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|Title||Matrix Stimulation of Water Disposal Wells Using Viscoelastic Surfactant-based Acid|
|Authors||H.A. Nasr-El-Din, A.M. Al-Mohammad, A.A. Al-Shurei, N.K. Merwat, Saudi Aramco; M.M. Erbil, M. Samuel, Schlumberger|
SPE Asia Pacific Oil and Gas Conference and Exhibition, 18-20 October 2004, Perth, Australia
|Copyright||2004. Society of Petroleum Engineers|
Application of matrix stimulation treatments in horizontal wells with openhole completions is a challenging task. Acid diversion is one of the key factors that determine the success of these treatments. This is especially true when the target zone has high permeability streaks. Proper acid distribution in carbonate formations can be achieved by increasing the viscosity of the acid in-situ. This can be done by using polymer/cross-linker or viscoelastic surfactant acid systems. The latter system was selected in the present study because of the tight nature of the reservoir and the presence of hydrogen sulfide in the disposal water. Both factors do not favor the use of polymer-based acids.
The surfactant used in the diversion stage is amphoteric. It carries a positive charge in the presence of live HCl acid. Once the acid reacts with the carbonate minerals, the pH of the solution rises and the concentrations of divalent cations (calcium and magnesium) increase. Both factors enhance the viscosity of the solution in-situ. This high viscosity will help in diverting the live acid into other un-stimulated zones. Once the acid stimulation is complete, the high viscosity of spent acid can be broken either by dilution with the injection water or by adding mutual solvent in the pre and post-flushes.
Viscoelastic surfactant-based acids have been successfully used to stimulate oil and gas wells over the last three years. This is the first time these acids were used to matrix acidize water disposal wells worldwide. No operational problems were encountered. Injectivity tests conducted after the acid treatment indicated that the treatment removed the damage; iron sulfide, calcium carbonate particles and trapped oil droplets. The skin factor decreased from + 22 before the treatment to nearly zero after the treatment. The pumping rate increased by a factor of three and the injectivity index increased by a factor of 14. The well maintained its injectivity up to the present time (more than seven months after the treatment) and still showing no signs of decline. An additional benefit of the viscoelastic surfactant acid system is its ability to decrease friction losses during acid pumping. Field data are given to support this new benefit.
One of the main goals of acid stimulation treatments is to remove formation damage induced by drilling mud filter cake. However, the stimulation fluid will flow through the path of least resistance where the permeability is high or the damage (skin) is low. Hydrochloric acid reacts with carbonate materials very fast creating high conductivity flow paths in the formation (also known as wormholes). As a result, most of the injected acid would flow through these paths leaving the majority of the formation untreated. Therefore, there is a need for a proper fluid diversion to enhance the outcome of the treatment. 1-4
Mechanical and chemical means have been utilized for diversion during matrix acidizing treatments with various degrees of success. Unlike cased wells, mechanical means e.g., isolation packers, are less efficient in horizontal wells with open hole completion. Polymer-based acids and foam are the most preferred chemical means for diversion during matrix stimulation of carbonate formations. However, there were some concerns raised when polymer-based acids were used in tight, sour formations.5,6
Viscoelastic surfactant-based fluids have been used for diversion in carbonate reservoirs. They were used to enhance the properties of foams used for diversion in water injectors.7 They were also used to increase the viscosity of HCl during matrix acidizing of horizontal oil producers.8
This paper presents the first application of a viscoelastic surfactant-based acid system to enhance acid diversion in a horizontal water disposal well with an openhole completion. The objective of the stimulation treatment was to increase the injectivity index by at least four folds. This was needed to avoid drilling additional wells for water disposal in this area.