WO2012123338A1

WO2012123338A1 - Bore hole fluid comprising dispersed synthetic polymeric fibers

Info

Publication number: WO2012123338A1
Application number: PCT/EP2012/054048
Authority: WO
Inventors: Herbert HOFSTÄTTER
Original assignee: Lenzing Plastics Gmbh
Priority date: 2011-03-11
Filing date: 2012-03-08
Publication date: 2012-09-20
Also published as: BR112013023276A2; AR089552A1

Abstract

A bore hole fluid for control of fluid loss, the bore hole fluid comprising a base liquid and synthetic polymeric fibers dispersable in the base liquid as an additive.

Description

Bore hole fluid comprising dispersed synthetic polymeric fibers

Field of the invention

The invention relates to a bore hole fluid, a bore hole arrangement, a method of controlling fluid loss in a bore hole, a method of use, and a method of producing a bore hole fluid.

Background of the invention

In drilling of wells for hydrocarbons such as oil and/or gas from

subterranean deposits or in drilling for geothermal energy, it is common practice to use a rotary drilling procedure in which a drill bit is rotated at the bottom of the bore hole by means of rotating hollow drill pipe which extends to the surface. The drill pipe is driven from the surface and a circulating fluid commonly referred to as a drilling fluid or drilling mud pr bore hole fluid is pumped through the drill pipe where it emerges through openings in the drill bit to cool the same and is returned to the surface in the annular space between the drill pipe and the walls of the bore hole. The bit might also be rotated by a downhole motor which is powered by the drilling fluid as well.

The drilling fluid, upon emerging from the well at the surface, may be mechanically and/or chemically processed to remove the cuttings and other undesirable contaminants and is normally treated chemically to maintain certain chemical and physical properties of the fluid depending upon particular drilling conditions encountered. The drilling fluid after being reconstituted is normally recirculated by pumps to be forced downwardly through the drill pipe, this circulation being generally continuous during drilling. Circulation of the drilling fluid may be interrupted occasionally such as when an additional section of drill pipe is added at the top of the string of pipe or when the entire length of drill pipe is withdrawn to replace or repair the drill bit.

The drilling fluid may be capable of performing many functions which are required in a successful drilling procedure and therefore may possess certain desirable chemical and physical properties. The drilling fluid may have sufficient viscosity to suspend and remove the cuttings from the bore hole and may have sufficient gel strength to hold solids in

suspension, especially when circulation of the fluid is interrupted. It also may have sufficient density to exert suitable pressure to the sides of the bore hole to prevent the entrance of fluids into the bore hole from the earth formation being penetrated, and it may have low fluid loss to prevent undue loss of fluid into the formation by its deposition on the bore hole sides such as by forming an impervious filter cake or deposit. Furthermore, a dense drilling fluid may be used to compensate for the pressure the borehole is exposed to by the surrounding earth formation. Although the rheologic characteristics of the drilling fluid may be adjusted by additives, for instance polymers, such additives tend to have a limited temperature stability.

WO 2010/069479 Al discloses compositions and methods for providing fluid-loss control in subterranean wells. Well-completion fluids contain fine particulate additives whose glass-transition temperatures are below the anticipated bottom hole temperature. The particles soften upon injection into the well, whereupon they soften and become deformable. The particles then migrate to the borehole wall and form a seal that reduces further fluid flow from the borehole into the formation. The additive may be supplied as a powder or in the form of a liquid suspension.

WO 2010/019535 A2 discloses a method for reducing lost circulation in drilling wells, employing composite materials as lost circulation materials. The composites comprise a thermoplastic polymer and cellulosic fibers. Optionally the composites may include other components such as calcium carbonate, clay, oil and other blending agents.

WO 2009/105745 Al discloses a field-responsive fluid which enters a semi-solid state in the presence of an energy field and which is improved by use of a plurality of energy field responsive particles which form chains in response to the energy field. The particles can be (a) composite particles in which at least one field-responsive member having a first density is attached to at least one member having a second density that is lower than the first density, (b) shaped particles in which at least one field- responsive member has one or more inclusions, and (c)

combinations thereof. The particles improve the field-responsive fluid by reducing density without eliminating field-responsive properties which afford utility. Further, a multi-phase base fluid including a mixture of two or more substances, at least two of which are immiscible, may be used. The multi-phase base fluid improves the field-responsive fluid because surface tension between the boundaries of the immiscible substances in conjunction with chains formed by field-responsive particles tends to stop or retard creep flow, resulting in an improved dynamic or static seal.

WO 2007/107015 Al discloses that lost circulation of drilling fluid is one of the most serious and expensive problems facing the drilling industry. The present invention relates to an improved drilling fluid for reducing or preventing lost circulation to an underground formation surrounding a well bore in the process of drilling a well . The drilling fluid comprises a base fluid and wax or waxy substance as a primary seepage loss agent. WO 2007/107015 Al also provides a method of reducing or preventing lost circulation to an underground formation surrounding a well bore in the process of drilling a well using the mentioned drilling fluid, wherein the primary seepage loss agent is added to the drilling fluid, either before or during drilling, and the drilling fluid is pumped down hole during drilling.

Conventionally used drilling fluid is not suitable for requirements in very deep bore holes, particularly when an efficient filter cake formation is desired.

Object and summary of the invention

It is an object of the invention to provide a drilling fluid which meets requirements in very deep holes and at the same time allows for an efficient filter cake formation.

In order to achieve the object defined above, a bore hole fluid, a bore hole arrangement, a method of controlling fluid loss in a bore hole, a method of use, and a method of producing a bore hole fluid according to the independent claims are provided.

According to an exemplary embodiment of the invention, a bore hole fluid (which may also be denoted as a bore hole suspension since it may comprise both liquid and solid components) for control of fluid loss is provided which comprises a base liquid and synthetic polymeric fibers dispersable or dispersed, as an additive, in the base liquid.

According to another exemplary embodiment, a bore hole arrangement is provided which comprises a formation having a bore hole formed therein, and a bore hole fluid having the above-mentioned features which at least partially fills the bore hole for controlling fluid loss towards a wall of the bore hole. According to a further exemplary embodiment, a method of controlling fluid loss in a bore hole is provided, wherein the method comprises filling at least a part of the bore hole with a bore hole fluid comprising a base liquid and synthetic polymeric fibers, as an additive, dispersed in the base liquid.

According to yet another embodiment, a mixture of a base fluid and an additive of synthetic polymeric fibers is used as a bore hole fluid for controlling fluid loss in a bore hole having a temperature at the deepest position of at least about 200 °C, particularly of at least about 300 °C.

According to still another embodiment, a method of producing a bore hole fluid for control of fluid loss is provided, wherein the method comprises forming synthetic polymeric fibers, and dispersing, as an additive, the fibers in the base liquid.

In the context of this application, the term "bore hole fluid" (which may also be denoted as drilling fluid or drilling mud or bore hole liquid) may particularly denote a fluidic substance which is used in a bore hole for purposes such as hole cleaning, fluid circulation, etc. In the course of completing oil and gas wells and the like, various types of fluids may be circulated in the wellbore. These fluids may be called bore hole fluids. However, solid or solidifyable cement slurries and granular solid lost circulation materials will not be considered as bore hole fluids due to the lack of basically liquid character. Hence, a bore hole fluid may have a substantially liquid character, i.e. may be non-solid and non-solidifyable as a whole at least under the pressure and temperature conditions of bore holes. As an additive, a bore hole fluid may contain solid particles such as the fibers. In other words, the flow properties of the bore hole fluid may be liquid outside and inside of the bore hole. For instance, a bore hole fluid can be used for applications such as circulation from a position externally of a bore hole through a tubular drill string towards an annulus between dring string and bore hole and back to the surface.

The term "bore hole" or wellbore may particularly denote a vertical, horizontal or slanted hole drilled in a formation such as a rock to access deeper regions of the formation in which exploitation fluids such as oil, gas or water may be located.

The term "fibers" may particularly denote elongated pieces of a given material, for instance roughly round or rectangular in cross-section, optionally twisted with other fibers. Fibers may have an aspect ratio which is larger than 2, particularly larger than 5, more particularly larger than 10. The aspect ratio is the ratio between the length of the fiber and a diameter of the fiber. Fibers may form networks by being

interconnected or interwoven. Fibers may have a substantially cylindrical form which may however be straight, bent, kinked, or curved. Fibers may consist of a single homogenous material and may hence be a non- composite material. The term "synthetic" may particularly denote produced by synthesis instead of being isolated from a natural source. Synthetic compounds are made artificially by chemical reactions. By providing fibers being synthetic, the bore hole fluid has defined properties and a high

temperature stability since a chemical synthesis produces fibers with high internal bonding strength. In contrast to this, many natural fibers do not have a sufficient temperature stability as required for use in a bore hole fluid. Hence, synthetic fibers may be fibers which do not occur in nature.

The term "polymeric" may particularly denote of or relating to or consisting of a polymer. A polymer may be denoted as a large molecule (such as a macromolecule) composed of repeating structural units typically connected by covalent chemical bonds. Such polymeric fibers are capable of providing a high resistance against deterioration and decomposition.

The term "control of fluid loss" may particularly denote the action of adjusting the degree and character according to which one or more liquid components propagate from a bore hole into surrounding material. By adjusting the properties of the fibers (concentration, size, material, etc.), it is controllable in which manner the fibers will fill small gaps in surrounding material of the formation, thereby clogging the gaps to thereby prevent liquid components of the bore hole fluid to get lost in the formation . Fluid hydrostatic pressure and pumping pressure may create a pressure differential between the wellbore and the surrounding formation rock. As a result, the liquid portion of the bore hole fluid may have a tendency to enter pores in the subterranean rock and migrate away from the wellbore. Such a filtration process can be denoted as fluid loss.

The term "base liquid" may particularly denote that the majority, particularly the large majority, of the bore hole fluid may be contributed by the base liquid. This may result in a basically liquid behavior of the bore hole fluid as a whole. The one or more further components of the bore hole fluid, however, has or have in contrast to this the character of additives only. Particularly, at least 60 volume % or weight %,

particularly at least 70 volume % or weight %, more particularly at least 80 volume % or weight %, of the bore hole fluid may be contributed by the base liquid.

The term "additive" may particularly denote a component that may be added to the base liquid to thereby form a mixture to modify its properties and in general, enhance its processing performance. Additives may include catalysts, viscosity influencing agents, accelerators, inhibitors and other ingredients. An additive may therefore be an ingredient or combination of ingredients added to the base liquid to fulfil a specific need. An additive may therefore denote a minority, particularly a small majority, of the bore hole fluid. Particularly, less than 20 volume % or weight %, particularly at least 10 volume % or weight %, more particularly at least 5 volume % or weight %, of the bore hole fluid may be contributed by an additive.

According to an exemplary embodiment, a bore hole fluid is provided which is specifically configured to meet the requirement of drilling fluids in reservoirs of fossil fuels to be exploited from reservoirs at very high depths. Such a drilling fluid may be used for removing drilling cuttings from the bore hole, for bore hole stabilization, for protecting the formation, etc. In view of the high temperatures in a bore hole, a stability against the high temperatures down in deep bore holes as well as a resistance against bacteria is needed and is conventionally challenging. Environmental compatibility of bore hole fluids and their capability to be recycled is of significance as well. By adding polymeric synthetic fibers to a main or base liquid, particularly the requirement of temperature stability down in the deep bore holes can be met, since synthetic fibers show an excellent resistance against high temperatures and can be made subject to temperatures of 200°C and more without being decomposed. At the same time, such synthetic polymeric fibers show an excellent resistance against bacteria of different types. Such fibers can further be produced on an industrial scale and allow for a cost-efficient operation of a well bore. Particularly for controlling fluid loss properties of a well bore operation, the mentioned combination of a base liquid and synthetic polymeric fibers has turned out to be highly efficient. Apart from the proper dispersion properties of such polymeric synthetic fibers in a base liquid, when such a bore hole fluid is pumped in a bore hole, the fibers have a proper tendency even at relatively low concentrations to

efficiently fills gaps and cracks in the surrounding formation to thereby prevent undesired draining of the liquid components of the bore hole fluid into the surrounding formation. Size, concentration and material of the fibers in the bore hole fluid are proper design parameters to properly adjust the fluid loss properties of the drilling fluid in accordance with specific requirements of an individual application. Apart from this, so- called filter cake which is formed by the fibers at a surrounding wall of a bore hole also contributes to the maintenance of the stability of the bore hole, while at the same time reducing or eliminating deteriorations of the formation. The low friction of synthetic polymeric fibers, i.e. an

advantageous tribologic behavior, also allows to use the bore hole fluid as a lubricant for drill string and drill bit. Also the transmission of hydraulic drive energy to the drilling bit is possible with this kind of material, as well as a communication of information with regard to the bore hole (so- called mud pulse telemetry). Furthermore, synthetic polymeric fibers are inert against many components of the bored formation and are therefore not prone to undesired interactions. Hence, according to an exemplary embodiment, a temperature stable filtration additive is provided which is capable of attaching to a bore hole wall in order to prevent, to an adjustable degree, fluid loss to a surrounding formation. In an

embodiment, apart from the control of the filtration properties, also a rheological control even at high temperatures is possible. When being pumped into a bore hole, the fibers may form some kind of bridge or filter layer between the formation and an interior of the bore hole.

Detailed description of embodiments of the invention

In the following, further exemplary embodiments of the bore hole fluid will be explained. However, these embodiments also apply to the bore hole arrangement, the method of treating a bore hole, the method of use, and the method of producing a bore hole fluid.

In an embodiment, the fibers may comprise fluoropolymer fibers, particularly polytetrafluorethylene (PTFE). Fluoropolymer fibers, i.e. fibers having Fluor as a component, are highly appropriate as an additive for the bore hole fluid, because a Fluor component within a polymer is basically inert and therefore chemically stable even under harsh, chemically aggressive and high-temperature conditions. On the other hand, Fluor polymers have a low friction in the present application of bore hole fluids. Particularly suitable is PTFE which can be produced at reasonable costs on an industrial scale. However, there are further fluoropolymers which are suitable for the fibers of the bore hole fluid as well, such as perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), or ethylene tetrafluoroethylene (ETFE). Furthermore, it has turned out to be advantageous that synthetic polymeric fibers, particularly

fluoropolymer fibers, are resistant against an atmosphere including C0₂, H₂S and other chemicals which are present in a bore hole or a

surrounding formation. Fluoropolymers are particularly suitable as a material for the fibers, because it is believed that the Fluor shields a carbon-carbon bonding of the fiber material, thereby rendering the material inert and therefore not prone to decomposition by chemicals, temperature or the like. Furthermore, particularly PTFE with its

hydrophobic and therefore water-shielding properties is advantageous, since any undesired interaction with water or the like can therefore be eliminated.

In an embodiment, the fibers are staple fibers. The term "stable fibers" or "pulp fibers" can denote short fiber pieces which have turned out in already performed viscosity measurements and fluid loss measurements as particularly advantageous also with respect to their dispersion properties in a base liquid. Such pulp fibers may be grinded fibers, particularly of PTFE, in which a majority of the fibers have a length of not more than 2 mm .

In an embodiment, at least about 50%, particularly at least about 70%, of the fibers have a length of less than about 0.5 mm . Due to the manufacturing process, the length of the fibers will in many cases be a distribution. However, if a majority of the fibers have a length of smaller than 0.5 mm, the corresponding dispersion properties in the base fluid, particularly in an oleophilic base liquid, have turned out to be

advantageous. At the same time, fibers of this length have turned out to properly seal the wall of a bore hole by forming a filter cake. In an embodiment, 10% to 30%, particularly 15% to 25%, of the fibers have a length in a range between 0.5 mm and 1 mm. In an embodiment, less than 10%, particularly less than 5%, of the fibers have a length of more than 1 mm. In an embodiment, at least 50% of the fibers may have a dimension in a direction perpendicular to the length of 10 μιη to 50 μηι, particularly of 20 μιτι to 40 μηι.

In an embodiment, at least about 50%, particularly at least about 70%, of the fibers have a length in a range between about 10 μηη and about 0.5 mm . In an embodiment, at least about 70% of the fibers have a length in at least one dimension of at least about 10 μηι. Without wishing to be bound to a specific theory, the present invention has determined that fibers with a length below 10 μηη do not contribute with the same efficiency to the formation of a filter cake as larger fibers. If however the size of the fibers becomes too large, both the dispersion properties and the formation of a filter cake can be deteriorated.

In an embodiment, at least about 50%, particularly at least about 70%, of the fibers have an aspect ratio of larger than five. The aspect ratio may be the ratio between length and diameter of a fiber. Without wishing to be bound to a specific theory, it is presently believed that a sufficiently high aspect ratio of the fibers is advantageous in terms of the formation of an efficient filter cake for filling small cracks and gaps in a surrounding formation. However, it turned out to be advantageous that at least 50%, particularly at least 70%, of the fibers have an aspect ratio of smaller than 50. If the aspect ratio becomes too large, the low-friction pumping characteristic and the dispersion properties of the fibers may be deteriorated.

In an embodiment, a concentration of the fibers is less than about 20 gram fiber mass/liter bore hole fluid, particularly less than or equal to about 10 gram fiber mass/liter bore hole fluid. Although the fibers are a cost-efficient additive to the bore hole fluid, it is desirable for cost reasons and also for reasons of dispersion and pumping characteristic that the concentration of the fibers does not become too large. In this context, it has been surprisingly determined by the present invention that above about 10 gram fiber mass/liter bore hole fluid, no further significant improvement of the properties regarding the formation of a filter cake can be achieved. Therefore, already a very small mass contribution of the fibers in the bore hole fluid may be sufficient to meet at the same time the requirements of an efficient control of fluid loss as well as a proper adjustment of the dispersion and friction properties. In an embodiment, the fibers are made of a material being temperature- resistant at least up to about 200 °C, particularly at least up to about 300 °C. To distinguish fibers being temperature-resistant up to a certain temperature or not, it will be apparent for the skilled person that a simple test whether a specific fiber material meets this requirement is to place a corresponding fiber material in a base fluid under such temperature conditions and to analyze whether a thermal decomposition of the fibers occurs or not. Examples for fibers which meet such high temperature requirements are fluoropolymers or polymers of other halogenides (such as chlorine).

In an embodiment, the fibers are oleophilic, i.e. have a strong attraction to oils, that is to say are of a substance that mixes readily with oils. Such an embodiment, which is for instance fulfilled by fluoropolymer fibers, is particularly advantageous when the base liquid comprises or consists of one or more oils. In such an embodiment, the oleophilic fibers can be dispersed properly in the oil.

In an embodiment, the fibers consist of a single homogeneous material . In such an embodiment, the fibers do not comprise more than one material such as PTFE. Hence, no complex compositions or the like need to be formed, in contrast to this the fibers can be easily manufactured and have homogeneous behavior in the base liquid.

In an embodiment, the base liquid comprises or consists of oil,

particularly at least one of the group consisting of a synthetic oil, a natural oil, and a mineral oil. A synthetic oil may be formed of one or more chemical compounds being artificially made (synthesized). For example, chemically modified petroleum components may be an example for such a synthetic oil . Natural oils are in contrast to this naturally occurring oils such as vegetable oils. For example, rapeseed oil and/or sunflower oil may be used as a basis for the bore hole fluid. Paraffinic oils, naphthenic oils and aromatic oils are examples for mineral oils.

In an alternative embodiment, the base liquid comprises or consists of water and/or an organic solvent, particularly an alcohol. Hence, also an aqueous solution may be used as a base liquid for the bore hole fluid. However, additionally or alternatively, organic solutions may also contribute to the bore hole fluid. An example are alcohols such as ethylene, propylene, etc.

In an embodiment, the bore hole fluid may comprise a fiber sinking inhibition additive, particularly potash (K₂C0₃). Such a fiber sinking inhibition additive may have a sufficient density to prevent the fibers from sinking to the ground. Potash has a relatively high density so as to increase the overall density of the bore hole fluid, thereby preventing a sinking of sometimes heavy or high density synthetic polymers such as PTFE. Therefore, the described additive also provides a contribution to the homogeneity of the bore hole fluid. For instance, such a fiber sinking inhibition additive may be provided with a concentration of 50 g/l to 200 g/l, particularly 120 g/l to 200 g/l.

In an embodiment, the bore hole fluid may comprise a pH control additive, particularly citric acid. By controlling the pH-value, also the chemical interaction of the bore hole fluid with a surrounding can be controlled properly. Other acids or, in other applications, alkalines may be used as well. For instance, the concentration of a pH control additive may be in a range between 1 g/l and 50 g/l, particularly in a range between 5 g/l and 20 g/l. In an embodiment, the bore hole fluid may comprise a viscosity control additive, particularly bentonite. Such a viscosity control additive may be used in the bore hole fluid for controlling viscosity and controlling the filtrate as well. It can be also advantageous to use such a material in combination with the fibers, since it can also contribute to the

maintenance of the fiber in a homogeneous distribution in the bore hole fluid. In an embodiment, such a viscosity control additive may be provided with a concentration between 10 g/l and 100 g/l, particularly between 20 g/l and 60 g/l. In an embodiment, the bore hole fluid may comprise a fiber emulgating additive, particularly soft soap or dish liquid. Such a fiber-emulgating additive may serve to adjust the surface tension properties of the bore hole fluid. Hence, it may promote a proper emulgation of the fibers. In an embodiment, such a fiber-emulgating additive may be added optionally in a concentration between 1 g/l and 50 g/l, more particularly between 5 g/l and 20 g/l.

In an embodiment, the bore hole fluid is used in the field of oil

production, gas production, water catchment or geothermic systems. However, other applications are possible as well.

In an embodiment, forming the synthetic polymeric fibers comprises providing a foil of synthetic polymeric material (for instance made of PTFE), forming filaments therefrom, cutting the filaments into strips, and grinding the strips into smaller pieces to thereby form the fibers. In an embodiment, the filaments are formed by guiding the foil along a needle roller. For example, foils of a thickness in an order of magnitude between 1 pm and 100 μιη, particularly with a thickness between 10 μιη and 50 μηι, may be manufactured, as known by those skilled in the art. This foil may be a basis for subsequent singularization into elongate filaments, which may for instance be performed by using a needle roller or the like. Afterwards, the long filaments may be cut into strips or short pieces, for instance with a length between 10 mm and 100 mm, particularly between 30 mm and 70 mm . This may be achieved for instance by using cutter plates which cut a conveyed filament into smaller pieces. Then, these strips or shorter pieces may be grinded to thereby form the fibers. These fibers still have an aspect ratio which significantly differs from 1, particularly is larger than 2 or even 5 so that no spherical particles but still elongated fibers are maintained. In order to ensure that the size of fibers does not exceed a threshold value to a significantly extent, any separation technology may be applied for separating larger from smaller fibers. In an embodiment, the grinded strips are sorted with regard to size, particularly in a perforated strainer. Using a perforated strainer and shaking a strainer of this type will allow only fibers with a size smaller or equal to a threshold value or cutoff value to fall through the recesses in the strainer and therefore be used for the subsequent mixing with a base oil.

According to an exemplary embodiment, the bore hole fluid which has been pumped into the bore hole and has worked therein can later be pumped out of the bore hole. The removed bore hole fluid can then be reconditioned, and the reconditioned bore hole fluid can be introduced in the bore hole again. That is, the bore hole fluid may be recycled by removing the bore hole fluid together with contaminants, for instance cuttings, out of the bore hole, removing the contaminants from the bore hole fluid, for instance by filtering or cleaning the contaminated bore hole fluid, and introduce or refill the cleaned bore hole fluid again into the bore hole. After being pumped into a bore hole and later on out of the bore hole, the bore hole fluid may be recovered or recycled for a further cycle. In this context, it may also be advantageous to perform a separation of base liquid and fibers from other impurities such as drilling cuttings. The separation of the liquid components from the mixture is technologically very simple, since a solid/liquid separation can be performed with very simple measures such as a filtration. The separation of the fibers from other solid particles such as drilling cutting can be performed, for instance by making use of the different sizes and/or densities of the fibers as compared to other components such as drilling cutting. For instance, when using PTFE fibers, the density of about 2.1 g/cm³ of the PTFE fibers is different from an average density of for instance 2.6 g/cm³ of drilling cuttings. By centrifugation or other methods, a separation of the two components is possible. Also the different viscosity properties of the low friction synthetic polymer fibers on the one hand and other material on the other hand can be used for such a separation. Also different sizes may be used for the separation, for instance by using a strainer or the like.

According to an exemplary embodiment, the bore hole fluid has a temperature stability which is higher than 150°C, more particularly higher than 250°C and preferably higher than 300°C. Even higher temperature stability values, like more than 350°C may be possible.

Advantageously, a bore hole fluid is provided which comprises at least one base liquid and synthetic fibers being temperature-resistant up to 300°C or more, wherein at least 50% of the fibers may have an

extension in at least one direction of at least 10 μηι. In an embodiment, about 1 m³/min to 10 m³/min, particularly 3 m³/min to 5 m³/min, bore hole fluid may be pumped through the bore hole. It has turned out that PTFE is particularly capable of being used as a fiber material for such a bore hole fluid, because the friction with bore hole tools is sufficiently small under such conditions.

Brief description of the drawings The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited :

Fig. 1 illustrates a bore hole arrangement into which a bore hole fluid according to an exemplary embodiment of the invention is pumped.

Fig. 2 illustrates a bore hole arrangement into which a bore hole fluid according to an exemplary embodiment of the invention can be pumped.

Fig. 3 shows a lost circulation scenario in which a bore hole fluid according to an exemplary embodiment of the invention can be used.

Fig. 4 shows an ensemble of fibers for use as an additive of a bore hole fluid according to an exemplary embodiment of the invention. Fig. 5 shows a table and a diagram illustrating the length distribution of staple fibers as an additive for a bore hole fluid according to an exemplary embodiment of the invention.

Fig. 6 is a diagram illustrating a dependency between the fluid loss properties and a fiber concentration as well as a fiber length of fibers of a bore hole fluid according to an exemplary embodiment of the invention.

Fig. 7 is a diagram illustrating a dependency between a concentration of pulp fibers of a bore hole fluid according to an exemplary embodiment of the invention and fluid loss properties of the bore hole fluid.

Fig. 8 is a flow-chart illustrating a method of producing a bore hole fluid for control of fluid loss according to an exemplary embodiment of the invention.

Detailed description of the drawings

The illustrations in the drawings are schematically. In different drawings similar or identical elements are provided with the same reference signs.

In a rotary drilling process, the drilling bit is rotated and a drilling fluid is circulated in order to transport the drilling cuttings out of the bore hole. The drilling fluid or drilling mud, also denoted as bore hole fluid, fulfils besides from the transportation of the drilling cuttings, a wide range of functions like bore hole stabilization, reservoir protection, environmental compatibility and safety and cost-effectiveness. A proper bore hole fluid should be designed in a way to fulfil all these complex requirements and to be stable against temperature and bacteria. However, drilling activities shift towards deeper reservoirs, up to a depth of 10000 m and more.

Thermal stability of the bore hole fluid, in particular additives thereof, is a fundamental issue under these conditions. Even high temperature resistant additives may be prone to bacterial degradation. Access to deep and ultra-deep reservoirs therefore essentially depends on the availability of thermal and bacterial stability of bore hole fluids. According to an exemplary embodiment, these conditions may be met by adding synthetic polymer fibers, more particularly fluoropolymers such as PTFE fibers, as an additive to a base liquid to form a temperature stable and bacteria robust bore hole fluid.

In an embodiment, such a bore hole fluid may be a water-based mud. The base liquid may be fresh water, seawater, brine, saturated brine or formation brine. Further components of water-based muds are weighting agents, clay, polymers, thinners, surfactants, inorganic chemicals, bridging particles, and loss circulation material.

It is also possible to use oil-based muds which may use as a base liquid diesel oil or mineral oil . It is also possible to add brine, viscosifier, emulsifier, wetting agents, dispersants, filtrating agents and/or loss circulation material.

Also synthetic-based drilling fluids may be used.

In the following, referring to Fig. 1, a schematically illustrated bore hole arrangement 100 according to an exemplary embodiment of the invention will be explained.

Fig. 1 shows a formation 102 which comprises an arrangement of interconnected rocks or stones 104 which have in between gaps or small channels 106. Within the formation 102, a bore hole 108 has been drilled. Drilling bit and many components required for drilling the bore hole 108 are not shown in Fig. 1 but are known to a person skilled in the art. A drill string 110 is inserted into the bore hole 108. Via an inner lumen of the drill string 110, a bore hole fluid 112, which will be explained below in more detail, may be pumped into the bore hole 108. This is indicated by an arrow 114. The bore hole fluid 112 may at least partially fill the bore hole 108 for controlling fluid loss towards a surrounding wall 116 of the bore hole 108. After having pumped the bore hole fluid 112 into the bore hole 108, and after the bore hole fluid 112 has fulfilled its function there, the bore hole fluid 112 may be again pumped up and out of the bore hole 108, as indicated by arrows 118. In the shown embodiment, the bore hole fluid 112 is pumped out of the bore hole 108 via a separate annular lumen. Fig. 1 furthermore shows a detailed view 150 of a boundary between the bore hole 108 filled with the bore hole fluid 112 and the surrounding formation 102. As can be taken from the enlarged view 150, the bore hole fluid 112 comprises a base oil 130 as well as an additive consisting of synthetic polymeric poiytetrafiuorethyiene (PTFE) fibers 132. The fibers 132 are dispersed in the base oil 130.

One of the tubular or cylindrical fibers 132 is shown in larger detail in Fig. 1. It has a length L of about 150 μηη and a diameter of about 25 μηι . The cross-section of the fiber 132 is basically circular, wherein also other cross-sectional geometries are possible, for instance a rectangular (for example square) cross-section. The aspect ratio of the shown fiber 132 is 6, i.e. the ratio between 150 μηη and 25 μιτι (ratio between length and diameter). By using PTFE fibers 132 as an additive to the base oil of the bore hole fluid 112, the bore hole fluid 112 is temperature-stable even at the conditions at the bottom 222. Furthermore, the bore hole fluid 112 is not prone to decomposition by bacteria which may impact the bore hole fluid 112 along its entire propagation path. Also, the low frictional properties and the chemical inertness properties of the PTFE fibers 132 are advantageous. Beyond this, the fibers 132 in the bore hole fluid 112 also contribute to the efficient formation of a filter cake 124 for closing the gaps 106 adjacent to the bore hole 116. The filter cake 124 is an aggregation of the fibers 132 (together with material from the formation 102) which here locally have a higher concentration as compared to the remainder of the bore hole fluid 112.

The bore hole fluid 112 shown in Fig. 1 can also be utilized in the context of a rotary drilling arrangement 200 as shown in Fig. 2.

A mud pump 202 pumps a drilling fluid 112 through a standpipe 204, a rotary host 206, a swivel 208 and a kelly 210 into a drill pipe 110. The bore hole fluid 112 is passed through drill collars 212 down to a drill bit 214 where the bore hole fluid 112 may also serve for driving the downhole motor of the drilling bit 214. Through an annulus 216 between bore hole wall 116 and drill pipe 110, the bore hole fluid 112 (including now some drilling cuttings and other impurities) is conducted towards a shaker 218. From there, the material can be stored in a mud tank 220. After purification or recycling, the drilling fluid 112 may then be pumped again in a new cycle through the arrangement 200.

In some applications, the depth of the bore hole 108 below ground can be in the order of magnitude of 5 km or 10 km . With the temperature rising by about 3°C per 100 m, the temperature and the pressure conditions at the bottom of the bore hole, denoted with reference numeral 222, can be very high.

The bore hole fluid 112 also contributes to the cleaning of the bore hole 102, preserving well bore stability, balancing formation pressure to prevent well control issues, minimizing formation damage, cooling and lubricating the drill string 110 and the bit 214, transmitting hydraulic power to the bit 214, provide information about the well bore to an outside of the bore hole 102, and reduction of friction. The function of bore hole stabilization by the bore hole fluid 112 can also be seen in more detail in Fig. 3 in which a lost circulation scenario 300 is illustrated. In the lost circulation scenario 300, bore hole fluid exiting via ports 302 in the drill string 110 into the bore hole can be lost in fracture openings as indicated by reference numeral 304. When using synthetic polymeric fibers as an additive to the bore hole fluid 112, they can, alone or in combination with one or more additives, even close fracture openings which then reduces or eliminates loss circulation.

Fig. 4 shows an image taken from an ensemble of staple fibers or pulp fibers used as an additive for a bore hole fluid according to an exemplary embodiment of the invention. The staple fibers shown in Fig. 4 are PTFE fibers. As can be taken from Fig. 4, the PTFE fibers are of different size, however only a very small amount of them exceeds a size of 1 mm (as can be derived by comparison with the 5 mm bar in Fig. 4).

Fig. 5 shows a table 500 and a diagram 550 relating to the staple fibers shown in Fig. 4. By visual inspection of the individual staple fibers, the staple fibers have been ordered or classified into different groups of length . As can be taken from Fig. 5, more than 3/4 of the analyzed fibers have a length of at least 17 μηη and less than 500 μηι. These so-called staple fibers or pulp fibers have turned out to be extremely advantageous in terms of dispersion properties in a base liquid, as well as proper viscosity properties and proper filter control properties. Moreover, they show an excellent temperature stability and bacteria stability and allow for a low friction pumping of a corresponding bore hole fluid through downhole tools.

Fig. 6 shows a diagram 600 having an abscissa 602 along which three different masses of fibers per volume bore hole fluid are plotted. For each of these three concentrations of 5 g/l, 7 g/l and 15 g/l, four bars can be seen which relate to a sample having a mud without fibers, having fibers of a length of 5 mm, having fibers of a length of 10 mm and having pulp fibers (similar to Fig. 4 and Fig. 5). Along an ordinate 604, the fluid loss is plotted in ml within 15 minutes. Fluid loss was measured with a standard API filter press. 240 ml of each sample are filled into the probe. 7 bar air pressure is applied for a period of - in this case - 15 minutes. The total amount of filtrate was measured. The filter cake was

investigated with emphasis on visual aspects. As can be taken from the diagram 600, fluid loss can be significantly reduced by the addition of the fibers. However, it is a surprising result of the analysis that already with a relatively small concentration of 5 g/l of fibers, a significant improvement of the fluid loss properties was seen. As can furthermore be taken from Fig. 6, it is presently believed that pulp fibers show a better performance than longer fibers particularly at moderate to high concentrations of fibers such as 7 g/l and 15 g/l . Apart from this proper fluid loss control performance, the pulp fibers have also turned out in the experiments to be particularly appropriate in terms of dispersion effects in the base liquid.

Fig. 7 shows a table 700 illustrating a dependency of the variation of fluid loss in a 30 minutes experiment depending on the concentration of pulp fibers in the bore hole fluid. As can be taken from Fig. 7, in comparison with mud without fibers, the fluid loss can be significantly reduced by adding pulp fibers. However, it is also an astonishing result that already a small concentration of 10 g/l of pulp fibers significantly reduces the fluid loss properties. Hence, in a very cost-efficient way and without significantly disturbing the influence of the base liquid, the addition of pulp fibers is highly advantageous.

Different samples of bore hole fluids according to embodiments of the invention have been investigated :

For instance, a composition of 160 g/l K₂C0₃, 12 g/l citric acid, 10 g/l Polypac UL, 40 g/l bentonite, 10 g/l soft soap, 10 g/l fibers and 50 drops rapeseed oil has been used. The Polypac UL can also be omitted.

Another possible composition is 160 g/l K₂C0₃, 12 g/l citric acid, 10 g/l Polypac UL (which can also be omitted), 40 g/l bentonite, 12 g/l soft soap, 15 g/l fibers and 60 drops rapeseed oil .

In still another composition, 160 g/l K₂C0₃, 12 g/l citric acid, 10 g/l Polypac UL (which can be omitted), 40 g/l bentonite, 8 g/l soft soap, 7 g/l fibers and 45 drops rapeseed oil have been used.

In still another composition, 160 g/l K₂C0₃, 12 g/l citric acid, 10 g/l Polypac UL (which also can be omitted), 40 g/l bentonite, 7 g/l soft soap, 5 g/l fibers and 40 drops rapeseed can be used. For one or more of the mentioned compositions, it is possible to vary the values of the parameters (g/l, drops) by ±20%, particularly by ± 10%.

The experiments conducted with the different lengths of fibers lead to the result that pulp fibers are, at the present level of understanding, the most promising alternative for use in drilling fluids. Fluid loss and Theological parameters of different concentration of pulp fibers were therefore measured. Fluid loss measurement was conducted over a period of 30 minutes. Each experiment was performed twice to reduce the error of measurement. To reduce the value of fluid loss, the amount of bentonite can be increased, for instance from 40 g/l to 80 g/l .

Fig. 8 shows a flow diagram 800 showing different method steps of a method of manufacturing a bore hole fluid according to an exemplary embodiment of the invention.

In a block 805, a planar two-dimensional PTFE foil is manufactured. For instance, such a foil may have a thickness of 20 μηη to 25 μηι .

In a block 810, the manufactured PTFE foil is guided over a needle roller in order to form filaments (i.e. elongated strips of PTFE) of the guided foil . During this method step, the thickness of the foil does not change.

As can be taken from a block 815, the filaments received from block 810 can be cut into shorter piece using a cutter, for instance to a length of 60 mm .

In a subsequent block 820, these pieces may be grinded to form so- called staple fibers or pulp fibers. Downstream the corresponding mill, the individual fibers may then be sorted with regard to size in a block 825, for instance using a perforated strainer. This may ensure that basically no fibers are obtained which have a length of more than 1 mm . Subsequently, the obtained fibers are dispersed in a base oil such as rapeseed oil. This is performed in a block 830. At the end, optionally one or more additives 835 as described above can be added to thereby obtain the bore hole fluid. Finally, it should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The words "comprising" and

"comprises", and the like, do not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. In a device claim

enumerating several means, several of these means may be embodied by one and the same item of software or hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

C l a i m s

1. A bore hole fluid for control of fluid loss, the bore hole fluid

comprising :

a base liquid;

synthetic polymeric fibers dispersable in the base liquid as an additive.

2. The bore hole fluid according to claim 1, wherein the fibers comprise fluoropolymer fibers, particularly polytetrafluorethylene.

3. The bore hole fluid according to claim 1 or 2, wherein the fibers are staple fibers.

4. The bore hole fluid according to any of claims 1 to 3, wherein at least 50%, particularly at least 70%, of the fibers have a length of less than 0.5 mm .

5. The bore hole fluid according to any of claims 1 to 4, wherein at least 50%, particularly at least 70%, of the fibers have a length in a range between 10 μιη and 0.5 mm.

6. The bore hole fluid according to any of claims 1 to 5, wherein at least 70% of the fibers have a length in at least one dimension of at least 10 pm .

7. The bore hole fluid according to any of claims 1 to 6, wherein at least 50%, particularly at least 70%, of the fibers have an aspect ratio of larger than five.

8. The bore hole fluid according to any of claims 1 to 7, wherein a concentration of the fibers is less than 20 gram fiber mass /liter bore hole fluid, particularly less than or equal to 10 gram fiber mass /liter bore hole fluid.

9. The bore hole fluid according to any of claims 1 to 8, wherein the fibers are made of a material being temperature-resistant at least up to 200 °C, particularly at least up to 300 °C.

10. The bore hole fluid according to any of claims 1 to 9, wherein the fibers are oleophilic.

11. The bore hole fluid according to any of claims 1 to 10, wherein the fibers consist of a single homogeneous material.

12. The bore hole fluid according to any of claims 1 to 11, wherein the base liquid comprises or consists of oil, particularly at least one of the group consisting of a synthetic oil, a natural oil, and a mineral oil.

13. The bore hole fluid according to any of claims 1 to 11, wherein the base liquid comprises or consists of at least one of water and an organic solvent, particularly an alcohol.

14. The bore hole fluid according to any of claims 1 to 13, comprising a fiber sinking inhibition additive, particularly potash.

15. The bore hole fluid according to any of claims 1 to 14, comprising a pH control additive, particularly citric acid.

16. The bore hole fluid according to any of claims 1 to 15, comprising a viscosity control additive, particularly bentonite.

17. The bore hole fluid according to any of claims 1 to 16, comprising a fiber emulgating additive, particularly soft soap or dish liquid.

18. A bore hole arrangement, comprising

a formation having a bore hole formed therein;

bore hole fluid according to any of claims 1 to 17 at least partially filling the bore hole for controlling fluid loss towards a wall of the bore hole.

19. A method of controlling fluid loss in a bore hole, wherein the method comprises filling at least a part of the bore hole with a bore hole fluid comprising a base liquid and synthetic polymeric fibers dispersed in the base liquid as an additive.

20. A method of using a mixture of a base fluid and an additive of synthetic polymeric fibers as a bore hole fluid for controlling fluid loss in a bore hole having a temperature at the deepest position of at least 200 °C, particularly of at least 300 °C.

21. The method of claim 20, wherein the bore hole fluid is used in the field of one of the group consisting of oil production, gas production, water catchment and geothermic systems.

22. A method of producing a bore hole fluid for control of fluid loss, the method comprising :

forming synthetic polymeric fibers;

dispersing the fibers in the base liquid as an additive.

23. The method of claim 22, wherein forming the synthetic polymeric fibers comprises:

providing a foil of synthetic polymeric material;

forming filaments;

cutting the filaments into strips;

grinding the strips to thereby form the fibers.

24. The method of claim 23, wherein the filaments are formed by guiding the foil along a needle roller.

25. The method of claim 23 or 24, wherein the grinded strips are sorted with regard to size, particularly in a perforated strainer.