Acid placement

There are physical—chemical differences between fats of the same fatty acid composition, depending on the placement of the fatty acids. For example, cocoa butter and mutton tallow share the same fatty acid composition, but fatty acid placement on the glycerin backbone yields products of very different physical properties. 

Proteins are made up of combinations of the twenty common amino acids. Placement of each amino acid is specified by a three-nucleotide codon. Four different nucleotides are available to code taken three at a time, they can form sixty-four combinations. Some amino acids have more than one codon (a phenomenon known as redundancy). Translation also requires a start codon and stop codon. 

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. 

Given the different factors that we can influence to increase zone coverage, selection of an acid placement method sometimes requires creativity and always requires being open to all options and combinations thereof. With respect to selection of acid placement methods, service company/operator guidelines and charts have proven repeatedly to be of limited use. 

The inchnation that we have as engineers to develop and use standardized selection methods for certain sets of well and treatment conditions is not appropriate for acid treatment placement and diversion. Therefore, only limited information in that regard will be provided here. One is instead encouraged to draw upon one s creative and cooperative talents to develop an understanding of and facility with acid placement design. Remember, however, that no single method or combination of methods will be effective in all cases. 

The importance of treatment placement was evident in the earliest acid treatments. In his patent, Frasch noted the need for a ruhher packer for isolation, so that acid could be selectively injected into the formation. Although this need has been recognized from day one, the absence of proper acid placement is probably still the biggest reason why add jobs fail (besides improper well diagnosis). A well-conceived, properly designed treatment in all other aspects (formation damage assessment, selection of acid types, concentrations, volumes, and additives) can go for naught if the treatment is not properly placed. The zone of interest must be suffidently contacted by stimulation fluids. 

The goal in acid placement is to equalize the acid injection rate per unit area Q/A) across the entire treatment interval, divided into n sections ... 

Mechanical placement of acid was the first means thought of for improving contact with the interval to be treated. Currently, the following general acid placement methods exist ... 

Packer systems for acid placement in horizontal wells do exist, however, and can be effective. The limitation that does exist with such systems is the number of placement settings or injection stages that can be applied. Typically, the rubber packer sealing elements can withstand 10-15 settings in a particular well condition before the risk of packer failure becomes high. The number of settings depends on the packer and downhole conditions. 

Perhaps the greatest need in the acidizing industry is for an effective, nondamaging, inexpensive diverter or acid placement method. Inasmuch as treatment conditions are so different from one well to another, such a diverter is probably a fantasy. However, it is possible to continue improving on existing methods and, certainly, on well assessment and selection of acid placement technique. For commercially available chemical diverter types currently in use and widely accepted industry guidelines, see table 6-9. 

Three effective methods of acid placement in carbonates are used alone or in combination ... 

For successful acidizing of horizontal wells producing from carbonate formations, the keys are treatment placement and efficient use of acid fluids. It is necessary to utilize one or more methods of acid placement or diversion, to impart treatment coverage over as much of the target interval as possible. This is especially challenging in open-hole completions or those utilizing slotted or predrilled uncemented liners. Such completions preclude use of ball sealers and most solid-particulate diverters. 

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