Acid treatment design

Preflush and cooling Fresh water lObpm (through tubing and annulus—to cool wellbore temperature to below 200°F) 

Overflush Fresh water or 3% NH I Several hours at maximum rate 

For carbonate scale removal, 15% HCl is most common. HF should not be pumped in such cases. Unfortunately, the scales formed in geothermal wells may be mixed and layered scales, composed of carbonate, sulfate, and silicate. Therefore, a combination of HCl and HF (or of acid stages) is required in order to at least partially dissolve the scale. 

Despite their high HF strength, these mixtures are less corrosive because of the low total acid strength. HF itself is not a strong acid but is highly reactive with siliceous minerals and scales. 

Examples of treatment procedures using such formulations are given in tables 7-3 and 7-4. Table 7-4 presents the more recently employed, simplified procedure, which eliminates an acid preflush ahead of HF injection. 

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Coulter, A.W., Alderman, E.N., Cloud, J.E., and Crowe, C.W. "Mathematical Model Simulates Actual Well Conditions in Fracture Acidizing Treatment Design," SPE paper 5004, 1974 SPE Annual Fall Meeting of AIME, Houston. 

Anderson, M.S. Fredrickson, S.E. "Dynamic Etching Tests Aid Fracture Acidizing Treatment Design," SPE/DOE paper 16452, 1987 SPE/DOE Low Permeability Reservoirs Sumposium, Denver. 

Before delving into acid treatment design, chapter 3 presents a general discussion of formation damage. Assessment of formation damage is the most important aspect of acid treatment candidate selection and treatment... 

Much has been written over the years on the subject of formation damage. For background on formation damage, the best place to start is with the landmark paper by Krueger. Another occasionally forgotten but very important work is that of Maly. This chapter is concerned with formation damage issues as they relate to acid treatment design. 

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. 

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. 

Before delving into the systematic approach to acid treatment, well candidate selection, and acid treatment design, it is important to have a feel for the common reasons for acid treatment failure and the causes that are usually avoidable. Acid treatment design cannot be cookbooked. One should not attempt to reproduce a boilerplate acidizing procedure throughout an entire field, as convenient as that may seem. 

Nevertheless, the approach to an acid job can be reproduced—and should be. When a systematic approach to acid treatment design is followed, the controllable potential causes of failure can be addressed and eliminated to a great extent. The chance of success is thereby greatly increased. 

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. 

All possible steps (i.e., maximum-step acid treatment design) are discussed in the following sections. 

Nevertheless, the location of minerals is of greatest importance. Scanning electron microscopy (SEM) and thin-section analysis are additional tools that are useful in identifying locations of quartz, clays, feldspars, carbonates, and other minerals. These are all factors considered in acid treatment design. Expertise is available through the major service companies, who should be contacted to help in treatment development. Input from company geologists should also be sought. 

In 1984, McLeod introduced his acid-use guidelines to the industry. This was a breakthrough in formation-based acid treatment design. The original guidelines focused on mineralogy, which was—and still is—often overlooked. McLeod s guidelines are shown in table 6-5. 

Determination of the proper fluid placement method is thus a key factor in acid treatment design in both carbonates and sandstones. Treatment success can hinge on it. More often than not, some method of placing or diverting acid is required in order to distribute acid across the zone or zones of interest. This is especially true in matrix acidizing. 

Service company and operator representatives should, however, always discuss and work out a mutually acceptable method of placement and/or diversion to be included in an acid treatment design. The procedure may be chosen on the basis of prior experience and examples. The remainder of this chapter is intended to provide such a basis for placement/diversion selection... 

Naturally, during injection, add takes the path of least resistance, as does any injected fluid. Anyone who has worked on acid treatment design or treatment execution has no doubt heard or said that before. The paths of least resistance will be those formation sections or layers with the highest permeability and the least damage. Higher-permeability streaks or natural fractures will accept fluid more readily than zones of lower permeability or injection capacity. 

None of these acids react appreciably with sand (SiO ). They will react with iron compounds or minerals containing iron. Acetic acid forms a complex with iron in solution and helps prevent iron precipitation. Iron content in the formation—and its source—must be considered in carbonate acid treatment design. In formations with high iron content, it may be necessary to cut back on HCl concentration or substitute part or all of the HCl with acetic acid. 

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