WO2001073253A1

WO2001073253A1 - Method and cleaning fluid for use in well cleaning and subsequent treatment of the used cleaning solution, as well as use of a particular fluid mixture

Info

Publication number: WO2001073253A1
Application number: PCT/NO2001/000140
Authority: WO
Inventors: Farzad Nabati Shoghl
Original assignee: Bj Services As
Priority date: 2000-03-31
Filing date: 2001-03-30
Publication date: 2001-10-04
Also published as: NO20001693D0; AU2001248922A1; NO20001693L

Abstract

In well cleaning, a cleaning fluid mixture is circulated in the well, cleaning off and removing dirt and particles of polluting matter that adhere to the interior surface of the borehole wall. Used, dirtied cleaning fluid mixture is considered to be special waste requiring aftertreatment/ disposal. The invention suggests the use of an unstable cleaning fluid mixture, that is an invert emulsion based on base oil, containing soap and a water based activating fluid, which emulsion retains its mixed nature during the well cleaning in order to ensure a good cleaning result but, in the used, dirtied condition, allows not permanently miscible phases such as oil and water to separate following a certain period of rest, e.g. a few days, in a tank or other suitable vessel - as a result of its inherent tendency to decompose - with the lighter oil phase floating on top in a reusable, non-dirty condition, and with the underlying water phase after this period of rest containing all the dirt, soap, activating agent etc. and alone constituting the special waste - in an amount less than approximately 40 volume percent of the total cleaning fluid mixture.

Description

METHOD AND CLEANING FLUID FOR USE IN WELL CLEANING AND SUBSEQUENT TREATMENT OF THE USED CLEANING SOLUTION, AS WELL AS USE OF A PARTICULAR FLUID MIXTURE

This invention regards a method for use in well cleaning, primarily cleaning of the interior surface of the casing in subsea wells ("casing cleaning"), in which a cleaning fluid mixture circulates in the well for cleaning off dirt particles and particles of various other foreign matter which adhere and attach to said interior surface (or in the case of an unlined well, to the formation surface defined by the well bore), which particles during the circulation of the cleaning fluid mixture are successively absorbed into the cleaning fluid and transported out of the well along with the cleaning fluid at the conclusion of the cleaning operation.

Thereafter, the cleaned-off dirt and other particles of polluting matter assume a partly floating, partly settled state in the cleaning fluid.

The invention also, regards a cleaning fluid for this general purpose, as well as use of a particular fluid mixture. Particles of polluting foreign matter and ordinary dirt particles on the internal wall of the well may detach accidentally and enter the formation, which would damage this or plug it up. Dirt particles that are accidentally scraped off the borehole wall when equipment/tools are lowered into the well may cause the equipment/tools to jam and become stuck, or it may cause particles of polluting matter to come between the equipment/tools and fitting surfaces in the well, thus preventing the desired sealing effect.

As indicated at the beginning, the circulation of a cleaning fluid mixture in wells to be cleaned is already known in the case of well cleaning.

Known cleaning fluid mixtures for well cleaning purposes include, among other things, a detergent that is soluble in water or oil based drilling mud.

The detergent may contain a soap based on a fatty acid such as sorbital ester dissolved in water, to which is also added polymers of hydroxyethyl cellulose or xanthen in order to increase the viscosity and thereby effect an improvement in the ability to transport solid particles of polluting matter.

After the completion of well cleaning by circulation of cleaning fluid in accordance with the above, the cleaning fluid contains the cleaned-off dirt particles and particles of other polluting foreign matter that may have adhered to the borehole wall.

This used, polluted and dirty cleaning fluid must in accordance with regulations be treated as liquid special waste, and as such must be subjected to aftertreatment or be disposed of in a safe manner.

In ordinary well cleaning operations, it is a case of very large amounts of cleaning fluid to be treated or disposed of. Aftertreatment or disposal of cleaning fluid volumes of this order of magnitude finds expression in a considerable expenditure of time and very high costs.

The object of the invention is partly to provide a rational method for use in well cleaning by simplifying and making more efficient the arrangements for aftertreatment/disposal of the cleaning fluid, and partly to provide a suitable well cleaning fluid that in addition to its advantageous nature in matters relating to the technicalities of cleaning, also exhibits particular characteristics that make it simple to treat in a quick and satisfactory manner, or to dispose of.

The invention also regards use of a fluid mixture for well cleaning purposes, in which the amount of the used, dirty fluid mixture which requires aftertreatment/disposal is reduced substantially in comparison with conventional and other known techniques. Thereby a first fluid mixture fraction consisting of one component may after a certain rest period be separated out substantially free of dirt and polluting particles, the latter concentrating in a second fraction/component in which the dirt and the particles of polluting foreign matter partly float, partly settle.

Said object is realised through characteristics as stated in the description below and in the subsequent claims. Non-limiting examples of embodiments of the invention are explained in greater detail below.

Example

A so-called invert emulsion was prepared according to the present invention to be used for well cleaning purposes. The invert emulsion was prepared from oil, soap, sodium carbonate (calcined soda) and water, and the oil would make up the bulk of the volume of the emulsion.

A highly characterising feature of this emulsion and similar emulsions that may be used for well cleaning purposes in accordance with the present invention consists in the emulsion being unstable, so that it will separate into its two main phases, oil and water, following a rest period of a few hours. In the specific example stated above, soap and sodium carbonate will follow the water phase.

In the case of used, dirty emulsion, the dirt and the other polluting particles of foreign matter also follow the water phase.

The well cleaning is carried out in a known manner per se, by circulating the inventive invert emulsion (or other unstable solution/mixture) through the well, whereupon the used, dirtied emulsion is stored in a tank or other vessel until the emulsion, following a certain rest period, has separated into its principal components, the phases of oil and water, whereupon the oil which, due to its lower specific gravity floats on the top, may be pumped into another tank. The remaining water phase containing soap, sodium carbonate and all the dirt that has been cleaned off from the borehole wall during the well cleaning, is sent to aftertreatment or disposal.

The oil that has separated out from the rest of the emulsion and been pumped away has a purity that allows it to be reused.

The reuse of a component of the cleaning fluid without or through unessential aftertreatment represents cost savings in the well cleaning operation, however the more important aspect is the significant reduction in the amount of fluid that will require aftertreatment/disposal.

In the testing that was carried out in connection with the development of the invention, it was found that the water phase could advantageously make up thirty to forty percent of the fluid mixture/emulsion.

A cleaning fluid mixture in the form of an oil based emulsion was according to the invention prepared from the following components, which were mixed to form an unstable solution/mixture (invert emulsion):

a) an activating agent consisting of water mixed with approximately 3% sodium carbonate (C0₃2Na) (calcined soda) ,

b) oil of the same type as or equivalent to that which is used as base oil (with or without additives) in the drilling mud in the well,

c) soap based on a fatty acid such as sorbital ester. The cleaning fluid mixture was prepared in the following manner:

1) The activating agent was prepared by mixing sodium carbonate into water until the liquid activating agent s contained approximately 3% sodium carbonate,

2) soap was mixed into base oil until this oil based mixture contained approximately 8.5% soap,

3 ) the above mentioned prepared liquid activating agent was added to the mixture of oil and soap until the finished o cleaning fluid mixture (invert emulsion) contained approximately 30% activating agent.

The mixture was emulsified by circulating it through a suitable pump or by another known method (this mixing operation must take place under the influence of shear s forces; thus stirring will normally be insufficient). The emulsion that was prepared in accordance with the above example of an embodiment is an invert emulsion that separated into the phases of oil and water after a few days. Experiments performed in connection with the invention have o shown the separation of the unstable emulsion into oil and water to take up to 63 hours.

Base oil is more Newtonian, and provides a better particle transportation ability than that of the corresponding component in conventional well cleaning fluid, i.e. polymer 5 (hydroxyethyl cellulose or xanthen). Both exhibit viscosity increasing effects that favours particle transportation. After the well cleaning has been completed, the used, dirtied cleaning fluid mixture is given an opportunity to rest until it separates, and the base oil may be recovered while all the dirt follows the remaining part of the invert emulsion.

Claims

C l a i m s

1. A method for use in well cleaning and subsequent treatment of the used, dirtied cleaning fluid that is circulated in the well for well cleaning purposes and in this cleans off dirt and attached particles of other undesired, polluting types of foreign matter, which during the circulation of the cleaning fluid are successively absorbed into the cleaning fluid in order to be transported away from the well with the now used, o dirtied cleaning fluid upon completion of the well cleaning, which cleaning fluid requires aftertreatment/ disposal, c h a r a c t e r i s e d i n that the well cleaning is carried out by means of an unstable cleaning fluid mixture such as an invert emulsion, an s unstable liquid solution or an unstable fluid mixture that retains its mixed nature during its intended task, the well cleaning, during which the components of the cleaning fluid mixture, e.g. oil and water, activating agent and soap, exist in an at least approximately o homogeneous state, and that the used, dirtied cleaning fluid in said approximately homogeneous state with the cleaned-off dirt particles etc. floating therein is transported out of the well in a known manner, after which the used cleaning fluid mixture is allowed to rest 5 for a period, for instance in a tank, whereby the unstable cleaning fluid mixture separates into its not permanently iscible principal components, so that the lightest phase, e.g. oil, remains floating on top of the heavier phase, the water, in which dirt particles etc. o (including activating agent and soap) congregate and remain floating or settle, whereby the lightest phase is simply removed in a clean, reusable or slightly dirtied condition, while the heavier phase with the dirt particles etc. is treated or disposed of, e.g. in a manner that is known per se, the heavier phase constituting for example less than half of the total 5 volume of the cleaning fluid mixture.

2. A cleaning fluid for use in well cleaning, c h a r a c t e r i s e d i n that the cleaning fluid consists of an unstable mixture of temporarily miscible liquids containing detergent/soap, the nature of which o unstable fluid being such in regard to miscibility that it ensures the at least approximately homogenous mixing of the cleaning fluid during its actual task, cleaning off dirt coatings etc. from the interior surface of the borehole wall; however in the used, dirtied state, after s a relatively short rest period of e.g. a few days, it decomposes and separates into its principal components, an upper floating oil layer essentially free of polluting matter and a lower water layer containing dirt particles as well as soap and possibly an activating o agent etc.

3. A cleaning fluid according to claim 2, c h a r a c t e r i s e d i n that the cleaning fluid consists of an oil based invert emulsion containing detergent/soap and possibly a water based activating 5 fluid.

4. A cleaning fluid according to claim 3, c h a r a c t e r i s e d i n that the oil is a base oil of the same type as or equivalent to the type used in the drilling mud in the well that is to be cleaned; o that the detergent/soap is a soap based on a fatty acid such as a sorbital ester, and that the cleaning fluid in addition contains an activating agent consisting of water mixed with approximately 2-6 volume percent sodium carbonate (calcined soda).

5. A cleaning fluid according to claim 4, c h a r a c t e r i s e d i n that the base oil constitutes 55-70 volume percent of the total cleaning fluid mixture, and that the water fraction corresponds to 25-45 volume percent.

6. A method of preparing a cleaning fluid according to claims 2-5, c h a r a c t e r i s e d i n that the oil based starting fluid is prepared by mixing soap, for instance soap based on a fatty acid such as a sorbital ester, into oil, preferably base oil, until this oil s based starting fluid contains 5-10 volume percent soap, an activating fluid being prepared separately by mixing sodium carbonate into water until the activating fluid contains 1-5 volume percent sodium carbonate, and that the prepared activating fluid is added to and mixed into 0 the oil based starting fluid until the cleaning fluid mixture contains approximately 25-45 volume percent of the water based activating fluid, and that the components that form part of the resulting cleaning fluid mixture are mixed to form an emulsion, that is 5 under the influence of shear forces, e.g. in a pump.

7. Use of an unstable cleaning fluid mixture such as an oil based invert emulsion containing at least water and detergent/soap, in well cleaning, the used, dirtied cleaning fluid mixture decomposing and separating into o oil and water phases due to the unstable nature of the mixing, after a period of rest while in the used, dirtied condition, with the oil layer floating on top and being essentially pollution free, and with the cleaned-off dirt etc. in the underlying water which preferably constitutes less than 50 percent of the total volume of the cleaning fluid mixture, thus reducing the amount cleaning fluid mixture requiring aftertreatment/ disposal to less than 50 percent.