Reservoir Engineering

Wellbore Cleanout and Scale Removal

Learning Objectives

After completing this topic “Wellbore Cleanout and Scale Removal, you will be able answer to:

  • Wellbore Cleanout and Scale Removal
  • wellbore cleanout and scale removal tool
  • wellbore cleanout and scale removal system
  • wellbore cleanout and scale removal process
  • wellbore cleaning procedure
  • Materials Used in Wellbore Cleanout
  • Wellbore Cleanout Procedure
  • Scale Problems
  • Scale Types
  • Scale Removal
  • Scale Prevention

By completing this topic and then applying the learnings in your job, you will be able to attain Basic Application Competence Level in Acidizing and Other Chemical Treatments.

Wellbore Cleanout: General Description

Wellbore cleanout is generally a soaking and agitation treatment designed to open up perforations and formation damage occurring in perforations or in the immediate wellbore vicinity. This technique allows the treating solution to dissolve sludge and acid-soluble precipitants (scale) and recover mud filtrate, formation fines, and other materials that might restrict production. Removing these contaminants permits the well to produce at its natural potential.

Special attention is required for wellbore cleanout on the increasingly rare occasions when oil-external emulsion muds and true oil-base muds are used. Emulsions used for drilling are usually fairly low-solids, oil-external, oil-wet, clay-stabilized emulsions. Oil-base muds are usually prepared by gelling a viscous, highly asphaltic oil and adding to it blown asphalt, fine calcium carbonate, and other materials that impart additional fluid-loss properties. An approach used to successfully remove formation damage caused by oil-base muds is discussed later.

A well is cleaned out prior to production testing (or before it is placed on initial production) or whenever a producing interval begins to “clog up.” This type treatment is designed only to remove near-wellbore contaminants. The treating solution is not intended to penetrate the formation beyond the immediate surface of the perforations or face of the productive interval.

Materials Used in Wellbore Cleanout

Prior to the selection of a treating solution, formation fluid samples should be taken to determine the presence of chlorides, sulfides, sulfates, and pH. If a sample of scale and/or sludge can also be obtained, a more thorough laboratory analysis can be performed.

Many types of treating solutions may be used for wellbore cleanout. The constituents are based on type of damage, formation type, general formation conditions, and the treating technique.

Pure Solvents: Xylene or toluene alone is most effective in removing paraffinic and asphaltic sludges and oil-base mud. However, care must be taken in the selection of a hydrocarbon solvent when the reservoir is dry gas. Solvents such as kerosene and diesel may cause relative permeability effects which can seriously hinder production. For these applications, aromatic solvents, such as xylene and toluene, are preferred. They are more easily removed from the formation than are straight-chain solvents.

Mutual Solvent Systems In acidizing, mutual solvents are used in the postflush to:

  • reduce surface and interfacial tensions to promote cleanup and fluid load recovery
  • strip oil and sludge layers from acid-soluble materials
  • prevent emulsions from forming
  • break existing emulsions
  • assist in preventing or removing water blocks

Commonly available mutual solvents are

  • Amoco A-Sol: a blend of water-soluble and oil-soluble alcohols
  • EGMBE: ethylene glycol monobutyl ether
  • Amoco A-Sol A-28: a blend of alcohols
  • Amoco Super A-Sol: a blend of alcohols and xylene
  • A variety of proprietary service company mutual solvents (e.g., MS-10, MS-12, MS15 (BJ Services), MS-200 (Nowsco), Musol A (Halliburton), MAS (Serfco), and WSA-2, EZ-Sol and MAS (Western)).

All these mutual solvent/acid systems can break through a thin sludge coating and allow acid to contact the surface of the solid covered by sludge. However, mutual solvent/acid systems do not disperse sludge.

To remove thick sludge, an aromatic solvent soak must usually precede a mutual solvent/acid treatment. A two-hour xylene or toluene soak can soften sludge layers and allow mutual solvent/ acid penetration. Plain acid following an aromatic solvent soak generally has no positive effect.

Oil-base muds present a special problem in acid treatment. Many of these muds contain emulsifiers that can develop very stable emulsions with acid or spent acid. In addition, mud solids and drill cuttings provide the fines to stabilize the emulsions. An aromatic solvent soak followed by mutual solvent and acid usually disperses it completely. The ideal mutual solvents for this operation are Super A-Sol, A-Sol, or EGMBE, and the acid should be weak intensified acid (1/2% HF), if possible. The treatment area is usually confined to the wellbore and open natural fractures, since the fines, which are necessary for the emulsion, do not penetrate into the formation.

To avoid problems in completing a well drilled with an oil-base mud, circulate the well with solids-free mud and then flush with xylene. The xylene dilutes the mud and causes any remaining solids to drop out of suspension. These solids can be circulated out or removed by an appropriate mutual solvent and acid.

Acid-External Emulsified Acid This acid system is usually composed of 10% xylene, with a surfactant which disperses the aromatic solvent in the acid, and 90% acid phase that is usually 15% HCl. Acid nonemulsifiers are not used because they may make the acid dispersion unstable. Moreover, performance experience in the field shows that nonemulsifiers have little effect even when injected into oil-producing intervals.

Stability of the acid-external emulsified acid is influenced by three factors: the degree of agitation during preparation, concentration of the xylene phase, and strength of the acid phase. Increasing the xy1ene or acid strength decreases stability. Should separation occur, the aromatic solvent can be readily redispersed by circulating.

5 to 15% HCl This solution, with a high concentration of anionic surfactant (1 to 3% by volume), is widely used for well-bore cleanout. The high surfactant concentration reduces surface tension and makes the acid highly adaptable to both carbonate and sandstone formation as a first-stage wellbore treatment for water-base mud removal. Even when diluted with 20 volumes of brine, surface tension properties remain very low.

Another characteristic of this solution is that clay solids, which normally have a tendency to cluster when contacted by acid, are effectively dispersed for easier removal. In addition to shrinking hydrated mud by acid reaction, this mixture softens and allows the removal of harder and more tenacious filter cakes.

Weak Intensified Acid (1/2% HF) Weak intensified acid, consisting of 1/2% HF, is useful in unplugging perforations and establishing flow for HCl or HCl-HF acid mixtures. HF is especially useful for dissolving clays and silica, making it a good choice for sandstone acidizing. In carbonates, however, HF must be used in extremely low concentrations to avoid formation of insoluble precipitants such as calcium fluoride.

Wellbore Cleanout Procedure

The number of soaking and agitation applications depends on the amount of damage that has occurred in perforations or in the immediate wellbore area. Treating solutions designed for suspension, aromatic-solvent acid dispersions, or cleanup types are normally used in a soaking action. The treating solution is placed, or spotted, across the perforations or producing interval, and is allowed to set for a fixed period of time. The treatment load is then recovered.

Agitation can be accomplished by one of three methods. One technique is to spot the treating solution across the perforation, allow a short soaking period, and then wash it back through the annulus by reciprocating or rotating the work-string. Another method is to pressure-up, staying below fracturing pressure, and then release the pressure very quickly. This action is sometimes referred to as “backsurging the perforations.” The third method is to spot the treating solutions across the perforations, allow it to soak for awhile, and then swab back through either the casing or the tubing, or jet the treatment back with nitrogen.

With any of these techniques, the treating solution may have to be applied several times before the formation is opened for fluid entry. The use of several wellbore cleanout applications allows for a matrix or fracturing acidizing treatment to be performed without fear of pushing unwanted plugging material into natural formation permeability.

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