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Monday, September 13, 2010

Reworking of a gas or oil well can occur for up to 20 years and can involve these processes



Acidizing is the second of the three methods discovered to promote well stimulation. Acidizing is injecting an acid under pressure into a formation to enlarge the pore spaces and passages in order to increase productivity.


Acidizing was first performed in 1932 by the Pure Oil Company in cooperation with Dow Chemical Company. By 1934 acidizing was commonly used in addition to shooting, and these were the only two known methods for well stimulation until fracturing was invented in 1948. Today it is known that certain kinds of formations respond better to acidizing than the other methods. Acidizing is still widely used to:

· Dissolve rocks in the productive formations

· Open new channels to the wellbore

· Reduce formation resistance to the wellbore

Description of the Process

In the acidizing process, acids such as hydrochloric acid, formic acid, and acetic acid are pumped into the wellbore under pressure so as to allow the acid to react chemically with the rock in order to enlarge existing channels or to create new ones. There are two basic types of acidizing treatment:

Low pressure acidizing to avoid fracturing the formation and allow the acid to work through the natural pores of the rocks

High-pressure acidizing, also called “acid fracturing”, where sufficient volume and pressure is maintained in order to keep the fractures open while the acid is injected

The acid used for the treatment must create products of the reaction that are soluble and which can be easily removed from the well. Since large volumes of the acid are used, it must be relatively inexpensive and safe to handle. Additives are used with the acids for several purposes, but the most important purpose is to inhibit the acid from attacking the steel tubing or casing in the well. There are several types of additives used:

Inhibitors are used to prevent the acid from working on steel for several hours

Surfactants are mixed in small amounts with the acid to make it easier to pump the acid into the formation and prevent the acid and oil from forming emulsions

Sequestering agents are added to control the precipitation of iron deposits from spent acid solutions and from scales of iron sulfide, iron oxide, and iron carbonates found in well tubing and casing

Suspending agents are used to suspend the fine clay and silt particles that may remain in the well

Oil reservoir rocks most commonly acidized are limestone-calcium carbonate, dolomite (a mixture of calcium and magnesium carbonates), and calcareous sands.

The service of acidizing the formation of a well begins with the perforating of the well casing with an acid jet gun, or, if the casing is already perforated, the service begins with the setting up of all the equipment necessary for servicing the well and ends with the removal of the equipment required to perform the acidizing service.

Non-Taxable Services

The following are services which are not subject to the oil and gas well servicing tax as not all acidizing treatments are considered to be acidizing of the formation:

Acidizing to recover fish or stuck pipe. During drilling operations it is not uncommon for the drill pipe to become stuck in the hole. This may be caused by sloughing of the material around the borehole, insufficient mud circulation to remove the bit cuttings from the hole, or numerous other reasons. Through the proper use of acids, it is possible sometimes to dissolve the material causing the sticking and thereby release the drill pipe so that the drilling may continue. Similarly, the casing sometimes becomes stuck while being lowered in the borehole due to the same conditions causing the drill pipe to stick. Acids may be used to dissolve the material so that the running of the casing may continue.

When tools or other objects are dropped or “lost” in the borehole, acids may be used as an aid to fishing tools. In this case, dissolving of the formation or other substances may allow the fishing tools to function as desired. The goal is to recover the lost objects so that drilling may continue.

Acidizing to clean screens. In some wells it is necessary to use screens or strainers or pipes positioned in the well to prevent encroachment of undesirable substances. Acid is frequently used to clean these screens or strainers of drill mud, carbonate scales, sulfate scales, iron sulfide, or other substances which have plugged the screens or strainers.

Acidizing to dissolve mud sheaths. A function of drilling mud used in drilling or rework operations is the forming of a mud cake or sheath on the wall of the hole in order to prevent sloughing of the formations into the borehole. While this is desired during the course of drilling operations, this mud cake must be removed from the productive formation when completing the well so that the natural fluid flow in the formation will not be restricted. Acid mixtures, commonly called mud acid, are used to dissolve and remove this mud cake. The objective of such acid mixtures is to dissolve the mud sheath and not the productive formation.

Acidizing soluble metals. Pipe or special tools made from certain metals are soluble in acids. Should their removal from the wellbore be desired, it is accomplished through the use of acids.

Acidizing to dissolve paraffin deposits in the tubing. This acidizing is not for the purpose of acidizing the formation nor is it performed during the drilling, completion, and/or reworking or reconditioning of the well.



Fracturing is one of the methods of well stimulation in addition to shooting and acidizing. In fracturing, commonly called a “frac job,” sand and a fluid mix are forced into the formation to open cracks. When the fluid is removed, the sand particles remain in the cracks in order to keep them propped open so that the liquid hydrocarbons can flow into the wellbore.


Fracturing was the last of the three known methods of well stimulation to be introduced. It was first used in the Hugoton gas field in western Kansas in 1948, and has since gained wide acceptance in the oil and gas industry. It is commonly called “hydraulic fracturing’ due to the use of hydraulic equipment to create the extremely high pressures needed to crack open or break the tight formations. Today, hydraulic fracturing is used to accomplish four basic jobs:

Overcome wellbore damage

Create deep penetrating reservoir fractures in order to improve the productivity of a flowing well

Aid in secondary recovery operations in order to enhance the productivity of a well

Assist in the injection or disposal of brine and industrial waste material


Description of the Process

The fracturing process consists of the application of hydraulic pressure against the formation by pumping fluid (gel) laden with some type of particle into the well. The formation is actually split by the pressure of the fluid. The amount of pressure, sometimes as high as 10,000 pounds per square inch (psi), and the equipment necessary to furnish the pressure vary according to the requirements of each situation. The fluid pumped or injected into the well contains sand or other solids (glass beads, walnut shells, etc.) called “propping agents” which are deposited into the cracks and fissures created by the pressure. The sand or other solids injected into the well act as props after the pressure is released. The most common fracturing fluid is diesel fuel, but refined oil, crude oil, salt water, acids, and emulsifiers are some other fluids used.

Some examples of ways in which hydraulic fracturing can be applied to a producing well are:

With a single packer in the hole

Pulling the tubing so that the fluid can be pumped down the open casing. This allows much more fluid to be pumped down the hold without exceeding the rated maximum pressure of the wellhead fittings.

Pulling the tubing and using a “straddle packer.” This makes it possible to isolate one zone from another and selectively fracture several zones by simply changing the position of the packer

Where perforated casing is involved, sealing balls may be injected into the fluid. The high velocity of the fluid will transport the balls to perforations which are “taking fluids,” seal them off, and direct the hydraulic fluid to other perforations in order to create multiple zones. If the perforated zone is to be abandoned, various-sized balls are used to plug the perforations. When the injection is stopped, all remaining excess balls will fall to the bottom of the well.

The service of fracturing a well begins with the setting up of all the equipment required to perform the fracturing service, and it ends with the removal of that equipment.

Non-Taxable Services

The only fracturing service not subject to the oil and gas well servicing tax is:

· Fracturing of injection or disposal wells

Thank goodness they don't tax injection wells!



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