Acid jobs

The aim of a matrix acidizing job, which, in carbonate reservoirs, generally consists in injecting plain hydrochloric acid at pressures below the formation parting pressure, is to decrease 5 down to zero. 

Comparison with Actual Skin Histories during Acid Jobs... 

Well A.. This oil producer was to be converted into a water injector. Though its production period had been relatively smooth, with no sign of production damage build-up, the operator wanted to be sure to obtain a maximum injectivity index before initiating water injection. The well data and acid job main parameters are summarized in Table I. 

Occasionally an acid job may leave a quantity of acid trapped in the formation, or chemicals used in i frac or other workover job may be trapped Such trapped materials may break out weeks or even months later and cause temporary treating upsets. 

Stimulation of horizontal and multi-lateral wells presents a very challenging problem. With a very long target zone(s), acid diversion plays a very important role in determining the efficiency of the acidizing job. Without a proper means for acid diversion, most of the injected acid... 

The American Petroleum Institute recommends conducting acid sludge test API RP 42 before performing acidizing jobs in the field [39], These tests are used to determine the type and concentration of the surfactant (nonionic or anionic) needed to break acid-in-oil emulsions in a reasonable period of time. It is very important to perform the tests using live and spent acids at reservoir temperature. Spent acid should be prepared in the lab using formation rock. The live and spent acids should include acid additives as per field formula. The crude oil sample should be fresh, and free from water and oilfield chemicals (e.g., scale inhibitors, demulsifiers, etc.). 

The droplet size of the dispersed phase plays a key role in the effectiveness of the acidizing job. Too fine or coarse droplets will adversely affect the efficiency of the stimulation job. The acid-in-diesel emulsion was examined under the microscope and the drop size distribution of the dispersed phase determined using a phase contrast technique. 

A larger volume of acid is needed to conduct the matrix-acidizing job... 

Acid job Refers to when acid is poured or pumped down a well to dissolve limestone and increase fluid flow. 

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. 

The popular perception is that acid jobs have a high failure rate. That is unfair, as success is very much site specific and largely dependent on practices employed. In any case, acidizing is often considered a hit-or-miss prospect. This reputation is understandable, if based on single-well results alone, ratber than collective multi-well or fieldwide results, and if the factors leading up to—and during—the acid treatment are not considered. 

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. 

It is then necessary to determine if a treatment placement method must be employed mechanical, chemical diversion, or a combination. The final steps are taken after the design procedure is determined and the acid job is to be pumped. Proper treatment execution and quality control practices... 

While there are multiple but limited number of reasons why acid jobs fail, the two most frequently encountered are that... 

A. Inspect all tanks that will be used to hold acid or water. The tanks must be clean. Small amounts of dirt, mud, or other debris can easily ruin an acid job. 

Special precautions must be taken in a geothermal acid job, especially in the remote or jungle locations common to geothermal fields. The safety of local inhabitants who reside near the well sites is of paramount concern. All empty chemical containers must be cleaned and disposed of immediately. 

These treatments were conducted in naturally fractured Monterey shale. Based on DST results, formation damage was suspected. Evaluation of previously unsuccessful acid jobs in the same zones, drilling records, workover data, and laboratory testing suggested that formation damage was due to one or more of the following ... 

Incorporation of quality control measures during all aspects of an acid job can make the difference between success and failure. Quality control monitoring during the actual pumping of the treatment only is not sufficient. In addition, quality control steps must be planned and executed during rig-up of equipment, before pumping, and after pumping. 

Unfortunately, this potential problem was discovered after the acid job, not before. Inspection of tanks after pumping, conducted in the interest of assigning blame, showed that the nearly empty preflush tank contained the material from the prior cement job. This was suspected to be the principal culprit in the stimulation treatment failure. 

Currently available corrosion inhibitors were formulated for carbon and low-alloy steels and their performance with standard materials is snfficiently known. Recently some data on the corrosion behavior of corrosion-resistant alloys dnring acidizing jobs and using standard corrosion inhibitors, became available. However, only few data have been published, restricted to low or medium temperatures, and medium acid concentration. 

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