Matrix Stimulation of Water Disposal Wells Using Viscoelastic Surfactant-based Acid

Abstract

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. Introduction 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.

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“…The surfactant used was amphoteric and the rheological properties and field cases using this system were given in previous publications. [24][25][26][27][28] Tables 3 and 4 give the acid formulations used. The acid volume used in both wells was 10 gals/ft of open hole.…”

“…21,22 Positive field results were reported when this acid system was used to stimulate oil producers, water injectors and water disposal wells. [23][24][25][26] Recently, viscoelastic surfactant-based acids were used to stimulate OH sections that could not be reached because of CT lock-up limitations. Openhole sections of up to 2,000 ft were successfully stimulated by bullheading the treating acids at the CT lock-up depth.…”

A Novel Technique to Acidize Horizontal Wells with Extended Reach

“…13,14 Positive field results were reported when visco-elastic surfactant based acids were used to stimulate oil producers, water injectors and water disposal wells. [15][16][17][18] The present study addresses a combination of CT-tractor as a means of mechanical diversion in the wellbore and viscoelastic surfactant-based acid as a means of chemical diversion in the formation. This study marks the first time such a combination was used to acidize horizontal wells with extended reach.…”

Optimization of Surfactant-Based Fluids for Acid Diversion

“…This diversion can be accomplished through mechanical, chemical means or both. [6][7][8][9][10][11][12] Many chemical means exist including polymer gels, foams, oilsoluble resins, rock salt, and surfactant gels. 6 Surfactant gels have recently become an increasingly popular choice for viscosifying acidic fluids.…”

“…13 Amphoteric surfactants have been the subject of several lab 16 and fields studies. 7,10,11 The present study will focus on cationic viscoelastic surfactants. There are several advantages of the using the cationic surfactant tested in the for carbonate acidizing.…”