WO2004011771A1

WO2004011771A1 - Oil well treatment

Info

Publication number: WO2004011771A1
Application number: PCT/GB2003/003332
Authority: WO
Inventors: Harry Trenouth Russel Montgomerie; Ping Chen; Thomas Hagen; Rex Man Shing Wat; Olav Martin Selle
Original assignee: Champion Technologies, Inc.; Statoil Asa
Priority date: 2002-07-31
Filing date: 2003-07-31
Publication date: 2004-02-05
Also published as: AU2003246977A8; GB0217737D0; NO20051041L; AU2003246977A1

Abstract

A new use of polyaspartate is described for reducing particle migration in rock formations. The use of polyaspartate has beneficial effects in oil wells, where particle migration is responsible for blocking of pores in rock formations and consequent losses in productivity. Also disclosed is a method of treating a rock formation by well intervention, characterised in that the rock formation is contacted with a dispersion of a polyaminoacid prior to, or simultaneously with, the well intervention. The method reduces the amount of particle migration during well intervention.

Description

OIL WELL TREATMENT

This invention relates to the prevention of particle migration in rock materials. More particularly, this invention relates to the prevention of clay particle migration in oil- producing rock formations.

Oil production relies on the escape of oil held under pressure in subterranean rock formations. By drilling a bore into the rock formation, the pressure which traps the oil in the rock is vented, allowing oil to escape through the bore. One of the factors which influences the rate of oil production is the permeability of the rock formation. The permeability of the rock formation depends on the size of pores and also the size of internal capillaries in the rock formation. Any constriction in the capillaries or blockage of the pores will cause a reduction in the permeability of the rock, resulting in a reduced rate of oil production.

A disturbance of the rock formation may cause fine particles to be dislodged in the rock formation. When fine particles are dislodged they may become entrapped in fluids moving at a high flow rate, and migrate through the rock formation. The migration of particles through the rock formation is undesirable, because the moving particles can deposit in capillaries and/or plug pore spaces, causing a reduction in rock permeability and hence a reduction in the rate of oil production.

Well interventions introduce foreign liquids, such as water and various aqueous solutions, into the rock formation. Typical well interventions are scale inhibitor treatments, corrosion inhibitor treatments, acid treatment or fracture stimulation. These types of intervention may cause particle migration. However, particle migration may occur without any well intervention. For example, fluid moving with a high flow rate may be sufficient to cause particle migration. Alternatively, water breakthrough, whereby aqueous impurities become mixed with the oil, may cause particle migration.

Clay particle migration is a particular problem in oilfields. Nearly all oil producing sandstone contains some clays, occurring as a coating on individual sand grains and/or discrete particles mixed with the sand. The clays most frequently found in hydrocarbon zones are bentonite, illite, kaolinite and chlorite. Among these, bentonite is classed as a swelling clay, since it swells on contact with low salinity aqueous liquids. Illite, kaolinite and chlorite may be classed as non-swelling clays.

Clay swelling exacerbates the problems of particle migration. Clay swelling increases the mobility of pre-existing fine particles formerly cemented in place. Furthermore, the expanded clay itself is more likely to undergo disintegration and subsequent migration in the presence of rapidly flowing fluids.

Clay particle migration is not expected in oilfields during normal production. However, clay particle migration frequently occurs during well interventions such as squeeze treatments. If particle migration occurs, serious losses in oil production are experienced, together with losses in associated revenue.

Several methods for controlling clay particle migration have been proposed. For example, a 2% aqueous solution of KC1 can control clay swelling. Acid treatment may be effective in removing clay particles such as kaolinite. In addition, quaternary ammonium salt polymers are also known to be effective in reducing clay particle migration (Borchardt J.K., "Cationic Organic Polymer Formation Damage Control Chemicals", Oilfield Chemistry).

It is an object of the present invention to provide a new method of preventing or reducing particle migration in a rock material.

Accordingly, the present invention provides the use of a polyaminoacid for the prevention or reduction of particle migration in a rock material.

As used herein, the term "polyaminoacid" means any polymeric substance comprised of repeating amino acid units, which may be the same or different. A preferred example of a polyaminoacid used in the present invention is polyaspartate (or polyaspartic acid). Polyaspartate is highly biodegradable and has low toxicity. It is therefore used advantageously in the present invention. Polyaspartate has been identified previously as a scale inhibitor (see, for example, US 5,152,902), and has been used to a limited extent in oil wells for its scale inhibition properties. However, its use as a scale inhibitor has been limited due to its poor return profile.

More recently, polyaminoacids, such as polyaspartate, were shown to improve the retention of other scale inhibitors, such as phosphonates, on rock materials (copending UK patent application). This improved retention means that scale inhibitor squeeze treatments are required less frequently in oil wells.

Hitherto, polyaminoacids had not been proposed for reducing particle migration in rock materials. Whilst not wishing to be bound by theory, it is believed that the polyaminoacid has the effect of binding particles, especially clay particles, together, thus preventing particle migration.

The new use of polyaminoacids, in accordance with the present invention, will make such compounds more attractive for use in oil wells. It is envisaged that treatment with a polyaminoacid could be at any stage in oil production. Preferably, the treatment with a polyaminoacid will be prior to or during a well intervention, in order to mitigate against potential particle migration, which may occur during well intervention.

For example, polyaspartate may be employed as a pre-flush before a standard phosphonate or phosphate ester squeeze treatment. When used as a pre-flush, the polyaspartate will immobilise clay particles in the rock formation and prevent the particles from migrating during the subsequent squeeze treatment. Hence, the use of polyaspartate as a pre-flush will lead to more efficient squeeze treatments, which do not suffer from undesirable losses in productivity due to blocking of pores in the rock formation.

Accordingly, the present invention further provides a method of treating a rock formation by well intervention, characterised in that the rock formation is contacted with a dispersion of a polyaminoacid prior to said well intervention, thereby reducing the amount of particle migration during said well intervention. In another aspect of the present invention, there is provided a method of treating a rock formation by well intervention, characterised in that the rock formation is contacted with a dispersion of a polyaminoacid simultaneously with said well intervention, thereby reducing the amount of particle migration during said well intervention.

Many types of well intervention will be known to the skilled person. Examples of well interventions are scale inhibitor treatments, corrosion inhibitor treatments, acid treatments and fracture stimulation.

Alternatively, a scale inhibitor squeeze treatment may be performed using solely polyaspartate, making use of the dual properties of polyaspartate to act as a scale inhibitor and also to reduce particle migration.

Preferably, the new use of a polyaminoacid, in accordance with the present invention, will be in oil wells where the rock formation comprises a clay material. The clay material may be, for example, bentonite, illite, kaolinite or chlorite.

The polyaminoacid is preferably employed as an aqueous dispersion or solution. Preferably, the concentration of polyaminoacid is in the range of 1 to 50% w/v. It has been found that a concentration in the range of 1 to 50% w/v provides an effective coating of polyaminoacid on the rock material, which is enough to prevent or reduce particle migration. More preferably, the concentration of polyaminoacid is in the range of 5 to 20% w/v, and more preferably about 10% w/v.

The present invention will now be explained in more detail below with reference to the following Figures in which:-

Figure 1 shows the Productivity Index of well A-48 before and after a scale treatment using phosphonate.

Figure 2 shows the Productivity Index of well A-51P, which was subjected to acid treatment and subsequent scale treatment using phosphonate. Figure 3 shows the Productivity Index of well A-29, which was subjected to acid treatment and subsequent scale treatment using phosphonate.

Figure 4 shows the Productivity Index of well A-28, which was subjected to six scale treatments using polyaspartate.

Measurement of Productivity Index

The Productivity Index provides a standardized measure of the rate at which oil is being produced by any given oil well. PI is defined as follows:

PI = Production Rate/(P_reservoir - Pwell bore)

where Preservoir is reservoir pressure Pweii bore is near well pressure

Production Rate is total liquid flow rate

In the following Examples the Productivity Index was determined at different stages in a number of oil wells in the Heidrun Field.

Comparative Example 1

The PI of well A-48 in the Heidrun Field was evaluated over a period of 7 months. Initially, the PI was measured at about 35 Sm³/d/bar, remaining relatively constant for the first 3 months. After about 3 months, the well was subjected to scale inhibitor squeeze treatment using a phosphonate scale inhibitor. The PI was observed to fall sharply to about 20 Sm³/d/bar. The dramatic reduction in PI was believed to be due to clay migration, caused by the inhibitor squeeze treatment.

The results of Comparative Example 1 are summarised in Figure 1. Comparative Example 2

The PI of well A-51P in the Heidrun Field was evaluated over a period of 14 months. Initially, the PI was measured at about 350 Sm³/d bar, remaining relatively constant for the first 6 months. After about 6 months, water was injected into the rock formation to maintain reservoir pressure (water breakthrough). This resulted in a decrease in PL The decrease in PI was believed to be caused by clay particle migration, which may block pores in the gravel pack and near well bore area.

After about I VΛ months, when the PI had fallen to about 20 Sm³/d/bar, the well was treated with acid (Mud Acid: mixture of 1-6%HF and 7.5-16% HC1; and Clay Acid: mixture of 4-7.8% HBF₄, 0.3-0.6% HF and 0.15-5.5% HC1). The acid treatment was designed to remove clay particles such as kaolinite and test whether such particles were, indeed, responsible for the fall in PI. The effect of the acid treatment was to increase the PI, indicating that the earlier reduction in PI was due to the migration of clay particles and blocking of pores.

One week after the acid treatment, when the PI had risen to a level of about 150 Sm³/d/bar, the well was subjected to a scale inhibitor treatment using phosphonate. A decrease in PI was observed immediately following the scale inhibitor treatment, consistent with the results of Comparative Example 1.

Comparative Example 3

This Comparative Example was conducted in a similar manner to Comparative Example 2, but on a different well. The PI of well A-29 in the Heidrun Field was evaluated over a period of 14 months. Initially, the PI was measured at about 140 Sm³/d/bar, remaining relatively constant for the first 2 months. After about 2 months, water breakthrough resulted in a decrease in PI. This decrease in PI was believed to be caused by clay particle migration. After about 7 months, when the PI had fallen to about 60 Sm /d/bar, the well was treated with acid. The effect of the acid treatment was to increase the PI to about 140 Sm³/d/bar, indicating that the earlier reduction in PI was due to the migration of clay particles and blocking of pores.

Three months after the acid treatment, when the PI had risen to a level of about 170 Sm³/d/bar, the well was subjected to a scale inhibitor treatment using phosphonate. A decrease in PI was observed immediately following the scale inhibitor treatment, consistent with the results of Comparative Examples 1 and 2.

Example 1

The PI of well A-28 in the Heidrun Field was evaluated over a period of 17 months. Initially, the PI was measured at about 125 Sm³/d bar, and fell steadily for the first 3 months to about 50 Sm /d/bar. After about 3Vz months, the well was subjected to milling and treatment using 10% w/v polyaspartate. A rapid rise in PI was observed, in contrast to the previous scale inhibitor treatments using phosphonate. Within 1 month, the PI had risen to about 130 SmVd/bar. The well was subjected to five subsequent scale inhibitor treatments using polyaspartate at intervals spaced about two months apart. Unlike the previous Comparative Examples, no sudden reduction in PI was observed after each scale inhibitor treatment. The PI remained at around the 100 Sm /d bar level throughout. It was concluded that the polyaspartate acted as a clay stabiliser, having the effect of confining clay particles together, thus preventing particle migration and blocking of pores.

The results presented herein demonstrate the ability of polyaspartate to prevent or reduce particle migration in rock materials. The skilled person will readily understand that the present invention may be used advantageously in oil wells for avoiding the problems associated with clay particle migration.

It will, of course, be understood that the present invention has been described by way of example, and that modifications of detail may be made within the scope of the invention.

Claims

1. Use of a polyaminoacid for the prevention or reduction of particle migration in a rock material.

2. Use as claimed in claim 1 wherein the polyaminoacid is polyaspartate.

3. Use as claimed in claims 1 or 2 wherein the rock material is an oil-producing rock formation.

4. Use as claimed in claim 3 wherein the rock formation comprises a clay material.

5. Use as claimed in claim 4 wherein the clay material comprises bentonite, illite, kaolinite or chlorite.

6. Use as claimed in any one of claims 3 to 5 wherein the rock formation is part of an oil well.

7. Use as claimed in any one of the preceding claims wherein the polyaminoacid is used in a concentration of 1 to 50% w/v.

8. A method of treating a rock formation by well intervention, characterised in that the rock formation is contacted with a dispersion of a polyaminoacid prior to said well intervention, thereby reducing the amount of particle migration during said well intervention.

9. A method of treating a rock formation by well intervention, characterised in that the rock formation is contacted with a dispersion of a polyaminoacid simultaneously with said well intervention, thereby reducing the amount of particle migration during said well intervention.

10. A method as claimed in claims 8 or 9 wherein the well intervention is a scale inhibitor squeeze treatment, a corrosion inhibitor squeeze treatment, acid treatment or fracture stimulation.

11. A method as claimed in claims 8 to 10 wherein the polyaminoacid is polyaspartate.

12. A method as claimed in claims 8 to 11 wherein the rock formation comprises a clay material.

13. A method as claimed in claim 12 wherein the clay material comprises bentonite, illite, kaolinite or chlorite.

14. A method as claimed in claims 8 to 13 wherein the polyaminoacid is used in a concentration of 1 to 50% w/v.