NO170102B - PROCEDURE FOR THE PREPARATION OF A SOLID-FREE, Aqueous Liquid from a Liquid Contaminated with Solid Substances Such as SUSTAINABLE, SAND AND DRILLING SLAM - Google Patents

Description

The present invention relates to the use of salt solutions with high density in wellbores, and more particularly the invention relates to a method for producing a solids-free, aqueous liquid from a liquid contaminated with solids such as rust, sand and drilling mud.

Well fluids such as aqueous salt solutions with high density are used in well systems used in the production of petroleum. The term well fluid as used here is intended to denote fluids with a continuous water phase and which fluids may contain fresh water, seawater or salt solutions and vary with regard to amounts of dissolved substances such as salts, corrosion inhibitors and gases. These solutions have been used both as drilling fluids, completion and • packing fluids, especially in deep wells which are subjected to high formation gas pressure at elevated temperatures. These fluids can be formed from sodium, calcium, zinc salts with chlorides, bromides and potassium. These aqueous liquids may include corrosion inhibitors and other salts such as sodium ash. The density of these well fluids depends on the particular salt or mixture of salts and their concentration in the aqueous well fluid. Generally, salt-type well fluids have a density in the range 0.958-2.28 g/l.

The salt-type well fluid must be solids-free in its use as a well fluid. If there are solids in a packing or completion fluid, they can cause serious damage in a producing formation by plugging pore cavities in it or even by perforations and channels provided to get fluid flows between the formation and the wellbore. If there are solid substances in a packing liquid, the substances will precipitate after some time in the packing. As a result, these solid deposits will make it difficult to disconnect the casing from the packing resulting in expensive well repair work.

The high density salt solution can be prepared on the well side by dissolving the prescribed amount of salt in the aqueous phase, which phase mainly consists of fresh water or sea water, but can also include various inhibitors to prevent pitting, corrosion, etc. Mixtures are circulated or agitated in surface sludge system facilities until there are no undissolved salt solids. Naturally, the problems with the addition of salts to be dissolved in the aqueous well fluid become increasingly difficult when the density increases, both in terms of requirements and time, manpower and equipment.

For now, sellers will deliver to the well side the manufactured high density salt solution with a desired density and combination of selected ingredients. It is desired that these well fluids are clean and free of solids. The delivery of saline usually requires several changes of containers. For example, the salt solution is transported from sales tanks to truck transport, offshore supply ships and to the rig mud system. In most cases, the brine is contaminated by unwanted solids, including residual water-wetted solids and/or oil-based drilling mud, bulking agents such as barite, rust, salt, mud and sand, ferrous and ferric deposits and other insoluble materials. Contaminated liquids such as sludge bases, lubricants and diesel oils can also be present in the sludge system and be mixed into the salt solution. Usually these polluting liquids are occluded or absorbed on the undissolved solids.

If the quantity of solids in the salt solution is small in quantity, the rig management can be used for their removal, usually in a stepwise flow pattern through cartridge filters. The labor and rig time costs of filtering the brine are usually prohibitive (eg, $100,000 per work shift) unless the solids are (1) less than 0.01% by weight of the well fluid, (2) granular, and ( 3) not gel-like as is common in the case of HEC, polymer or bentonite sludge contamination.

Contamination of salt solution by drilling mud components is most common as the salt solution is usually handled on the rigs in parts of the mud system. The mud system usually absorbs contamination during washing of the well drill to remove residual mud and cement particles immediately before the introduction of the completion/packing salt solution of the high density salt type. Only a small amount of washing liquid that is mixed into the salt solution causes the solids content to be too high. The salt solution must then be treated to remove the solids. Residual solids must be less than 5 pm in maximum dimension otherwise they cause formation sealing.

As mentioned, the use of cartridge, plate or layer-type filters is impractical unless the solids content of the salt solution is very low. Furthermore, rig time when using equipment and labor is limited and only available for critical operations, namely optimal drilling of the wellbore. As a result, brines with large solids contaminants must either be discarded or returned to facilities for purification. As the salt solution is very expensive (e.g. $300 - $900 per barrel) it cannot be thrown away. Furthermore, the saline solution must be handled carefully so that these are not spilled away, as the environment is damaged by strong, aqueous saline solutions.

It has also become a practice to clean the rig mud system of residual mother mud before the introduction of the salt solution by various washing and manual cleaning techniques in an attempt to reduce the extent of filtration. For example used on offshore rigs flushing with seawater and crew with scrapers, brooms, etc. to try to remove residual drilling mud components. This technique for priming and cleaning the rig's mud system is very hazardous (slippery, wet, alkaline and narrow working ice areas) and burdensome labor costs. In addition, the cleaned mud system still has residual drilling mud that is hidden in hooks, but which is mixed with the high density salt solution as it passes through it. During cleaning of the mud system, operation of the rig must be stopped for between 5 and 13 hours on average. The cost of cleaning varies from about $3,000 to $8,000 per hour. By avoiding this cleaning procedure, the rig's downtime will be saved in the amount of $40,000.

As a practical result, current rig practice, especially offshore, requires full flow filtration (usually in cartridge filters) of the brine so that solids levels of 0.2% or less are desired immediately before the brine is introduced into the wellbore.

Although the salt solution can be made solids-free on the rig, it is also necessary to clean the well system of drilling fluid, mud particles or other contaminants before introducing the salt solution into it in order to keep the salt solution in a solids-free state during use as discussed above. One of the main problems with the removal of drilling fluid and mud is from the well drilling equipment which includes the production pipe or well pipe and the annulus between it and the casing or surrounding wellbore.

Many chemical detergents have been proposed and used to remove drilling fluid and mud from the well drilling equipment prior to introduction of the solids-free salt solution. For example circulating chemical washing methods have been used using water with surfactants, thickeners and gelling agents, chemical auxiliaries such as sodium tetraphosphate, weight increasing agents such as barite under turbulent flow conditions to achieve effective mud and fluid removal from the well drilling equipment. In some cases, the chemical detergents are used as aqueous spacers for increased drilling fluid removal, often in the same way as used to displace drilling fluid before a cement slurry used in the well cementing operation.

Generally, the chemical washing methods are adapted for turbulent or laminar flow with aqueous phases of fresh and salt water as continuous solid and for plug flow gel-forming water-based phases are used. The gelled water phase or plug was sufficiently viscous to attempt to reduce sedimentation of drilling fluid from it, especially the solids. If oil-based drilling fluid had been displaced, diesel oil and an emulsifier are added to increase the removal of drilling fluid and its mud cake. Some chemical washing methods combined turbulent and plug-flow aqueous phases to increase the displacement of drilling fluid and mud from the wellbore equipment, particularly prior to cementing operations or introduction of completion brine. These chemical washing methods were used, in some operations, by a single pass through the well drilling equipment to remove drilling fluid so that rig time is saved.

A method has been developed for the removal of contaminating solids from salt-type (salt solution, aqueous drilling, completion and high-density packing fluids prior to their placement in a wellbore and for the removal of substantially all of the drilling mud and mud, including solids such such as barite, bentonite, cement, oil materials, as well as other contaminants from the well system prior to the introduction into it of solids-free completion and packing brine to prevent recontamination of the brine. As a result, the brine is kept substantially free of solids during use in the well system. The total rig "down time" and cleaning time for salt solution when performing the present method is significantly reduced. Especially greater rig "down time" savings have been achieved with deep offshore wells with large angles at directional wellbores (e.g. 70°).According to the present invention, a method for production is provided ing a solids-free, aqueous liquid from a liquid contaminated with solids such as rust, sand and drilling mud, by introducing into the liquid small effective amounts of an aliphatic alcohol of 5-8 carbon atoms and a surface-active chemical aid to agglomerate solids therein, and separating the agglomerated solids from the liquid before it is used in a solids-free state, and this method is characterized by the use of a surface-active chemical aid which includes a surface-active agent having a molecular weight in the range of 150-500 with predominantly hydrophobic properties and is selected from aliphatic amines, aliphatic amides and aliphatic amine oxides, the amine or amide having an alkyl group with 8-18 carbon atoms.

In the preferred embodiment of the invention, the alcohol is 2-ethylhexanol and the surfactant is the amide reaction product of a monovalent fatty acid with a secondary or tertiary amine. Each is used in an amount of 0.5 volume-# of the well fluid. The agglomerated solid contaminants are thus separated from the well fluid before its introduction into the wellbore in a solids-free state.

A treated water is produced by adding a surfactant and alcohol to pure water by agitation and shear mixing. The treated water is circulated through the well system until substantially all of the drilling mud and fluid and other contaminants therein are suspended in the treated water during circulation in the well system to aid in the displacement of the contaminants. The liquid can be pure water or a plug of gelled aqueous waste containing either a viscous polymer or a combination of bentonite, water and diesel oil. The spacer plugs and treated water can in particular be composed so that the former is moved by laminar flow and the latter by turbulent flow in the well system at the same linear velocity ranges. The treated water and the liquid are displaced from the well system to a suitable drainage area by the solids-free salt solution previously prepared. The well system is clean and the salt solution will be kept uncontaminated during use in it.

The figure is a schematic flow chart illustrating the drilling mud equipment on a well system which includes devices for producing solids-free salt solution before its introduction into a wellbore and for circulating treated water through the well system before introducing the salt solution.

With reference to the drawing, a well system 11 is shown schematically which comprises surface equipment 12 and well drilling equipment 13 which forms part of the drilling mud system which can be found on offshore oil well rigs. The well system 11 can also comprise a filtration unit 14 which contributes to the removal of solids to provide a high-density salt solution with a low solids content, e.g. 0.2 wt-# or less. The well system 11 can include other devices, or devices in a different arrangement and even be used when carrying out the present improved method.

For example, the surface equipment may include mud pumps 16 to circulate drilling mud and through the wellbore equipment 13, and the circulating loop from this equipment may have a drill roof/desanding/fuel mud screen 17, a drilling mud tank 18 with powered mixers 19 and a suction drilling mud tank 21. The drilling mud tank 18 may contain inlets 22, 23 and 24 for the addition of various burnt materials such as solid contaminated salt solution, chemicals and clean or sea water. The term clean or seawater is intended to denote water that can be fresh or salty, such as from the world's oceans, but with a relatively low solids content, e.g. less than 200 ppm. In normal practice, the surface equipment 12 will be to receive the salt solution from a source such as a barge or seagoing ship transport and to process the salt solution to a solids-free state for placement in the wellbore equipment 13.

The well fluid contaminated with solids is placed in a suitable container (not shown) which can be exposed to air or closed if desired. A mixer (not shown) is provided to the container so that the materials used in the present method can be thoroughly mixed with the well fluid. The mixer can be either an impeller type or a centrifugal forward and reverse loop type. In addition, the container is provided with a suitable mechanism (not shown) for removing agglomerated solids from the liquid phase. For example, the mechanism can be a rotary rake device to remove the solids over an inclined discharge ramp as used in air rotary cell. Alternatively, the container can be equipped with both a rake or decanter mechanism for separating solids and liquid phase. Generally, the container can be operated at ambient temperature at which the well fluid is secured.

The well fluid is assumed to be heavily loaded with solids which can be sand, formation particles and degraded material, oil, pipe lubricants, rust insoluble carbonates, mud liquids and solids such as barite, emulsifiers, diluent, cement and other solid materials in various mixtures and quantities which can be found in the well circulation system.

As the first step of the method, it is preferred to mix in a less effective amount of the aliphatic alcohol with the well fluid. The quantity of the alcohol is usually not required above about 2 volume-#. Usually, good results are obtained by using alcohol amounts above about 0.5 vol-#. In most well fluids, the alcohol is used in an amount of 0.5 volume-# and larger amounts, such as 1.0 volume-Æ, do not seem to significantly increase the desired results when removing solids. Generally, the solids removal results decrease when the amount of alcohol is simultaneously decreased below the 0.5 volume-# level.

After the alcohol has been thoroughly distributed in the well fluid, the next step in this method is to mix in the surface-active chemical aid. The quantity of the chemical aid is usually not more than about 2 volume-#. Good results are achieved by using chemical aid amounts above about 0.5 volume-#. In most well fluids, the chemical aid can be used in the amount of 0.5 volume-5e and larger amounts, such as 1.0 volume-#, do not seem to significantly increase the desired results in solids removal. Generally, solids removal results decrease when the amount of chemical aid is reduced substantially below the 0.5 volume-56 level. Larger amounts (eg, over 3 volume-#) of the chemical aid increase the amount of well fluid that is captured in the removed solids. The chemical aid and especially the surface-active agent seem to change the surface tension and the boundary film that surrounds the negatively charged solid particles and especially the bentonite components from the drilling mud. This effect produced by the chemical aid is mainly the agglomeration of the solid masses from the drilling fluid.

It has been found that the minimum effective amounts of the alcohol and surfactant chemical aid depend on their activity characteristics and the particular solids in the well fluid. Thus, this minimum effective amount is imperial and there does not appear to be a determinable relationship in these amounts between a particular alcohol and a particular surfactant chemical aid from the groups defined below.

After the alcohol and chemical aid have been distributed in the well fluid, it is left in a motionless state. The solids are removed from the liquid phase by agglomeration in a gel-like soft mass which can float to the surface or settle to the bottom of the container depending on the density of the agglomerated mass of solids. These solids remain stable in this agglomerated mass for longer periods of time (eg a week) but can be redispersed if the well fluid is subjected to new mixing operations. The mass of solids is transported from the aqueous phase by the raking device or by decanting or both in some examples when part of the solids mass floats and the other part of the mass sinks to the bottom of the container.

If the alcohol is added first to the well fluid and then followed by the chemical aid, an immediate clarification of the liquid phase generally occurs when the mixing operation is interrupted. Addition of the chemical aid before or together with the alcohol sometimes requires a long standing time for clarification of solids from the liquid phase. Clearing of solids is usually completed within minutes.

In either case, once the liquid phase has been clarified and the agglomerated mass of solids has been removed therefrom, the resulting well fluid will be essentially solids-free, especially from particle sizes greater than 5 pm in maximum dimension .

The alcohol is 2-ethylhexanol which is also known as 2-ethylhexyl alcohol and octyl alcohol. The chemical "abstract service" name is l-hexanol-2-ethyl. This alcohol can be obtained from sources of preferred solvents and its low evaporation rate and solubility make it suitable in the present process. It has low water solubility and low surface tension properties, which is an advantage for being able to easily separate this from the salt solution that has been cleaned of solids.

A good source for the alcohol is the suppliers of vinyl resin plasticizer manufacturers. Obviously, the alcohol need not be chemically pure, but it will usually be 99.0 vol-# pure alcohol with minor amounts of organic acids and aldehydes that do not interfere with the process.

The 2-ethylhexanol can be obtained commercially and it has a relatively high COC flash point of 84° with a specific density of about 0.83 at 25°C.

The surfactant chemical aid comprises a surfactant and usually comprises a carrier solvent such as a minor amount of an aromatic hydrocarbon, corrosion and pitting inhibitors and other additives desired to be added to the aqueous well fluid. The surfactant has a molecular weight in the range from 150 to 500 with predominantly hydrophobic properties. The surfactant is selected from the group consisting of aliphatic amines, amides and aliphatic amide oxides, in which the amine and amide oxide have an alkyl group between 8 and 18 carbon atoms.

Preferably, the surfactant is the amide reaction product of a monovalent fatty acid and a secondary or tertiary amine. More preferably, the fatty acid can be given by the formula CnH2n+iC00H in which n is an integer between 12 and 18. The fatty acid can be selected from the group of olein and dimerized olein, linolein, palmitinolein, palmitin, myristin, myr-stolein and stearic acids. The oleic acid amide products give good results.

The secondary and tertiary amines are selected from normal aliphatic amines which react with monovalent fatty acids to form fatty amides which are generally used as nonionic emulsifiers. Good results are obtained when these amines are selected from the group consisting of diethanol and triethanolamines and mixtures thereof.

One surfactant gives excellent results with 2-ethylhexanol, which is a product of Witco Inc. and available under the trade name "Witcamide 1017" (surfactant). This product is indicated to be the amide (salt) reaction product of oleic acid and diethanol and triethanolamines. It has a specific gravity of 1.0 (same as water) and is yellow with a PMCC flash point above 93° and is a non-hazardous product according to current Department of Labor definitions.

The theory of the effect of the alcohol and the surface-active chemical aid in the present method cannot be determined with certainty from the currently available information. It is believed that the alcohol serves to destabilize the dispersed solids by stripping their electrophoretic charges, and the surfactant then acts to collect the solids and composite oily materials into a loose solids system that can be removed by careful liquid/solid phase separation techniques which does not include cutting or mixing energy during the removal of the solids. For example, the liquid phase can be decanted from the solids. Alternatively, the solids can be carefully removed by a raking device such as used in air flotation cells.

It is preferred that the alcohol is added first and thoroughly mixed into the aqueous well fluid before the addition of the surface-active chemical aid. However, with combinations of specific alcohols and surface-active chemical aids, these materials can be added together and good results in the removal of solids can be achieved by this method. Today, there are no known regulations to help with the selection of these materials that can be used together in the well fluid and give the same good results as provided by separate but subsequent addition of the alcohol and then of the surface-active chemical aid. Likewise, with specific ingredients, the surface-active chemical aid can be mixed primarily with the well fluid and then the alcohol can be added with good removal of the solids in the process. Today, there are no known guidelines for help in choosing which surface-active chemical aid and which alcohol will provide the desired good removal of the solids from the well fluid by the additional measure. Unless the alcohol is first mixed into the well fluid and then followed by the addition of the surface-active chemical aid, some experimentation will be required to determine which of these materials can be added together or in the opposite order and even provide the desired good solids removal by the present method .

In general, the present method can be used to remove solids from all types of aqueous well fluids. In general, the presence of corrosion inhibitors, anti-pitting compounds etc will not cause any problems when removing solids. Some of the materials used in the production of drilling mud can interfere with the process, so that increased amounts of alcohol, surface-active chemical aids are required, or in an extended separation of the solids from the aqueous phase. These interfering materials can be removed prior to performing the present method steps. For example, the well fluid may have a significant amount of polyelectrolytes or polymers such as cellulose-based organic fluid loss agents (eg HEC). In these cases, the polymer can be removed by early treatment of the well fluid with a strong oxidizing agent such as hydrogen peroxide before carrying out the method on the well fluid.

An inlet to the pump 26 can be used to introduce the solids-free salt solution into the filtration unit 14. The filtration unit is connected with valved lines to the suction drilling mud tank 21 and mud pumps 16 so that the salt solution can be transported by a centrifugal pump 26 through a filter 27 (e.g. e.g. cartridge type) to a salt solution suction drilling mud tank or vessel 28. Mud pump 16 can thus introduce the salt solution into the wellbore. The filtration unit provides the opportunity for further removal of solids in the process, but can be avoided when sufficient contaminants are removed by the alcohol and the surface-active agent.

The drilling equipment 13 can include the drill head, casing, production pipe, gaskets, valves and other well-related devices, such as drilling safety valves and surface mud pipes, etc.

Several drain pipes 29-32, with auxiliary control valves are included in the well system 11 so that liquid from either the surface equipment 12 or the filtration unit 14 can be discharged to a suitable outlet in a pollution-free and environmentally safe area.

Before introducing the solids-free salt solution into the well system 11, the well system must be cleaned of drilling mud and liquid both with regard to solids and oil material and other contaminants. For this purpose, in an embodiment of the invention, the drilling mud and fluids from the well system are displaced by circulating through this a separate volume of a spacer liquid consisting of clean water introduced into the mud tank 19 from the inlet 24. This water is circulated by the mud pumps 16. A major part of the drilling mud is removed from the well system and transported in the clean water, which water can be diverted through one or more of the drainage pipes to a suitable drainage area.

Today, treated water is produced, preferably in the sludge tank 18, by adding together clean or sea water, a surfactant and an alcohol. The treated water is subjected to agitation and shear treatment by the mixer 19 while it is continuously circulated through the well system in both the surface and the wellbore equipment. The treated water displaces the now polluted water from the well system via one of the outlet pipes 29-32. It is important that the treated water is circulated through the parts of the well system in which the salt solution is to be transported. The treated water is circulated in the well system 11 until essentially all of the remaining drilling mud and liquid is suspended in it.

Typically, the treated water is present in a chemical water ratio of 4 barrels (208.2 liters each) mixed with each 500 barrels (208.2 liters each) circulated in the well system. Ever barrel consists of a 50/50 composition of the surfactant and the alcohol. As a result, the treated water has a volume concentration each of about 0.8 # of surfactant and alcohol. In most cases the concentration of the chemicals need not be greater than 1 # and a 0.5% concentration works well.

As mentioned, the alcohol is an aliphatic alcohol with between 5 and 8 carbon atoms and the surface-active agent is a surface-active chemical aid with a molecular weight in the range of 150 to 500 with predominantly hydrophobic properties. The surfactant is selected from the group consisting of aliphatic amines, amides and aliphatic amine oxides in which the amine and amide have an alkyl group with between 8 and 18 carbon atoms. The surfactant may be an amide such as the amide reaction product of oleic acid and diethanol and triethanolamine as previously described.

The alcohol and the surface-active agent can be selected and dosed as described above, for the steps relating to the removal of contaminants from the high-density salt solution, but in a preferred embodiment the alcohol is 2-ethylenehexanol and the surface-active chemical aid is bishydroxyethylcetylamine and each chemical is used in amounts of 0 .5% by volume of clean/seawater as used in the production of the "treated water.

Other alcohols that are suitable include pentanol, n-hexanol, and octanol.

Different amines can be used in this process. For example, the alkyl polyamines available under the trade name "Acquiness" such as "Acquiness MA401A" can be used. It is understood that this amine is mainly bishydroxyethylcetylamine.

Other examples of amines that are suitable for the invention are cocosamine, octylamine, dioctylamine, decylamine and dodecylamine. Cocosamines can generally be denoted by the formula CZtôCCItô )io^2-NH2 and it is prepared from monoethenized fatty acid derivatives derived from coconuts. The "coconut" group C12H25 is not a group containing a specific number of carbon atoms, but is a number of individual groups containing different numbers of carbon atoms. However, the C12H25 group is in greater abundance than any other group.

The cocoamine can be a condensation product, i.e. ox-alkylated cocoamine such as ethoxylated cocoamine with between 2 and 15 moles of ethylene oxide. More particularly, the condensation product is formed by allowing the cocoamine to undergo condensation with a plurality of moles of ethylene oxide in a manner known in the art. In general, the condensation product of one mole of cocoamine with between 2 and 15 moles of ethylene oxide can be used with good results. Preferably, the condensation product is formed by condensation of 10 mol of ethylene oxide per mol of cocosamine. Expressed on the basis of molecular weight, ethoxylated cocosamine can have an average molecular weight between 285 and 869, but preferably has an average molecular weight of about 645.

The circulating treated water mainly removes all remaining drilling mud (both solids and oils) and other contaminants from the well system. Drilling mud and fluids are transported in an agglomeration of gel-like soft masses of solids in a relatively stable suspension. The treated water provides an excess cleaning of the well system and removes remaining drilling mud, liquid and other contaminants in the pipes, sieve, drilling mud tank, valves, pumps and other components of the well system. As a result, this equipment retains no significant amounts of these contaminants both on the surface and in the wellbore. In other words, all remaining contaminants from the previous clean-water circulation step are now suspended in the treated water that is circulated in the equipment 12 and 13. No extensive manual cleaning by the rig personnel is necessary. The treated water is removed from the equipment and transports the remaining drilling mud in the suspension.

While the treated water is still being circulated in the well system, it is displaced via drainage pipes 29-32 to a suitable non-polluting and safe drainage area. The displacing liquid is solids-free clean water added through inlet 24. After the well system has been filled volumetrically with solids-free clean water, the solids-free salt solution previously prepared as described above can be arranged for introduction into the wellbore.

By using the mud pump 16, the solids-free salt solution is transported from the drilling mud tank 28 to the well drilling equipment (e.g. production pipes, casing spaces and wellhead devices) and it volumetrically displaces the solids-free clean water through the drainage pipe 32. The well system is then prepared for subsequent operations once the well drilling equipment down in the wellbore has been filled with solids-free salt solution.

It is preferred that the fire pipe or drill pipe, as the case may be, is moved up and down and rotated in the wellbore during circulation of the treated water. The pipe in inclined wells can be moved up and down about 10 meters with periodic rotation and this movement function accelerates the removal of drilling mud from the pipe and the wellbore and its suspension in the treated water.

In some cases, an improved cleaning result is obtained if the alcohol and the surfactant are added in two parts to clean water to produce the treated water. For example, half of the chemicals are added after the circulation has continued for 30 minutes, or when the well pipe is to be moved up and down and rotated in the well hole.

The use of circulating clean and treated water in the well system is advantageous as only moderate amounts of water are required. It has been found that the volumes of clean water, treated water and solids-free water used in this process are in the range of 250 to 1000 barrels. This feature is important in water-poor areas.

In an alternative embodiment of the invention, aqueous spacer plugs can be used before, after (or both) the treated water as a spacer liquid instead of pure water. The main purpose of the forward transported aqueous spacer fluid is (1) to reduce the gravity treatment of the treated water with either drilling fluid or solids-free brine, (2) to retain and transport well fluid solids such as drilling mud, bottom mud, sand, oil, barite, ferro - and ferric precipitates and other undissolved solids from the well drilling equipment, in a similar way to pure water used in the above-described embodiment. The main purpose of the following aqueous spacer fluid is to reduce gravitational mixing between the treated water and salt solution. Generally, the present method removes drilling fluid from the wellbore very efficiently so that only the first part of the circulating salt solution (e.g. 100 barrels) has some solid contamination.

An aqueous spacer fluid containing a thickener is prepared to provide a non-Newtonian flow that can be circulated in laminar flow in the well drilling equipment. For example, a small amount (e.g. 50 barrels) of fresh or salt water is placed in the sludge tank 18 and a thickening agent is added which gives a gelled form of the mixture. For example, suitable thickeners used in drilling mud products such as Polybrine, Polymix, Cellosize, Fuovis (trade name) are sold commercially by the Magcobar Division of Dresser Industries, Inc. For oil-based drilling fluids, this group has available a diesel thickener under the trade name Oilfaze which can be used in conjunction with diesel oil or selected crude oils, clays such as bentonite, emulsifiers and weight increasing agents (e.g. barite). Thickeners are added to the aqueous phase in amounts sufficient to give it a gelled state and to maintain this state under laminar flow conditions at the linear flow rate at which the following treated water is moved at such a rate under turbulent flow. In general, the aqueous spacer operates at a Reynolds number of about 100, where the treated water is at a Reynolds number of about 2000.

In other words, at a certain linear flow rate, the gelled aqueous spacer flows below or at the maximum velocity for plug (laminar) flow while the aqueous spacer flows at or below the minimum critical velocity for achieving turbulent flow in the well drilling equipment.

There are many suitable thickeners available from sludge product suppliers and they can be substitutes for the particular trade name products such as furnace linings discussed. These products have known rheological properties and an aqueous gelled phase can be produced with the desired non-Newtonian state.

In many cases, it is preferred that a weight-increasing agent can also be included in the gelled aqueous phase. For example, barite can be added to increase the phase weight up to 2.40 kg/l. Other thickeners such as "Bentone", "Attapulgite" or asbestos fibers can be used with good results. These materials also increase the effectiveness of the gelled aqueous spacer fluid and act as a mechanical scrubber and not only displace the drilling fluid and mud from the well drilling equipment, but also remove mud cake deposited on the walls of the well pipe, casing or wellbore.

Although the gelled spacer fluid acts as a true plug or mechanical swab, well effects cause it to be "strung out" on its passage through the wellbore armoring. For example, 50 barrels of the plug can be introduced into the well drilling equipment, but it may require an outflow of twice this volume due to the entrained drilling fluid, etc. mixed into it.

Since the drilling fluid is oil-based and especially as an inverse emulsion (water-in-oil), the gelled aqueous spacer fluid preferably comprises a liquid mixture of emulsifiers, wetting agents, lubricants, gelling agents and other additives such as arganophilic clay, HEC polymers, starch, poly- anionic cellulose, guar gum etc, which additives immediately form a stable inverse emulsion liquid when mixed with diesel oil and water under moderate shear mixing. For example, the aqueous inverse oil type spacer may be formed from water and organophyllic clay. The spacer prepares the mud-coated production pipe and the oil-based drilling fluid for effective displacement from well drilling equipment at the following treated water plug.

The aqueous spacer is transported from the drilling mud tank 18 and suction drilling mud tank 21 and is forced by the pumps 16 in the well drilling equipment 13. In deep wells, the aqueous spacer is introduced into the well pipe (production pipe or drill pipe) and displaces drilling fluid upwards from the annulus to the drill roof screen 17, or other suitable drains as with the drainage pipe 32. In other applications, the aqueous spacer can be introduced down through the annulus and force the drilling fluid up through the well pipe. In both cases, the aqueous spacer is in a gelled state and forms a separate plug that acts as a mechanical wiper by displacing the drilling fluid in front of it.

The linear velocity of the aqueous spacer plug displaced through the well drilling equipment depends on the hole diameter or annulus size and typically varies between 24.4 and 61.0 m/min to maintain the laminar flow condition of the non-Newtonian aqueous spacer having flow conditions of Reynolds number of about 100.

The treated water is circulated in the well drilling equipment, usually immediately followed by a gelled aqueous spacer. The treated water volumetrically displaces the plug of aqueous spacer and also provides a unique cleaning function by removing residual drilling fluid and other solids, such as rust, ferrous and ferric deposits, sand, mud, oil and other undissolved materials from the drilling equipment.

The treated water is produced, preferably in the drilling mud tank 18, by adding together clean (fresh or sea) water, surfactant and an alcohol. The treated water is subjected to agitation and shear mixing by the mixer 19. The amount of treated water depends on the capacity and strength of the well drilling equipment 13, the formation pressure and the physical condition of the well, (ie open high-pressure perforations or casing plugs that will not allow reduction of hydrostatic head) and can be produced in a quantity of e.g. 25-1000 barrels.

The treated water is usually composed in a chemical-to-water ratio of 4 barrels (199 liters each) mixed with each 500 barrels (159.0 liters each) of water added to the drilling mud tank 18. Every barrel is composed of a 50/50 mixture of the surfactant and the alcohol. As a result, the treated water has a volume concentration of surfactant and alcohol of each 1.0%>. In most cases, the concentration of the chemicals need not be greater than 2%, and 1 # concentration usually works well. If desired, other materials can be added that increase the performance of the treated water, such as sand, walnut shells, etc. The alcohol and surface-active agent can be chosen according to the version described above.

The treated water is transported from the drilling mud tanks 18 and 21 and is forced by the mud pumps 16 into the well drilling equipment immediately after the plug of gelled aqueous spacer. The flow of the treated water drives this plug ahead of it, which plug volumetrically displaces the drilling fluid from the well drilling equipment 13. In addition to its function as a displacement fluid, the treated water removes residual drilling fluid and its mud cake from the well surfaces when pumped under turbulent flow conditions.

The treated water has a Reynolds number of about 2000 and turbulent flow conditions are obtained in the well drilling equipment at linear flow rates between about 24.4 and about 61 m/min. The turbulent flow condition is intended to include actual turbulence above the critical velocity of the treated water and also the condition at the upper limits of linear flow which is essentially of the same effect as turbulent flow with respect to the cleaning result relative to the drilling fluid. Thus, the treated water and the plug gelled aqueous spacer can be transported in a range of flow velocity where the treated water is in a turbulent flow and the plug is moved at linear flow conditions.

As in the previous embodiment, the treated water removes essentially all of the remaining drilling fluid (both mud, solids and oils) from the well system. The drilling fluid is carried in an agglomeration of gel-like soft masses of solids in a relatively stable suspension. The treated water provides a thorough cleaning of the well drilling equipment and removes residual drilling fluids and mud in the well drilling equipment and no significant quantities of components of the drilling fluid escape due to its cleaning effect. In other words, all of the remaining drilling fluid and mud from the previous displacement is then suspended by the gelled aqueous spacer in the treated water. No cleaning operation using mops etc by the rigging staff is now necessary. The treated water is removed from the equipment and transports contaminants in suspension.

While the treated water is still circulating in the well drilling equipment, it is displaced via drain pipes 29-32 to a suitable non-polluting and safe drainage area. It can also be processed in the borehole view, if desired. The treated water is immediately followed by the second plug of gelled aqueous spacer which can be one of the same composition and volume as the previously described spacer. However, the second gelled aqueous spacer may have a different composition.

In some operations, the second plug can be made more compatible for volumetric displacement of the treated water by the subsequently introduced solids-free saline solution. For this purpose, the thickener is preferably a high molecular weight polymer used in clear salt solution systems, such as hydroxyethyl cellulose (HEC). A fluid loss control agent may also be added. For example, the mixture of HEC and the fludium loss control agent can be provided by the trade name product "Polybrine", a product of the Macobar Division of Dresses Industries. In all cases, the thickener and other additives must be operable at the temperature conditions in the well drilling equipment, so that they act in the correct way as a plug of gel-formed aqueous spacer.

The second plug of gelled aqueous spacer is introduced into the well drilling equipment 13 by the mud pump and is immediately followed by solids-free salt solution previously prepared as described above using the mud pump 16, the solids-free salt solution is transported from the drilling mud tank 28 to the well drilling equipment and it volumetrically displaces the first plug of gelled aqueous spacer, the treated water and the other plug of gelled aqueous spacer through the drain pipe 32. The salt solution is introduced into a clean well system free of drilling fluid, solids or other contaminants. The salt solution is pumped into the well drilling equipment at the same linear flow rate as the treated water so that the plugs of gelled aqueous spacer are moved under laminar flow conditions. Preferably, there are no interruptions to the circulation of these fluids, but temporary interruptions to the circulation can be tolerated as the plugs of gel-forming aqueous spacers reduce to a minimum cross-contamination between the drilling fluid, treated water and salt solution which is displaced volumetrically through the well drilling equipment.

The well system is now ready for subsequent activities once the well drilling equipment 13 is filled with the solids-free salt solution.

In a current field test on an oil-based mud, the present method was used to complete a well with solids-free salt solution. The following data define this process: (1) well depth 5.334 m with 177 mm casing and 3t drill pipe with a drill pipe volume of 128 barrels, a ring toms

volume of 378 barrels and a total volume of 506 barrels,

(2) first plug of gelled aqueous spacer was 47 barrels of 8.7 vol-# of "Bariod EX Spot" in clear

water weighted with barite to 1.74 kg/litre,

(3) the treated water was 5.5 barrels of seawater containing 1.74 vol # of 50/50 ratio of 2 ethylhexanol and bishydroxyethyl cetylamine, (4) the second plug of gelled aqueous spacer was 10 barrels of 0.01 wt- # of HEC in clear water, (5) the following salt solution had a density of 1.20 kg/liter of solids-free calcium chloride solution.

The first plug was introduced into the drill pipe followed by the treated water at a maximum annulus velocity of 41.5 m/min. Then the second plug was introduced followed by the saline solution at an annulus velocity of 114 m/min. The well equipment was displaced within about 2Vi hours.

Although about 10 barrels of a 1.74 kg/liter "Invermul" drilling mud were carelessly introduced before the second plug, the last portions of the treated water were clear as originally introduced. The well nut armor was free of solids so that the salt solution was kept solids-free and then provided the previously described cleaning process in a single pass.

The first portions of the treated water displaced from the well equipment brought up tons of dirt and several hundred pounds of cement even as circulation was interrupted several times during the process. It is believed that 12 hours of rig time was saved by using this method instead of previous chemical washing procedures and the well equipment was essentially made free of solids before introducing the salt solution.

In another field test, in the well drilling equipment, a 1.86 kg/liter "Invermul" oil-based mud was displaced by a reverse flow of about 636 liters per minute. The first spacer plug was introduced on the drilling mud, which spacer was 3180 liters formed from 416.4 liters of Baroid "Easy Spot" emulsifier and diesel oil and weighted at 1.86 kg/litre. A second spacer plug followed the first spacer and was used in a quantity of 2385 litres. The second spacer was seawater containing 1427 kg/m<5> of "Nut Plug" (ground walnut shell) with half a barrel of a 50/50 mixture of surfactant and alcohol as used in the treated water according to the previous example. A third distance plug was still used which followed the second distance plug. This third spacer was 6360 liters of a high viscosity (120 Saybolt) blend of HEC polymer with 2853 kg/m<5> of fracturing sand. Then, treated water was introduced in an amount of 1590 and immediately followed by the solids-free salt solution. This treated water was used in an amount of 1590 liters formed from fresh water containing half a barrel of the surfactant and the alcohol from the previous example.

About 3,975 liters of the salt solution that was first circulated through the well drilling equipment was diluted and therefore discarded. The balance of the salt solution was made solids-free by filtration for about 90 minutes in circulation in a filter system.

A storm interrupted the procedure described above for 24 hours. It was noted that the "Nut Plug" floated to the top of the second spacer. However, when the circulation was again carried out, the blow of the treated water behaved as expected.

In other field tests, the present method uses only the first spacer plug and the slag of the treated water before the circulation of solids-free salt solution through the well drilling equipment for good removal of drilling mud. When oil-based sludge is present, it is preferred that the first spacer plug contains or is followed by a few barrels of an organic-based solvent which may contain water-in-oil emulsifier.

The use of the treated water and aqueous spacer plugs in the well system is advantageous as only smaller quantities of water are required. It has been found that the volume of treated water and gelled aqueous spacers used in this process is in the range of 15,900-159,000 liters and is generally of less volume than a comparable volume used in the first embodiment (ie using pure water instead of spacers). This feature is important in water-poor areas and with regard to drainage problems.

From the above it will be apparent that a method has been described for removing contaminants from a high density salt-type completion and packing brine and for removing drilling mud and fluids and other contaminants from a well system prior to the introduction of the solids -free salt solution. Various changes and modifications can be carried out when carrying out the method by the person skilled in the art without deviating from the inventive concept. It is intended that such changes are covered within the scope of subsequent patent claims. The present description is intended to be illustrative and not to limit the present invention.

Claims (10)

1. Process for producing a solids-free, aqueous liquid from a liquid contaminated with solids such as rust, sand and drilling mud, by introducing into the liquid small effective amounts of an aliphatic alcohol with 5-8 carbon atoms and a surface-active chemical aid to agglomerate solids therein, and separating the agglomerated solids from the liquid before it is used in a solids-free state, characterized in that a surface-active chemical aid is used which includes a surface-active agent having a molecular weight in the range of 150-500 with predominantly hydrophobic properties and is selected from aliphatic amines, aliphatic amides and aliphatic amine oxides, the amine or amide having an alkyl group with 8-18 carbon atoms. 2. Process according to claim 1 for the production of a high-density drilling fluid, characterized in that one or more of the sodium, calcium or zinc salts with chloride and bromide and mixtures thereof are used in the drilling fluid, and that the agglomerated solids are separated from the fluid before introduction into a wellbore in a solids-free state. 3. Method according to claim 2, characterized in that the amide reaction product of diethanolamine and triethanolamine with an organic monobasic fatty acid with the general formula CnH2n+iC00H where n is a whole number between 12 and 18 is used as surfactant. 4. Method according to claim 2, characterized in that the alcohol and the surfactant are used in a 50/50 volume mixture. 5 . Method according to claim 1, particularly aimed at removing contaminants from a well system by circulating treated water therein, characterized in that the treated water is produced by adding the surface-active agent and the alcohol to clean water and "subjecting the treated water to agitation and shear treatment , and that the solids-free, aqueous liquid is then circulated in the well system to displace the treated water from the well system. 6. Method according to claim 5, characterized in that it includes the following further steps: a) displacement of the contaminants from the surface equipment which is connected to the well system by circulating therein a spacer liquid introduced before the treated water until a major part of the contamination has been removed from the surface equipment and transported in the spacer fluid, the spacer fluid being displaced from the surface equipment by the treated water; b) displacement from the surface equipment to a suitable disposal area of the treated water containing the contaminants by means of the spacer fluid in front of the solids-free well fluid without interrupting the circulation through the surface equipment; and c) displacing the spacer fluid from step b) with the solids-free well fluid. 7. Method according to claim 6, characterized in that the surface-active agent and the alcohol are each added in a volume that is less than about 1% to the pure water to produce the treated water. 8. Method according to claim 6, characterized in that bishydroxyethyl cetylamine and the amide reaction product of oleic acid and diethanol and triethanolamines are used as surfactants. 9. Method according to claim 6, characterized in that the distance liquid used is one that contains a viscosity modifying agent for the formation of a non-Newtonian liquid that is circulated as a plug in a gel state in laminar flow, and that the treated water is produced as a Newtonian liquid that is circulated in turbulent flow conditions through the surface equipment. 10. Method according to claims 1-9, characterized in that 2-ethylhexanol is used as aliphatic alcohol.