Sandstone acidizing

Unfortunately, the results of this first attempt were very discouraging. The reaction of the strong acid solution in the formation caused substantial sand production into the wellbore. Consequently, use of HCl-HF was not very popular for the next 20 years. In fact, Halliburton did not offer HCl-HF until the mid-1950s.  

Interestingly, Halliburton was apparently unaware of a patent application filed onMarch 16,1933, byjesse Russell Wilson ofthe Standard Oil Company of Indiana. This patent concerned the use of HF for treating sandstone formations. Another patent application was filed the same day by James G. Vandergrift, employing a mixture of mineral acid and HF.  

The Wilson patent application indicated an understanding of sandstone acidizing and its purpose far ahead of its time. His description ofthe problem, as recounted in Acidizing Fundamentals, is as follows  

Wilson s process called for generation of HF in the wellbore or in the formation. In hindsight, his anticipation of add-removable formation damage due to solids plugging is remarkable. Many sandstone acidizing treatments have been pumped since, without such understanding of purpose and potential. 

Dowell did introduce a mixture of 12% HCl-3% HF, called mud acid, in 1939. The purpose of this mud add was to remove drilling-mud filter cake from the wellbore. However, use was limited to wellbore treatment only. Successful treatments were pumped in the Gulf Coast area.  

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Earlier corrosion inhibitors limited the maximum strength of the acid to 15% by weight. Improved corrosion inhibitors (see below) have made the use of higher acid concentrations, such as 28% HCl more common. More dilute solutions may initially be injected in sandstone acidizing to reduce the formation of insoluble sodium and potassium fluorosilicates by displacing saline formation water before injection of hydrochloric acid. 

Mixed acid systems are blends of mineral acids and organic acids. Combinations that have been used in carbonate acidizing include acetic acid/HCl and formic acid/HCl. While these are less corrosive than hydrochloric acid alone, the organic acid may not react completely with the rock. Blends of formic acid and hydrofluoric acid have been used in high temperature sandstone acidizing and are less corrosive than HC1/HF blends. 

Retarded acids are primarily applicable to sandstone acidizing. Fluoroboric acid slowly hydrolyzes to form the more reactive hydrofluoric acid (109,110). The time required for this hydrolysis process may enable deeper penetration of the HF into the formation although one report contradicts these findings (111). Na TiF and similar salts also slowly generate HF in acid media (112). Phosphorous acid addition to hydrochloric acid has been used to reduce the HC1 reaction rate with limestone (113). 

Sutton, G.D. and Lasater, R.M. "Aspects of Acid Additive Selection in Sandstone Acidizing," SPE paper 4114, 1972 SPE Annual Fall Meeting of AIME, San Antonio, October 8 11. 

Chemical conditions. The occurrence of glauconites in sandstones (acid) and carbonates (basic) indicates that pH is not of great importance to their formation. They are found in sedimentary rocks ranging in age from Precambrian to the present day. Hence, their formation seems to be constrained by chemical and physical conditions rather than by specific events. There is no apparent time-dependent reaction which transforms them into new phases. 

Hall, B. E. Dill, W. R. "Iron Control Additives for Limestone and Sandstone Acidizing of Sour and... 

The year 1965 featured another milestone in the development of sandstone acidizing technology. Smith and Hendrickson discussed the reactivity and kinetics of HF and the effects of common variables encountered in the field. HF reactions with rock minerals and secondary depositions were studied theoretically. In addition, core flow tests were conducted using Berea sandstone cores. 

The 1970s and early 1980s saw the rise of a proliferation of novel sandstone acidizing systems. Most were developed to provide certain benefits, such as the following ... 

In the 1980s and into the 1990s, developments in sandstone acidizing addressed treatment execution more than fluid chemistry. Techniques included nitrified and foamed acid treatments, high-rate/high-volume HF acidizing, and CO -enhanced HF acidizing. These are discussed in later chapters. 

In sandstone acidizing, the acids commonly used are as follows ... 

HF is used most commonly in combination with HCl. It should never be pumped alone. It may also be used in combination with the organic acids, acetic and formic, or in combination with acid blends, such as acetic-formic, HCl-acetic, and HCl-formic. Other organic acids—such as citric acid, glycolic acid, or proprietary acid compounds—may be combined with HF as well, in acid treatments of sandstone formations. Appendix A gives examples of successful sandstone acid treatment procedures. 

Sandstone acidizing is, for practical purposes, a method for removing acid-removable damage s) only. Except in rare cases, the production rate from an imdamaged well producing from a sandstone formation could be increased up to perhaps twice the original rate, at best. Acid-removable skin, discussed in detail in chapter 6, may manifest itself in the wellbore, in the perforations, or within the formation. 

The reprecipitation of reaction products is not a serious concern in carbonate acidizing, but it is in sandstone acidizing. Gdanski and Peavy,... 

Posttreatment fines migration is quite common in sandstone acidizing. It may be difficult to avoid in many cases. The reaction of HE with clays and other aluminosilicate minerals and quartz can release undissolved fines. Also, new fines may be generated as a result of partial reaction with high-surface-area minerals, particularly clays and certain zeolites, in which they more rarely occur. 

HF is the only common add that dissolves siliceous minerals appreciably. Therefore, sandstone acidizing formulations include HF or one of one of a variety of HF precursors, or compounds that generate HF. The most commonly used formulations are mixtures of HCl and HF. These are referred to as mud add, from the early days of sandstone acidizing. Acid concentrations can vary from the low end (e.g., 3% HCl 0.5% HF) to the high end (12%-15% HCl 3%-5% HF or more). 

Sandstone acidizing can be a very successful well stimulation method. However, the risk of failure is moderate to high. Fortunately, there are a limited number of reasons why acid treatments fail in sandstones. Success begins with the selection of a viable acidizing candidate well. Many poorly producing wells are not viable acidizing candidates. Conversely, many relatively prolific producers are the very best acidizing candidates. Once a viable candidate is selected, a systematic approach to the selection of fluids, additives, and acid placement technique must be taken. On-site quality control and posttreatment evaluation help to ensure successful results and improved future treatments. 

Reasons why acid jobs fail are discussed in detail in chapter 5. Using a straightforward, systematic approach to sandstone acidizing treatment design—to prevent failure and increase the success rate— is the subject of chapter 6. 

In reality, there are a limited number of reasons, or controllable causes, for sandstone acidizing treatment failure. All can be avoided with proper treatment planning and execution. This is not to say that a success rate of 100% —on a well-by-well basis— is readily attainable. It may be possible in a particular field or with a limited selection of wells. However, stimulation methods are not foolproof, and acidizing is no exception. 

First, one must identify and understand the causes of sandstone acidizing treatment failure that are avoidable. These should always influence the steps taken in the treatment dedsion and design process. 

Understanding that most treatment failures are due to one or more of the preceding reasons simplifies the process. It can also ease the mind when sandstone acidizing treatment design considerations seem hopelessly complicated. 

Sometimes add must be pumped above fracturing pressure, just to break down perforations and initiate flow. However, it is generally accepted that sandstone acidizing must take place in the matrix—within the pore spaces— to impart stimulation. 

The sandstone acidizing design approach to be taken is introduced and presented in this chapter as a six-step process, inspired by Harry McLeod. The six-step process to successful sandstone acidizing is as follows ... 

Sandstone acidizing treatment design can be overwhelming. It may seem that there are too many variables, too many issues to worry about, and too many choices. It is true that there are many variations to the acids, their concentrations and volumes, the additive choices, and the number of steps in an acidizing procedure. However, bear in mind that all sandstone acid treatments are variations of the following maximum-step procedure ... 

In the systematic approach to sandstone acid treatment design, it is best to work within this general design framework, which fits all cases. It is the variation within this framework that is subject to our ideas, choices, and creativity. 

It is the aim of sandstone acidizing treatments to reduce that portion of the total skin s) that is due to damage s). Skin damage must be present, but it must be acid removable, as manifested in the wellbore, in the perforations, and/or within the formation. In evaluating a well producing from a sandstone reservoir as a stimulation candidate, skin must be measured—or at the least, assessed as best as possible— to select the proper course of treatment (or nontreatment). 

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