FIELD OF INVENTION
This invention relates to the treatment of subterranean formations using acid and scale inhibitors. In a preferred aspect, the invention involves a one-step process for acidizing and scale treatment of a formation.
BACKGROUND OF THE INVENTION
The deposition of scale from both produced and source waters is common in oil-producing operations. Depositions occur downhole and in flow lines, separators, and other surface facilities. Scale is a problem in acid/scale squeeze operations where iron is present as an impurity either in the water or the reaction product of the acid with scale or oil field equipment. The reaction of the impure iron with scale inhibitors may produce solids that damage the formation or block production equipment. Because of the precipitation resulting from the mixing of the incompatible fluids, present procedures generally require two separate operations for wells that need both well work overs (e.g. acidizing) and scale squeezes. With two separate operations, the well is first treated with concentrated HCl, and the resulting spent acid is cleared out of the well. The second step consists of carrying out the squeeze operation with scale inhibitor. Another possibility is to treat the well sequentially by inserting pads of liquid between the HCl/spent acid and the fluids containing the scale inhibitor. These procedures are more expensive and result in more down time than a combined operation.
SUMMARY OF THE INVENTION
The method of the present invention involves the steps of
(a) injecting into a subterranean formation a treating solution of
(i) an acid that reacts with downhole materials and equipment to form CaCl2, and iron ions,
(ii) a scale inhibitor, and
(iii) an additive capable of preventing the scale inhibitor from forming a precipitate with iron ions; and
(b) after a suitable shut in time, producing fluid from the well.
Each of the three fluid components of the treating solution may be as follows:
(1) Acid: Typical well treating acids include aqueous solution of 1 to 38% HCl, with 15% HCl being preferred. Other acids include sulfamic acid hydrofluoric acid, acetic acid, formic acid, etc. and mixtures thereof.
(2) Scale inhibitor: The preferred oil field scale inhibitors include phosphate esters, phosphonates, sulfonates, and polyacrylates. Other scale inhibitors include copolymers and terpolymers of acrylates, sulfonates and phosphonates, phosphinico polycarboxylic acids (PPCA) and mixtures thereof. Particularly useful scale inhibitors include 2-hydroxyethyl imino bis methylene phosphonic acid, fatty amine phosphonates, triethanolamine phosphate ester, DETA phosphonate (pentaphosphonates) and TETA phosphonate (hexaphosphonates).
(3) The additive for preventing or inhibiting the formation of a precipitate of iron ions and the scale inhibitor may take a variety of forms such as chelating agents, or agents for maintaining the iron ions in the ferrous state. The preferred additive, however, is a reducing agent for reducing Fe+++ to Fe++. Example reducing agents include ascorbic acid, citric acid and thioglycolic acid, oxalic acid, and erythorbic acid and mixtures thereof. The preferred reducing agent is erythorbic acid.
The three water soluble or dispersible components may be used in one solution, or pumped in any sequence. It is preferred that they be used in one treating solution in the following concentrations (wt. %):
| || |
| || |
| || || ||Most |
| || ||Preferred ||Preferred |
| ||Range ||Range ||Range |
| || |
|HCl || 1-38 ||10-15 ||15 |
|Scale Inhibitor ||0.5-50 ||0.5-10 ||1-5 |
|Additive (e.g. reducing agent) ||0.05-50 ||0.1-5 ||0.1-1.0 |
In the oil field squeeze operation, the treating fluid may be premixed (“batched”) or mixed continuously (“on the fly”) during injection. All of the components may be blended together or they may be injected in any sequence of 1, 2 and 3.
(1) injection of a solution HCl
(2) addition of reducing agent
(2) injection of the scale inhibitor solution.
In either process, the precipitate preventative (e.g. reducing agent) reacts with the iron ions so that upon contact with the scale inhibitor, no precipitate is formed.
The amount of treating fluid squeezed into the formation will depend on several factors including the degree of scaling, the type of scale, length of perforations, etc. From 1 to 100 barrels per foot of perforations will be satisfactory for most treatments.
The squeeze operation may be carried out with other work over procedures and may be in accordance with procedures well known in the art, including the use of corrosion inhibitors and other well treating chemicals.
Once the well is returned to production chemicals squeezed into the formation will slowly be produced along with well fluids and will treat the well.