CA2620538A1 - Method and apparatus for removing suspended solids and turbidity from alkali metal formate brine fluids - Google Patents

Abstract

This invention provides a Method and Apparatus for economically removing suspended solids and turbidity from used alkali metal formate brine fluids to allow such fluids to be re-used rather than disposed. This is achieved by adding a strong base to the contaminated fluid to raise the pH of the water, thereby forcing positive metal ions in the fluid to begin acting as flocculants. After flocculation of colloidal particles suspended in the fluid is complete, clarified water is decanted for re-use and the settled solids are removed.

Description

METHOD AND APPARATUS FOR REMOVING SUSPENDED SOLIDS AND
TURBIDITY FROM ALKALI METAL FORMATE BRINE FLUIDS
FIELD OF THE INVENTION

This invention relates to a method and apparatus for quickly and economically removirig suspended solids from used alkali metal formate brine fluids to allow for their re-use rather than disposal. The invention is particularly useful for cleaning and recycling of high density brine drilling fluids such as potassium, caesium and sodium formate brines which have been contaminated with excess amounts of suspended solids and colloidal matter during use.

BACKGROUND AND PRIOR ART

Alkali metal formate brine fluids have been used in industry for a number of years, with one of the largest users being the oil industry, where they are commonly used as drilling or completion fluids.
One common property of these brines which makes them valuable in such applications is their ability to produce a clear high density liquid which doesn't contain solids.
This is particularly useful in the oil drilling and production industry where high density fluids are commonly used for a variety of purposes including displacing lighter weight fluids in drill holes during drilling and completion operations.

When brine fluids are used in these applications they often pick up undesired contaminants from the drill hole such as very fine solids which subsequently become suspended in the brine water. When the total suspended solids (TSS) concentration in the brine fluid reaches a specified threshold, the used brine is typically disposed and replaced with new brine, since excess solids in the water can have negative effects on the drilling operations. Due to the high cost of producing some of these brine fluids, as well as concerns over disposing such products into the environment, companies using these brine fluids would prefer to be able to clean and re-use them if it was economically feasible, rather than disposing of them when they become contaminated.

Various inventions have been developed over the years to address this problem including filtration technologies, use of organic chelants to remove metal impurities, and addition of various chemicals to try to precipitate out various contaminants. All of these have offered varying degrees of success in achieving this objective, but retain a number of limitations and disadvantages.

With filtration technologies, many brine fluids used in the oil industry typically become contaminated with extremely small clay type particles commonly referred to as `colloidal' solids.
These solids are so small that they fall into the sub-micron and nano size ranges and can't typically be removed economically with common filtration technologies due to their small size.

Other inventions have employed the use of multi-parameter chemical adjustments or adding chemicals aimed at precipitating out metal ions from the fluid. This is typically carried out in conjunction with various other process stages such as filtration to remove resulting precipitates from the fluid. One limitation of these processes is that they generally target metal contaminants in the water and don't address the issue of clay type colloidal particles which can comprise the majority of the solids suspended in the water.

One disadvantage common to almost all options is their cost. When the cost of cleaning the used fluid exceeds the cost of producing new fluids, then the recycling process normally won't be used.
Prior patents of background relevance to the current invention include the following;

US PAT. #7172703 - 02/06/2007 - Method of Reclaiming a Well Completion Brine Solution using an Organic Chelant US PAT. #4465598 - 08/14/1984 - Method of Treating Well Servicing Fluids US PAT. #4303624 - 12/01/1981 - Purification of Alkali Metal Chloride Brines US PAT. #4207152 - 06/10/1980 - Process for the Purification of Alkali Metal Chloride Brines METHOD BY WHICH THE PRESENT INVENTION OVERCOMES PRIOR
ART PROBLEMS

It is an object of this invention to provide a method and apparatus for removing suspended solids from alkali metal formate brines which is simpler, more economical, and effective in removing colloidal clay particles from the fluid. This is accomplished by adjusting the fluid environment to promote the flocculation and removal of these particles in one simple step.

It has long been known in other fields of industry that extremely small solids suspended in water will not settle out of solution through normal gravity forces, or float to the surface through normal buoyancy forces. This occurs when the particles involved are in the sub-micron and nano size ranges, at which point they are typically referred to as `colloidal' solids.
In the case of colloidal clay particles in water, it has also been known that these particles tend to carry a small negative electrical charge around their perimeter which repels other similarly charged particles suspended in the fluid, keeping them separate from each other. This property prevents the particles from coniing together as a larger group, which would then be dense enough to settle out. This property of colloidal clay particles is one which `flocculation' technologies have been designed to overcome in the past by encouraging the particles to coagulate and settle out. These processes typically work by injecting one or more chemicals into the fluid which are designed to introduce a propensity of strongly charged ions in the fluid which have an opposite charge to that of the clay particles. These strongly charged ions attract the oppositely charged clay particles in the water, causing them to `attach' to the flocculant ion in sufficient numbers to cause an increase in the density of the combined group, such that they eventually settle out of solution together as a group.

In conventional flocculation processes, a chemical compound known as a`flocculant' is typically added to the water to begin this process, along with various water conditioning chemicals designed to adjust the chemical environment of the water to one in which the flocculant will work best. This flocculant, when dissolved in water, releases strongly charged ions which carry an opposite charge to that of the target contaminant. One such product commonly used for this purpose is aluminum sulfate (A12012S3). When aluminum sulfate is dissolved in water it releases strongly charged positive ions (Al 3+), which attract negatively charged clay particles in the water to promote the flocculation process. Other chemicals and polymers have been developed for this purpose as well, but all generally seek to provide strongly charged ions in the water which are opposite in charge to that of the target contaminant.

Unfortunately, typical flocculation processes are relatively complicated, are capital intensive, are costly to operate, and consequently are not well suited for economically processing potentially low quantities of brine fluids. Costs of the process typically include the purchase, storage and handling of various flocculant and conditioning chemicals, determining optimal chemical balances required to achieve desired reactions, and attempting to ensure that the flocculant selected doesn't introduce new chemical species into the brine fluid which could result in the creation of by-product compounds which could negatively affect the physical properties of the brine.

To overcome these issues, the current invention has developed a method of removing solids from brine fluids without incurring the disadvantages noted above. This process involves greatly simplifying the flocculation process by activating existing elements already in the brine fluid to act as flocculating agents, thereby eliminating the need to add any new chemicals for this purpose. The process and methodology by which this is accomplished, and which comprises the embodiment of the present invention, is described below:

1) All alkali metal formate brine fluids which comprise the subject of this invention naturally contain one or more alkali metal atoms such as potassium, sodium, calcium or caesium.
Each of these, when dissolved in water, release metal ions into solution which carry a small positive charge (eg. Na+, K+, Ca+);
2) While these ions are not typically used as flocculants because of the small charge they carry, it has been found in the case of brine fluids that by adding a strong base to the brine fluid, such as sodium hydroxide (NaOH), and raising the pH of the brine flui(i to 12.80 or higher, the fluid environment is adjusted in a manner which promotes a cascade reaction in the fluid whereby positive ions already in solution begin acting as natural flocculants, and a strong floc begins forming in the liquid to remove the suspended solids.
3) This process happens immediately upon reaching the necessary pH threshold, and floc containing suspended solid particles quickly begins to settle to the bottom of the container, leaving only clarified water at the top. This clarified water is easily de-canted to provide a clean brine fluid which is free of most or all previously suspended solids, making it immediately suitable for re-use;
4) Because the resultant fluid has a high ph, however, and a high pH fluid may be undesirable for handling or corrosion reasons, an acid such as Hydrochloric Acid (HCL) can be added to the clarified fluid to easily adjust the pH to whatever final pH is desired.

This process improves on the prior art by eliminating one of the major steps in the flocculation process - that of having to purchase, store and add flocculant compounds to the water to achieve the desired treatment. This greatly reduces complexity of the operation, reduces capital and operating costs to much more competitive levels, and eliminates concerns regarding any by-products that could otherwise be formed by the introduction of new chemical species from use of the flocculant. The treatment process described herein is therefore reduced to the simple addition of a strong base material such as NaOH to the used brine fluid in sufficient quantities to initiate the flocculation process, and then decanting the clean water for re-use. The quantity of base material required for this purpose can be calculated arithmetically or done visually whereby the base compound is added until flocs beginning appearing in the water.

When the flocculated solids settle to the bottom of the container, and the clean water has been removed from the top, neutralizing acid can be added to both the clean and waste waters to bring the pH of the fluids back to a desired operating range.

Tests were carried out to confirm the efficiency of the described process.
These tests utilized a used potassium formate brine solution (KCOOH) as the raw material. This product had been returned from an actual oil well completion project, and was contaminated with suspended colloidal solids and a small amount of residual hydrocarbons. The used fluid was allowed to settle for a period of two weeks prior to conducting the tests described below to ensure that the settleable solids in the water had been removed, and any remaining solids being extracted by the new process would be primarily colloidal in nature.

Test Result:

= Raw Test fluid - Used potassium formate brine (KCOOH) = Brine Density - 1120 gms/L

= Total Suspended Solids - 153 ppm = Turbidity - 100 NTU

= pH of brine - 9.94 Solid sodium hydroxide (NaOH) was added to six separate test containers in varying concentrations representing 0.1%, 0.2%, 0.25%, 0.3%, 0.4% and 0.5% NaOH by weight. After dissolution of the NaOH in the brine water, the fluid in each test container was stirred to ensure mixing of the two compounds. Each sample was then observed for a period of time, and the reactions recorded.
Sample #1 (0.10%) - no flocculation occurred.
Sample #2 (0.20%) - flocculation commenced immediately upon being stirred.

Sample #3 (0.25%) - flocculation commenced immediately upon being stirred.
Sample #4 (0.30%) - flocculation commenced immediately upon being stirred.
Sample #5 (0.40%) - flocculation commenced immediately upon being stirred.
Sample #6 (0.50%) - flocculation commenced immediately upon being stirred.

At lower NaOH concentrations (#2 and #3), the flocculation process proceeded more slowly and less thoroughly than at higher concentrations.

At the three higher concentrations, all samples showed greatly increased water clarity within 10 minutes of mixing, but small floc particles remained in suspension for up to 6 hours. After settling for 6 hours, most of the floc had settled to the bottom of the container, and the clarified water on top (representing 85% of the water in the containers) was ready for decanting as clean water. The remaining 15% of water and floc at the bottom of the test containers was then added together and allowed to settle for a further 6 hours, following which a further 8% of the original brine was able to be removed as clarified water, resulting in an overall recovery rate of 93% in 12 hours.

The pH of each test sample resulting from addition of the NaOH was;
Sample #1: pH = 10.99 Sample #2: pH = 12.80 Sample #3: pH = 12.95 Sample #4: pH = 13.28 Sample #5: pH = 13.43 Sample #6: pH = 13.50 The clean water decanted from sample #6 was tested to determine the level of treatment which had been achieved, with the following results:

Clarified Water Quality (Sample #6):

= Fluid Density - 1120 gms/L (no change) = Suspended Solids Content- 6 ppm (96% reduction from original) = Water Turbidity - 5.4 NTU (94.6% reduction from original) It was noted during testing that gentle stirring of the water to achieve water mixing with the NaOH
resulted in all of the floc particles settling to the bottom of the treatment container after the required settling time, whereas mixing the water with air (which happens when a test container is shook vigorously) results in bubbles being entrained in the floc particles, causing many of them to initially float to the top of the container instead, making it more difficult to decant clarified water than if the flocs reside at the bottom.

The present treatment process improves on the prior art by:

= Providing a much faster and simpler treatment process which requires only one step to achieve clarified water;

= Eliminating concerns over possible addition of new chemical species to the brine which could produce compounds which impart negative fluid properties to the brine;

= Making the treatment process much more economical by eliminating the cost of significant amounts of chemicals, time and equipment that would otherwise be needed using alternate options.

LIST OF FIGURES AND DESCRIPTION OF THE INVENTION

In drawings which illustrate embodiments of the invention, Figure 1 is an elevation view as it would apply to one preferred embodiment.

The invention as illustrated consists of a process and apparatus for quickly and economically removing suspended solids from used alkali metal formate brine fluids which involves the following steps:

1) Addition of a strong base material (preferably NaOH) to the brine fluid to raise its pH to a level where existing positively charged ions in the fluid will begin acting as flocculants;
2) Storage of the brine fluid in a container for a period of time as required to allow the majority of floc created in the flocculation process to settle to the bottom of the container;
3) De-canting the clarified water from the top of the container;

4) Addition of acid to the brine to return the pH to a final desired level.

In one preferred embodiment of the invention as shown in figure 1, the contaminated brine fluid is placed in a primary treatment tank (1) constructed of a material resistant to high pH fluids. The treatment tank (1) may be at atmospheric pressure, with an open top or access openings in the top to permit easy access. A water sample is drawn from the tank to check its pH. A
calculation is made to determine the amount of base material required to raise the pH of the brine liquid to a level sufficient to initiate the flocculation process. In tests carried out on the brine described herein, this level needed to be 12.8 or greater. However, this may vary somewhat depending on the: liquid being treated, and a bench test is recommended each time a new fluid is to be treated by adding increasing amounts of NaOH to a measured amount of brine until the level at which the flocculation process begins can be identified. A strong base, preferably sodium oxide (NaOH), is then addeci to the brine by injecting it into the treatment tank (1) under pressure, or adding it by gravity through the top (2).
A means of mixing the brine fluid and base material in the treatment tank (1) is provided by use of a re-circulating pump (7) or a mechanical mixer located in the tank. Following mixing, the brine is allowed to sit for a period of 6 hours or more, or as otherwise needed to achieve a desired level of clarification. Sample and drain ports (5) can be installed at different elevations on the tank (1) to permit monitoring of the clarification process and removal of clarified water (3) when it has reached a desired clarity. Clarified water (3) from the primary treatment tank (1) is then decanted by means of a pump (6), or any other fluid movement equipment, to a final storage tank (8) for storage and subsequent reuse. Residual water and floc (4) remaining in the primary treatment tank (1) is left there until it builds up to a level where it needs to be removed and disposed.
An acid solution, preferably hydrochloric acid, can be added to the clarified water in the final storage tank (8) to adjust the pH to its final desired level. A means of mixing the clarified brine fluid and the acid material in the final storage treatment tank (8) is needed, and can be provided by means of a re-circulating pump (9) or a mechanical mixer located in the tank In another embodiment of the invention, the primary treatment and final storage tanks (1) & (8) can be any size or shape capable of holding the water to be treated, or a plurality of containers as appropriate to permit higher flows or better management of the product;

In another embodiment of the invention, the primary treatment and final storage tanks (1) & (8) can be pressurized vessels not open to the atmosphere, wherein all products to be added tc- or removed from the vessels can be accomplished through use of pressure injection systems, pumps or other commonly available fluid movement equipment;

In another embodiment of the invention, the means of mixing the brine fluids in the tanks can be by any common means of mixing fluids in an enclosed chamber including use of air injection systems;
In another embodiment of the invention, the material to be used for raising the pH of the brine water can be any chemical base capable of raising the pH of the brine to a level at which flocculation will begin;

In another embodiment of the invention, the acid to be used for adjusting the pH of the final treated product can be any acid capable of reducing the pH to the desired level. In this regard, it may be preferred in some cases to use acids which do not introduce new chemical species to the brine water, such as formic acid when dealing with formate brines, as this material is an existing component of formate brines;

In another embodiment of the invention, a floc concentrating tank may be added to the system to provide a repository in which the waste floc and water from the treatment process can be deposited for further concentration prior to disposal. One preferred method of doing this would be through the use of a tall slender tank in which the depth of floc in the tank can be increased by the shape of the tank so the self-weight of the floc can be used to compress the floc towards the bottom of the container, expelling clean water to the top which can then be decanted as additional treated fluid;

In another embodiment of the invention, concentration of waste floc material may be accomplished by means of conventional filters, screens or filter presses to reduce the water content of the floc prior to disposal;

In another embodiment of the invention, filter systems can be used to remove the eiitrained floc particles from the water after the flocculation process has taken place, for the purpose of speeding up the decanting process and minimizing storage requirements.

In another embodiment of the invention, the treatment system may be a closed system in which brine water enters one end of the process, base and acid products are injected into the system where required, and a treated product exits the discharge end of the process without operators having to handle the product along the process.

In another embodiment of the invention, more than one treatment tank can be placed in series between the primary treatment tank (1) and the final storage tank (8) to permit better control of the decanting process, and reduce the likelihood of floc particles escaping into to the final clarified brine product. Under this arrangement, one or more additional treatment tanks would. be installed between primary treatment tank (1) and the final storage tank (8). Clarified water decarited from the primary treatment tank (1) under this arrangement could accommodate small amounts of floc particles in the decant product, as such solids could settle out in the next stage of the treatment process. This would permit decanting from the primary treatment tank (1) to be less sensitive to whether some floc was entering the decant water, and allow operators to more quickly and easily decant the primary treatment tank (1) down to a level closer to the top of the main floc layer without concern. Any floc decanted in this manner would settle to the bottom of the new treatment tank, following which clarified water from there would be decanted to the final storage tank (8) as before.
Under this arrangement, residual water and floc settling to the bottom of the new treatment tank would be redirected back to the primary treatment tank (1) for further treatment after each decanting procedure.

ONE INTENDED USE OF THE INVENTION

One preferred use of the invention is to enable used alkali metal formate brine fluids from the drilling industry to be cleaned of suspended solids so they can be reused rather than disposed into the environment.

Claims (11)

1) A process for quickly and economically removing suspended solids from used alkali metal formate brine fluids using the following steps:

A) Addition of a strong base material to the used brine fluid, preferably sodium hydroxide, for the purpose of raising the pH of the brine to a level where existing metal ions in the fluid having slight positive charges will begin acting as natural flocculants;
B) Containment of the treated fluid in a suitable vessel to allow the floc created in the process time to settle to the bottom of the container, preferably 6 hours or more;
C) De-canting of the clarified water above the settled floc particles for re-use;
D) Adjustment of the pH of the final product to a desired level through addition of acid, preferably hydrochloric acid.

2) A process as defined in Claim 1 wherein the base material used to achieve the necessary pH is any base material capable of elevating the pH of the product to the level required to initiate the flocculation process;

3) A method and apparatus as defined in Claims 1 to 2 wherein the used brine fluid is placed in a primary treatment tank constructed of a material resistant to high pH fluids.
This tank may operate at either atmospheric pressures or be a pressurized vessel. A chemical base material will be added to the brine water in this container to raise the pH of the fluid to a level which initiates a flocculation process. A means of mixing the brine fluid in this tank is provided, through use of a re-circulating pump or common fluid mixing equipment. Following mixing of the brine and chemical base, the floc particles in the brine are allowed to settle for a period of 6 hours or more as required to achieve the desired water clarification. Sample and drain ports can be provided at different elevations on the tank to permit monitoring of the clarification process and removal of water when it has reached a desired clarity. Clarified water extracted from the drain ports are deposited into a final storage veseel for subsequent re-use. An acidifying solution, preferably hydrochloric acid, may be added to the clarified water in the final storage vessel to re-adjust the pH of the final product to the desired level. Floc collecting at the bottom of the primary treatment tank may be neutralized periodically with acid as well, and disposed of as waste;

4) A method and apparatus as defined in claims 1 to 3 in which the primary treatment tank can be any size or shape capable of holding the water to be treated, or a plurality of containers to permit higher flows or better fluid management;

5) A method and apparatus as defined in claims 1 to 4 in which all solutions to be added or removed from the container are accomplished through use of pressure injection systems, pumps or other commonly available fluid movement equipment.

6) A method and apparatus as defined in claims 1 to 5 in which the means of mixing the brine fluid or the final clarified brine water can be through use of an air injection system using compressed air;

7) A method and apparatus as defined in claims 1 to 6 in which the acidifying solution to be used for adjusting the pH of the final brine product may be any acid capable of adjusting the pH to the desired level. In some cases it may be preferred to use acids which do not introduce any new chemical species to the brine water, such as formic acid in the case of potassium formate solution;

8) A method and apparatus as defined in claims 1 to 7 in which a floc concentration vessel is provided to which waste floe and water from the treatment process can be deposited for further concentration prior to disposal. A preferred apparatus for doing this would be the use of a tall slender vessel in which the depth of the floc in the container would be maximized so the self-weight of the floc causes it to compress naturally at the bottom of the container, expelling clean water to the top, which can then be decanted as treated fluid;

9) A method and apparatus as defined in claims 1 to 8 in which final floc concentration can be accomplished through use of conventional filters, screens or filter presses to reduce the water content of the floe prior to disposal;

10) A method and apparatus as defined in claims 1 to 9 in which more than one treatment chamber may be used in series to simplify decanting of the clarified water. This system would utilize one or more additional treatment tanks downstream of the primary treatment tank.
Clarified water from the primary treatment tank would be decanted to the secondary treatment tank. This arrangement would permit small amounts of floc particles to remain in suspension in the fluid being decanted from the primary treatment tank, as any entrained floc would still be able to settle out in the second treatment tank. This would permit the decanting process from the primary treatment tank to be much less sensitive to whether some floc remained in the product, and thereby allow operators to decant the first tank down to a evel closer to the top of the main floc layer than would otherwise be possible.
Any remaining floc in the water that enters the secondary treatment tank would be allowed to settle to the bottom of this second tank, following which the clarified water from this chamber would be decanted to the final storage vessel. Residual water and floe remaining at the bottom of the secondary treatment tank after decanting would be returned to the primary treatment tank for further consolidation.

11) A method and apparatus as defined in claims 1 to 10 in which the entire treatment apparatus would comprise a closed system in which brine water enters one end of the process, base and acid products are injected where required, and a clarified brine exits the discharge end of the process without need for operators having to handle the product along the process.