend to generate pathways known as wormholes through a chemical reaction with target carbonate formations. Bottom hole conditions play an important role in the treatment selection since they influence the reaction rate, wormholes structure and wellbore integrity. This work shows an experimental study of the impact of acid type on wormholes generation in carbonate rocks at different temperatures. Three different acids were used which were hydrochloric (HCl), formic (HCOOH) and acetic (CH3COOH) acids. These acids were injected at a fixed rate of 2 cm3/min into Indiana Limestone core plugs at two different temperatures: 150 and 250°F using coreflood equipment. Non-destructive visualization and analyses were conducted by using the Computerized Tomography (CT) scanning technique, which provided wormhole patterns, slice images with a thickness of 625 micron as well as statistical comparisons. Furthermore, dynamic acoustic velocities and elastic parameters were determined at different confining pressures for the post treatment core plugs to compare their values. The use of different acids at the same temperature conditions formed different pathways and patterns of wormholes. In addition, the temperature effect has a clear relationship with the required pore volume to breakthrough (PVTB). At low temperature, HCl demonstrated an outstanding performance by having the least required PVTB, while at high temperature acetic acid showed an improved generated wormhole pathway compared to the other acid types. Nonetheless, the different acids did not show a significant impact on the dynamic mechanical properties of the rock samples. This work evaluated the effect of temperature on the rock wormholes patterns using different types of acids based on their dissolving power. The study concluded that the temperature has a significant effect on carbonate matrix acidizing operations. The study also focused on how the selection of the acid type can make a huge impact related to bottom hole conditions.