ACIDIZING FUNDAMENTALS PDF

Acid stimulation refers to using a stimulation fluid containing reactive acids to increase permeability both in production and injector wells, carbonate or sandstone, by dissolving various acid-soluble solids naturally present in the rock matrix or as formation damage. Stimulation by acidizing is an old production enhancement technique dating as far back as the nineteenth century [ 2 ] when Herman Frasch of Standard Oil patented the use of hydrochloric acid HCl to stimulate carbonate formations. Fracture acidizing is to pump the acid treatment above the fracturing pressure of the reservoir rock, which create long, open channels from the wellbore penetrating deep into the formation. Fracture acid stimulation is usually carried out on carbonate reservoirs, which have lower permeability than sandstone reservoirs. It can be used to either remove formation damage or stimulate undamaged formations to produce conductive channels within the fracture where oil and gas can migrate. A problem with fracture acid stimulation is that much of the acid is used up near the wellbore and is not available for etching the fracture faces farther from the wellbore.

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Acid stimulation refers to using a stimulation fluid containing reactive acids to increase permeability both in production and injector wells, carbonate or sandstone, by dissolving various acid-soluble solids naturally present in the rock matrix or as formation damage. Stimulation by acidizing is an old production enhancement technique dating as far back as the nineteenth century [ 2 ] when Herman Frasch of Standard Oil patented the use of hydrochloric acid HCl to stimulate carbonate formations.

Fracture acidizing is to pump the acid treatment above the fracturing pressure of the reservoir rock, which create long, open channels from the wellbore penetrating deep into the formation. Fracture acid stimulation is usually carried out on carbonate reservoirs, which have lower permeability than sandstone reservoirs.

It can be used to either remove formation damage or stimulate undamaged formations to produce conductive channels within the fracture where oil and gas can migrate. A problem with fracture acid stimulation is that much of the acid is used up near the wellbore and is not available for etching the fracture faces farther from the wellbore. In addition, the acid stimulation fluid follows the paths of least resistance and create typically long-branched passageways leading away from the fracture interface, which are called "wormholes".

Later injected fracturing fluid tend to leak off into the wormholes rather than lengthening the desired fracture. Leak off control techniques need to be utilized to avoid this. Matrix acid stimulation is pumped into the formation at or below the fracturing pressure. The objective of matrix acid stimulation in carbonate matrix is to allow the acid to dissolve channels to create wormholes in the near wellbore region, reaching as far as possible into the formation.

Carbonate matrix acid stimulation is also useful to treat carbonate cemented sandstones and formation damage from acid soluble species such as calcium carbonate CaCO 3 or sulfide scales, lost circulation materials, etc.

In sandstone matrix acidizing, the primary purpose is to remove acid-soluble damage in the well and near wellbore area. Treating an undamaged sandstone well with matrix acids does not usually lead stimulation unless the reservoir is naturally fractured.

The most common acid used in carbonate fracture or matrix acidizing is hydrochloric acid HCl. Lower concentrations can be used as pickling acids to clean up the well in a preflush to remove scale and rust or an afterflush. In high temperature applications, HCl does not produce acceptable stimulation results due to its fast reaction that leads to lack of penetration. Organic acids, like formic or acetic acids, can be used to offer a slower-reacting and thus deeper stimulating acid [ 5 ].

Sandstone reservoir is mainly composed of quartz and aluminosilicates such as feldspars. Migration of these particles fines into the pores of the near-wellbore area can reduce production and they will not dissolve in strong acids such as hydrochloric acid, but will dissolve in hydrofluoric acid HF. Although highly corrosive, HF is classified as a weak acid due to its low ionization in water and it is very toxic.

HF, or more usually HF-releasing chemicals such as ammonium bifluoride NH 4 HF 2 , is used for sandstone matrix acidizing, combined with hydrochloric HCl or organic acids. In sandstone acidizing, one has to be particularly careful of reprecipitation of reaction products, which could cause new formation damage [ 7 ]. They occur mostly if the well is shut-in for a long period of time.

The basic chemistry is HF reacts first with aluminosilicates to form fluorosilicates, which react further with clays to form insoluble sodium or potassium flurosilicates. Prevention methods include: 1 overflush with dilute HCl or NH 4 Cl to push the solution deeper into the formation 2 use delayed acid formulations that generate HF slowly 3 buffered acid system that allow for a deeper penetration.

CaF 2 and AlF 3 can also precipitate in the spent acid. There are several other ways that acidizing, both for sandstone and carbonate reservoirs, can lead to formation damage if not carried out correctly. These include:. For sandstone formation acid stimulation, the resulting permeability improvement peaks at certain acid volume and then drops as the volume increases in oil wells, but is proportional to acid volumes in gas wells.

Permeability improvement in gas wells is typically better than oil wells. Mange other additives can also be used, such as clay stabilizer, fines fixing agent, antisludging agent, demulsifier, scale inhibitor , H2S scavenger , drag reducer, foaming agent, etc. Treatment design and planning are often performed to ensure that the acid is placed across the entire interval.

The successful acid placement in matrix treatments of open-hole horizontal wells is even more difficult due to the length of zone and potential variation of the formation properties. A successful diversion technique is critical to place the acid to the location where damage exists. Either mechanical or chemical placement techniques can be used to improve contact of the acid solution with the interval.

The diverter is usually applied in a preflush to temporarily plug the zones of highest permeability zones, allowing the main flush to react with other less permeable or more damaged zones. On back production, the diverter is removed from the formation. These diverters include:. Did you find what you were looking for? Yes No. Toggle navigation. Everything Articles Users. Acid stimulation. Image used herein by limited permission with no further use, copying, license, copyright, or other rights extended, with all rights reserved.

Williams, J. Gidley, and R. Schecter, "Acidizing Fundamentals", Monograph Series, vol. Rae and G. Buijse, P. Klos, and G. Coulter and A. Jennings, "A contemporary approach to matrix acidizing" SPE , Help to improve this page What's this? Please post helpful feedback. By posting, you agree to be identified by your IP address.

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Acid stimulation

In the oil and gas extraction industry , acidizing is a technique used to extend the useful life of an oil and gas well. The process of acidizing involves pumping acid into the well in order to dissolve the rocks that line the contours of the well. Acidizing increases production rates by creating channels into the rock through which the oil and gas can flow into the reservoir. An additional benefit of acidizing a well is that it can help dissolve any loose debris found in the well. Acidizing is often employed to extract the remaining resources from oil wells that have reached the end of their productive lives.

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