Earth boring – well treating – and oil field chemistry – Preventing contaminant deposits in petroleum oil conduits
Reexamination Certificate
2000-11-02
2003-03-18
Tucker, Philip (Department: 1712)
Earth boring, well treating, and oil field chemistry
Preventing contaminant deposits in petroleum oil conduits
C507S933000, C507S934000, C507S269000, C507S277000, C507S274000, C507S256000, C507S258000, C166S300000, C166S307000
Reexamination Certificate
active
06534448
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
In one aspect, the invention relates to the treatment of a hydrocarbon-bearing subterranean formation with an acidic composition to facilitate the recovery of hydrocarbons from the formation. In this aspect, the invention more specifically relates to the acid treatment of such a formation in the presence of ferric ions. It still further relates to the treatment of a hydrocarbon-bearing subterranean formation with a specific aqueous mineral acid composition formulated to prevent the precipitation of ferric hydroxide and/or free sulfur in the formation and to resist the formation of sludge in crude oil in the formation.
In another aspect, the invention relates to the treatment of metal surfaces (for example, the internal metal surfaces of industrial equipment) with an acid solution to remove scale and other deposits therefrom. In this aspect, the invention more specifically relates to the acid treatment of such surfaces with a mineral acid composition formulated to prevent the precipitation of ferric hydroxide and/or free sulfur during the treatment.
2. Description of the Prior Art
Formation acidizing or “acidizing” is a well-known method for increasing the flow of fluid from a subterranean formation. The formation is contacted with an acidic composition to react with and dissolve materials contained therein for the purpose of increasing the conductivity of the formation. The flow of fluid from the formation is therefore increased because of the increase in formation conductivity caused by the dissolution of the material.
A common method of acidizing a subterranean formation comprises the steps of conducting an acid composition to the formation through tubing disposed in a borehole penetrating the formation, forcing the acid composition into contact with the formation and permitting the acid to react with and dissolve certain materials contained in the formation to thereby enlarge pore spaces within the formation and thus increase the permeability of the formation. The acidizing of calcareous formations, such as limestone formations, has been successfully conducted utilizing hydrochloric acid, certain organic acids such as acetic acid, citric acid and formic acid and mixtures thereof.
The object of formation acidizing—increasing formation conductivity—can be frustrated if precipitates are produced in the acid solution. The precipitates can fill and plug pore spaces in the formation with the consequent result of failing to increase and possibly even decreasing formation conductivity. Problematic precipitates include compounds containing iron, nonferrous metals, free sulfur and metal sulfides.
It is well known that in acidizing a subterranean formation contamination of the acid solution with dissolved iron is inevitable. For example, many acid solutions (e.g., hydrochloric acid solutions) dissolve rust, mill scale and other iron-containing scale from metal conduits (such as the tubing disposed in the borehole) and equipment associated with the well, and also dissolve iron-containing minerals (such as magnetite (Fe
3
O
4
)) present in the formation. As the acid reacts and spends, the pH of the solution increases. Once the pH of the solution reaches a level of about 2.5, dissolved iron present in the solution in the ferric, Fe(III), oxidation state begins to precipitate in the form of ferric hydroxides (e.g., Fe(OH)
3
, Fe(O)(OH), etc.). The ferric hydroxide precipitate can plug the formation and thus cause serious well damage. Ferrous hydroxide is much more soluble and typically not as much of a problem.
Wells containing quantities of sulfide and particularly hydrogen sulfide are sometimes referred to as “sour wells.” In these wells, regardless of what they are called, the combination of sulfide ions and iron creates precipitation problems. Sulfide ions reduce ferric ions to ferrous ions by the following reaction:
2
Fe
3+
+S
2−
→S
°⇓+2
Fe
2+
The resulting elemental or free sulfur can precipitate, plug the formation and thus cause serious well damage.
Unfortunately, attempts to control precipitation problems by maintaining the pH of the acid solution below a certain level, for example below 2.5, have failed. It simply is not feasible in most acid treatment operations to prevent the pH of the acid solution from increasing to a level of 2.5 or higher at some point in the operation. For example, when an acid such as hydrochloric acid is used to acidize a calcareous (e.g., limestone) formation, the acid typically spends to an extent such that the pH of the acid solution increases to a value of 4 or higher.
Another problem is the formation of sludge in crude oil in the formation. For example, strong hydrochloric acid solutions (e.g., ≧15% by weight) can cause the development of sludge when the acid contacts crude oil. The presence of ferric ions in the crude or acid solution makes controlling the sludge difficult or even impossible. The quality and quantity of precipitated sludge is also related to formation temperature, acid concentration, and the concentration of asphaltenes and maltenes in the crude oil.
The presence of acid soluble ferric ion in an acidizing composition can cause other problems as well. For example, the ferric ion can lead to increased corrosion, additive separation and emulsion formation.
Numerous acidizing compositions and methods for controlling precipitation and sludge during acidizing treatments have been developed heretofore. The effectiveness of such compositions and methods varies depending upon the type of acid used, formation characteristics and conditions and other factors known to those skilled in the art.
Precipitation out of the acid solution is particularly a problem when the acid solution includes one or more mineral acids. For example, organic acids alone do not dissolve a significant amount of iron scale until the associated temperature reaches around 400° F., which often does not occur. Furthermore, ferric ion is more difficult to reduce in mineral acid systems than in organic acid systems. As discussed below, in mineral acid systems, reducing agents alone typically do not sufficiently reduce ferric ion.
There is a continuing need for improved compositions and methods for controlling precipitation and sludge during acidizing operations, particularly when mineral acid systems are utilized. There is also a continuing need for improved mineral acid solutions for acidizing metal surfaces such as the internal surfaces of industrial equipment without generating damaging precipitate.
SUMMARY OF THE INVENTION
The present invention provides a mineral acid composition that is very effective for treating wells and carrying out other operations while at the same time controlling damaging precipitation problems. The invention also provides a method of treating a subterranean formation with a mineral acid solution in the presence of ferric ions. Precipitation of insoluble compounds such as ferric hydroxide and free sulfur from the acid solution and formation of sludge in association with crude oil is prevented.
In one aspect, the invention is an acidizing composition comprising an aqueous mineral acid solution, an electron donor agent and an electron transfer agent. As discussed below, each component plays a critical role in the performance of the composition.
The aqueous mineral acid solution comprises one or more mineral acids such as hydrochloric acid or a mixture of one or more mineral acids with one or more organic acids such as acetic acid.
The electron donor agent is preferably selected from the group consisting of (1) a thiol (mercaptan) compound having a carbon chain that includes an oxygen or oxygen containing functional group (e.g., HO—, RO—) (preferably in the beta position), (2) hypophosphorous acid (H
3
PO
2
), and (3) a hypophosphorous acid precursor (e.g., a salt of hypophosphorous acid).
The thiol (mercaptan) compound suitable for use as the electron donor agent is preferably selected from the group consisting of compounds of the formula HSCH
2
C(O)R
1
Dougherty, III Clifford C.
Halliburton Energy Service,s Inc.
Kent Robert A.
Tucker Philip
LandOfFree
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