In carbonate matrix acidizing, the critical design parameters are interstitial velocity and pore volume to breakthrough for wormhole propagation at the optimal condition (vi,opt and PVBT,opt). These two parameters are determined by the formation rock properties including permeability, porosity, temperature, pressure, and mineralogy, and the acid system that will be used. HCl has been the most commonly used acid in carbonate acidizing. However, the efficiency of wormhole creation using strong HCl solutions is sometimes low because the injection rate needed for optimal wormhole propagation is not attainable, especially when applied to long completion intervals and/or low permeability carbonates. High concentration HCl solutions also raise safety and environmental concerns. When retarded acid systems are used to overcome these challenges, the efficiency of acid stimulation often suffers from low reaction rate and low dissolving power. This study presents testing results of a modified acid system that has controlled reaction rate with favorable wormhole propagation characteristics, especially at low interstitial velocities.
Due to the increased activation energy barriers utilized in these modified-acid systems, it is possible to control the reaction rate of the hydrogen proton and optimize the wormholing effect based on the completion method and formation specifications. Single phase modified acid systems are applicable and can be tailored for limestone and chalk formations. Laboratory linear core flooding experiments and acid jetting experiments were conducted to study the wormhole efficiency with the new acid systems. Core flood tests were designed to generate the wormhole efficiency curves and compare the results from the tests with HCl under the same conditions. Jetting experiments were conducted to evaluate the structure of cavity and wormhole development, and the results were also compared with HCl injection at similar conditions. Results from the experimental study were then used to compare the acid systems through wormhole efficiency curves and CT scans of acid dissolution structures.
The experimental results showed clear advantages of the modified acid systems. The modified acids have similar or better wormhole efficiency parameters compared with HCl having comparable dissolving power. When combined with acid jetting, further improvement in wormhole growth in low permeability limestone is achieved. In addition to minimizing the hazardous exposure levels, corrosion rates, and negative HS&E properties compared with hydrochloric acid, the new acid system provides the positive aspects of solubilizing ability and reaction rates thus improving optimal wormhole conditions (lower vi,opt and PVBT,opt). This potentially allows for better stimulation results with less fluid required.